Functional Partitioning of the Human Lumbar Multifidus: An Analysis of Muscle Architecture, Nerve and Fiber Type

Distribution using a Novel 3D in Situ Approach

by

Alessandro Rosatelli

A thesis submitted in conformity with the requirements

for the degree of Doctor of Philosophy

Institute of Medical Science

University of Toronto

© Copyright by Alessandro Rosatelli 2010

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Functional Partitioning of the Human Lumbar Multifidus: An

Analysis of Muscle Architecture, Nerve and Fiber Type

Distribution using a Novel 3D in situ Approach

Alessandro L Rosatelli

Doctor of Philosophy

Institute of Medical Science University of Toronto

2010

Abstract

Muscle architecture, innervation pattern and fiber type distribution of lumbar multifidus

(LMT) throughout its volume was quantified. Musculotendinous (n=10) and neural

components (n=3) were dissected and digitized from thirteen embalmed cadaveric

specimens. The data were imported into Autodesk® Maya® 2008 to generate 3D

neuromuscular models of each specimen. Architectural parameters (fiber bundle length,

FBL; fiber bundle angle, FBA; tendon length) were quantified from the models using

customized software. The medial branch of the posterior rami (L1-L5) was traced

through LMT to determine its distribution. Using immunohistochemistry, Type I/II

muscle fibers were identified in 29 muscle biopsies from one fresh frozen specimen. The

total area and number of each cell type was calculated using Visiopharm® (image analysis

software). Architectural and fiber type data were analyzed using ANOVA with Tukey’s

post-hoc test (p ≤ 0.05).

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From L1-L4, LMT had three architecturally distinct regions: superficial,

intermediate and deep. At L5, intermediate LMT was absent. Mean FBL decreased

significantly from superficial (5.8 ± 1.6cm) to deep regions (2.9 ± 1.1cm) as did volume

(superficial, 5.6 ± 2.3ml; deep, 0.7 ± 0.3ml). In contrast, mean FBA increased from

superficial to deep. The medial branch of the posterior ramus (L1-L5) supplied the five

bands of LMT. Each medial branch in turn divided to supply the deep, intermediate and

superficial regions separately. The area occupied by Type I fibers was significantly less

(p< 0.01) in the deep (56%) compared with the superficial regions (75%).

Based on architecture and morphology, superficial LMT with the longest FBL and

relatively small FBA is well designed for torque production and controlling the lumbar

lordosis. Intermediate LMT with significantly longer FBL compared with the deep

region and with its caudal to cranial line of action may help to control intersegmental

stability. Furthermore, the absence of intermediate LMT at L5 and may contribute to the

higher incidence of instability observed at the lumbosacral junction. Deep LMT with its

short FBL, large FBA and proximity to the axis of spinal rotation may function to provide

proprioceptive input to the CNS rather than a primary stabilizer of the lumbar spine.

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Acknowledgments

If I have achieved anything thus far it is because I had the opportunity to work

with some truly great people. First and foremost I would like to thank my mentor and

research supervisor Dr. Anne Agur who has provided me support, advice and

encouragement throughout my graduate career. She has challenged me, pushed me and

made me strive far beyond what I perceived to be possible.

My sincerest thanks go to my advisory committee consisting of Dr. Bernie

Liebgott, Dr. Karan Singh, and Dr. Sharon Switzer-McIntyre for their expert advice,

assistance and guidance. I would like to recognize the participation of both my internal

and external examiners, Dr. Scott Thomas and Dr. Thomas Quinn. I also extend my

gratitude to Dr. Mike Wiley and Dr. Ian Taylor for reviewing this thesis and providing

much appreciated feedback.

I thank my fellow graduate students in the Division of Anatomy, Department of

Surgery: Soo Kim, Christopher Yuen, as well as Kajeandra and Mayo Ravichandiran

who were instrumental in software development and implementation.

Thanks go to the anatomy technical staff of Bill Wood, Terry Irvine, and Jerry

Topham for their expertise in preparing the cadaveric specimens used in my graduate

studies. Marianne Rogers at Mt. Sinai Hospital, spent many hours teaching me how to

use the image analysis software used to examine muscle biopsy specimens, I extend my

deepest gratitude. Your assistance was greatly valued.

Most importantly, I would like to thank my family for their unwavering love and

support. Their constant encouragement lifted my spirits and lightened the journey,

particularly when it was needed most. A special dedication goes to my parents from

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whom I drew strength by emulating their perseverance, desire, and dedication. Lastly, to

my wife Andrea, my best friend and most critical reviewer, thank you for believing in

me.

Acknowledgement is made to the AO/ASIF Foundation, Switzerland and the

Department of Surgery, University of Toronto for financial support.

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Table of Contents

Page

Abstract ............................................................................................................................. ii

Acknowledgments ............................................................................................................ iv

Table of Contents............................................................................................................. vi

List of Figures .................................................................................................................. xi

List of Tables .................................................................................................................. xiv

List of Abbreviations .......................................................................................................xv

Chapter 1: Introduction ....................................................................................................1 1.1 Contents of Thesis................................................................................................ 5

Chapter 2: Literature Survey ...........................................................................................6

2.1 Muscle Architecture............................................................................................. 6

2.1.1 Overview of Architectural Parameters.................................................. 6

2.1.1.1 Fiber Bundle Length (FBL) ....................................................... 9 2.1.1.2 Fiber Bundle Angle (FBA) ...................................................... 11 2.1.1.3 Muscle Volume and Mass........................................................ 11 2.1.1.4 Physiological Cross Sectional Area (PCSA) ........................... 12 2.1.1.5 Fiber Type Distribution ........................................................... 13

2.1.2 Functional Significance of Muscle Architecture ................................ 14

2.2 Why Study Human Lumbar Multifidus Architecture? ...................................... 17 2.3 Previous Studies on the Morphology, Architecture, Innervation and Fiber Type distribution of LMT .................................................................................. 19

2.3.1 Morphology of LMT........................................................................... 19 2.3.2 Architecture of LMT........................................................................... 20

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2.3.2.1 Qualitative/Descriptive Studies ............................................... 20 2.3.2.2 Quantitative Cadaveric Studies and their Results.................... 24

2.3.2.2.1 Quantification of FBL....................................................... 24 2.3.2.2.2 Quantification of FBA ...................................................... 25 2.3.2.2.3 Quantification of Volume ................................................. 29

2.3.3 Innervation of LMT ............................................................................ 29

2.3.3.1 Motor Control of Lumbar Stability.......................................... 30

2.3.4 Fiber Typing of LMT.......................................................................... 31

2.3.4.1 Cadaveric Investigations.......................................................... 32 2.3.4.2 In Vivo Investigation ............................................................... 33 2.3.4.3 Comparison of Cadaveric and In Vivo Measurements ............ 34

2.4 Functions of LMT based of Morphological, Biomechanical, Electromyographic and Clinical Evidence......................................................... 35

2.4.1 Morphological Evidence..................................................................... 35 2.4.2 Biomechanical Evidence..................................................................... 37

2.4.2.1 Role of LMT in Torque Production ......................................... 37 2.4.2.2 Role of LMT in Spinal Stability .............................................. 39

2.4.2.2.1 Role of LMT in Maintaining the Lumbar Lordosis .......... 39 2.4.2.2.2 Role of LMT in Controlling Shear Forces........................ 40 2.4.2.2.3 Biomechanical Models for the Stability Role................... 40 2.4.2.2.4 Role in Providing Stiffness to the Spine ........................... 44

2.4.3 Electromyographic Evidence .............................................................. 45

2.4.3.1 LMT Activity Involved in the Maintenance of Posture........... 46

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2.4.3.2 LMT Activity in Active Lumbar Movements ......................... 46 2.4.3.3 LMT Activity During Internal and External Perturbations of the Trunk.......................................................................... 48

2.4.4 Clinical Evidence ................................................................................ 49 2.4.5 Summary ............................................................................................. 50

Chapter 3 Hypothesis and Objectives............................................................................51

3.1 Hypotheses......................................................................................................... 51 3.2 Objectives .......................................................................................................... 51 3.3 Significance........................................................................................................ 52

Chapter 4 Methods ..........................................................................................................53

4.1 Digitization, modeling and quantification of architectural parameters of LMT ................................................................................................................... 53

4.1.1 Specimens ........................................................................................... 53 4.1.2 Serial dissection and digitization of muscle fiber bundles, tendons and spinal column ............................................................................... 53 4.1.3 Microscribe® 3G2 Digitizer ............................................................... 57 4.1.4 3D reconstruction and modeling of LMT ........................................... 58 4.1.5 Quantification of architectural parameters of LMT............................ 59

4.1.5.1 Fiber Bundle Length (FBL) ................................................. 59 4.1.5.2 Tendon Length ..................................................................... 59 4.1.5.3 Fiber bundle angle (FBA) .................................................... 60 4.1.5.4 Volume................................................................................. 61

4.1.6 Statistical analysis of architectural parameters ................................... 61

4.2 Digitization and modeling of the intramuscular nerve distribution of LMT ..... 62

4.2.1 Specimens ........................................................................................... 62

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4.2.2 Serial dissection and digitization of the medial branch of the posterior rami of L1 to L5................................................................... 62 4.2.3 Reconstruction, modeling and analysis of intramuscular nerve distribution ................................................................................ 65

4.3 Fiber typing of LMT .......................................................................................... 66

4.3.1 Specimen(s)......................................................................................... 66 4.3.2 Sectioning and Immunohistochemistry............................................... 68 4.3.3 Morphometric analyses of Type I/II fibers ......................................... 69 4.3.4 Statistical analysis............................................................................... 70

Chapter 5 Results.............................................................................................................72

5.1 Morphology and Architecture of Lumbar Multifidus........................................ 72

5.1.1 Superficial LMT.................................................................................. 72 5.1.2 Intermediate LMT............................................................................... 73 5.1.3 Deep LMT........................................................................................... 73 5.1.4 Architectural parameters..................................................................... 76 5.1.5 Tendon architecture ............................................................................ 79

5.2 Innervation of LMT ........................................................................................... 80

5.2.1 3D Model ............................................................................................ 80 5.2.2 Nerve distribution through LMT ........................................................ 83

5.3 Mean characteristics of muscle fiber type for LMT: Pilot Study ...................... 92

5.3.1 Fiber type distribution......................................................................... 92 5.3.2 Fiber type diameter ............................................................................. 96

Chapter 6 Discussion ....................................................................................................100

6.1 Introduction...................................................................................................... 100

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6.2 Morphology and Architecture.......................................................................... 101 6.2.1 Morphology....................................................................................... 102 6.2.2 Measurement of architectural parameters of LMT ........................... 104

6.2.2.1 Fiber Bundle Length (FBL) ............................................... 105 6.2.2.2 Fiber Bundle Angle (FBA) ................................................ 107 6.2.2.3 Volume............................................................................... 110 6.2.2.4 Physiological Cross Sectional Area (PCSA) ..................... 111

6.3 Innervation ....................................................................................................... 114 6.4 FiberType Characteristics ................................................................................ 115

6.4.1 Fiber Type Distribution..................................................................... 118 6.4.2 Fiber Size .......................................................................................... 122

6.5 Functional considerations ................................................................................ 123 6.6 3D Reconstruction and Modelling: Pros and Cons......................................... 127

Chapter 7 Conclusions .................................................................................................129

7.1 Functional paradigm ........................................................................................ 130

Chapter 8 Future Direction .........................................................................................132

Chapter 9 References ...................................................................................................134

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List of Figures

Page Figure 2.1. Muscle architectural parameters.................................................................. 12 Figure 2.2. Length-force curve of two muscles with different PCSAs but equal FBL. ............................................................................................................. 16 Figure 2.3. Length-force curve of two muscles with different fiber lengths but equal FBA and PCSAs................................................................................. 17 Figure 2.4. Drawing of the cervical vertebrae showing how the cervical musculature stabilizes the cervical spine similar to guywires stabilizing the mast of a ship........................................................................ 20 Figure 2.5. Illustrations of the fascicles of lumbar LMT as seen in a posterior- anterior view. ............................................................................................... 23 Figure 2.6. Posteroanterior view of the mean FBAs of the various fascicles of multifidus from the Ll to L5 spinous processes........................................... 26 Figure 2.7. Lateral view of the mean orientation of the fascicles of the multifidus from the Ll to L5 spinous processes. ........................................................... 27 Figure 2.8. Posterior view of lumbo-sacral spine showing typical orientation of fascicle of LMT............................................................................................ 37 Figure 4.1. Digitization of human LMT. ....................................................................... 54 Figure 4.2. Delineation of muscle fiber bundle of LMT (left, lateral view of lumbosacral spine). ...................................................................................... 55 Figure 4.3. Close up, lateral view of LMT originating from the L1 spinous Process showing a small segment of tendon (left, lateral view of spine). ... 56 Figure 4.4. Right, lateral view of digitized lumbar spine and sacrum as viewed in Autodesk® Maya®. ..................................................................................... 57 Figure 4.5. The Immersion Company Microscopic 3-G2 Digitizer............................... 57 Figure 4.6. Measurement of muscle fiber bundle length and tendon length of LMT (right, lateral view of lumbosacral spine). ................................................... 59 Figure 4.7. Calculation of muscle fiber bundle angle (right, lateral view of lumbosacral spine). ...................................................................................... 60

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Figure 4.8. Measurement of muscle volume.................................................................. 61 Figure 4.9. Flowchart outlining the process of serial dissection and digitization of the medial branch of the posterior ramus. ............................................... 64 Figure 4.10. Digitization of bony skeleton, 3D reconstruction........................................ 65 Figure 4.11. Lateral view of LMT showing deep (purple), intermediate (yellow) and superficial (red) regions. ....................................................................... 67 Figure 4.12. Typical microscopic view of the transversely sectioned LMT muscle. ...... 68 Figure 5.1. Digitization and three dimensional modeling of superficial, intermediate and deep regions of lumbar multifidus (LMT) of a cadaveric specimen, lateral views................................................................ 74 Figure 5.2. Digitization and three dimensional modeling of superficial segments of lumbar multifidus (L1-L5), of a cadaveric specimen, lateral views........ 75 Figure 5.3. Histogram of fiber bundle length (FBL). .................................................... 79 Figure 5.4. Views of the nerve supply to lumbar multifidus (LMT) by rotation of model............................................................................................................ 81 Figure 5.5. Views of the nerve supply to lumbar multifidus (LMT) at different magnifications.............................................................................................. 82 Figure 5.6. Views of the nerve supply to specific regions of lumbar multifidus (LMT). ......................................................................................................... 82 Figure 5.7. Lateral view of lumbosacral spine showing medial branches (L1 to L5) which supply the five bands of LMT………………………………………86 Figure 5.8. Lateral view of the lumbar spine showing the extramuscular course of the L1 medial branch (solid blue line) traversing the intersection (shaded blue area) formed between the transverse process (tp) and superior articular process (sap) of L2. ......................................................... 87 Figure 5.9. Dissection of lumbar multifidus showing extramuscular course of L1 medial branch, right lateral view. ............................................................... 87 Figure 5.10. Close up lateral view of the L2 lumbar vertebra and L1 medial branch. .... 88 Figure 5.11. Dissection of lumbar multifidus, right lateral view showing main trunk of L1 medial branch (yellow dotted line) giving off a nerve branch to supply deep LMT (a: red dotted line). ........................................ 88

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Figure 5.12. Lateral view of lumbar spine showing L1 medial branch giving off articular branch (solid red line) to supply the superior zygapophyseal joint. ............................................................................................................. 89 Figure 5.13. Lateral view of the lumbar spine showing medial branch dividing into three branches. ............................................................................................. 89 Figure 5.14. Dissection of lumbar multifidus (LMT), right lateral view showing medial branch (mb) of posterior ramus giving off branches to supply superficial (red), and intermediate (blue) regions of LMT attaching to the L1 spinous process and laminae............................................................. 90 Figure 5.15. Lateral view of digitized spine showing the L1 medial branch dividing into three branches to supply the three separate fascicle of superficial LMT attaching superiorly to the L1 spinous process................................... 90 Figure 5.16. Dissection of lumbar multifidus (LMT), right lateral view showing medial branch (mb) of posterior ramus giving off branches to supply fascicle of superficial LMT attaching superiorly to the L1 spinous process.......................................................................................................... 91 Figure 5.17. Lateral view of lumbosacral spine showing the L5 medial branch innervating deep and superficial regions of LMT........................................ 91 Figure 5.18. Mean area of Type I fibers for each region expressed as a proportion. ...... 93 Figure 5.19. Comparison of the mean area occupied by Type I fibers between the deep, intermediate and superficial regions of LMT..................................... 94 Figure 5.20. Mean areas of Type I fibers expressed as a proportion. .............................. 95 Figure 5.21. The relationship between mean Type I cell diameters and spinal level. ..... 97 Figure 5.22. The relationship between mean Type I fiber diameters and spinal level and region..................................................................................................... 98 Figure 5.23. The relationship between mean Type II fiber diameters and region. .......... 99 Figure 6.1. Lateral view of lumbosacral spine showing superficial and intermediate regions of LMT attaching to the L1 spinous process............ 104 Figure 6.2. Bar graph showing comparison of FBL values as a function of segmental level........................................................................................... 107 Figure 6.3. The net action of a given fascicle of LMT is dependant on the number and distribution of muscle fibers bundles. ................................................. 109

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List of Tables

Page Table 2.1. Muscle Length and fiber bundle length of selected upper and lower limb

muscles…………………………………………………………………… 10 Table 2.2. Architectural Data from Previous Studies………………………………… 28 Table 4.1. Spatial distribution of muscle biopsies taken from specimen……………… 67 Table 5.1. Summary of Mean FBL for LMT………………………………………….. 76 Table 5.2. Summary of Mean FBA for LMT………………………………………… 77 Table 5.3. Summary of Mean Volume for LMT……………………………………… 78 Table 5.4. Tendon length, FBL and muscle lengths of superficial and intermediate

regions……………………………………………………………………… 80 Table 5.5. LMT Type I fiber proportions (mean ± SD)………………………………. 92 Table 5.6. LMT fiber type diameters (mean ± SD)……………………………………. 96 Table 6.1. Comparison of LMT muscle fiber angles of different studies including the

current……………………………………………………………………… 109 Table 6.2. Comparison of PCSA of the current and previous studies………………... 113

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List of Abbreviations

2D Two dimensional 3D Three dimensional ALP Alkaline Phosphatase BSA Bovine serum antibody CNS Central nervous system CSA Cross sectional area CT Computed tomography ΔL Change in length of muscle fiber

lδ Change in length of sarcomere EMG Electromyography F Force FBA Fiber bundle angle FBL Fiber bundle length GNP Gross national product HCI Hydrochloric acid L Lumbar LBP Low back pain LMD Least mean diameter LMT Lumbar multifidus

m Mass ML Muscle length mp Mammillary process MRI Magnetic resonance imaging MVC Maximal voluntary contraction N Newton NA Not applicable NZ Neutral zone PCSA Physiological cross sectional area PSIS Posterior superior iliac spine θ FBA S1 1st Sacral vertebra Sa Sacrum SD Standard deviation sp Spinous process TL Tendon length tp Transverse process V Volume ZJ Zygapophyseal joint

1

Chapter 1 Introduction

Studies examining the microscopic properties of skeletal muscle fibers have

provided great insights into their function (Lieber & Friden, 2000). However, relatively

little work has been done to quantify the architectural characteristics of human skeletal

muscle. The architecture of a muscle consists of its external configuration and

dimensions, and the internal arrangement and morphology of the contractile and

connective tissue elements. Even though two muscles may have the same external

configuration they may differ significantly in function due to differences in the internal

arrangement of their contractile and connective tissue elements (Lieber & Friden, 2000).

Why is it important to study muscle architecture? Put simply, it is an important

determinant of function (Roy & Ishihara, 1997; Lieber & Friden, 2000). For example,

physiological cross-sectional area (PCSA) is calculated from the architectural parameters of

a muscle and is considered to be directly proportional to the maximum force or tension that

can be generated by that muscle (Gans, 1982). In contrast, fiber length is proportional to

fiber excursion and determines the range of lengths over which a muscle can generate

active force (Zajac, 1989).

Visualization of muscle architecture and geometry has for the most part relied on

data collected in 2D, either from cadaveric specimens (Friederich & Brand, 1990)or from

imaging techniques, such as ultrasound (Maganaris et al., 1998; Chow et al., 2000; Martin

et al., 2001). It is only recently due to advances in the area of computer modeling and

digitization that the study of skeletal muscle architecture has taken a three dimensional

perspective (Agur et al., 2003; Kim, Boynton et al., 2007).

2

One area which has received relatively little attention in regard to both the

quantification and visualization of muscle architecture has been muscles of the back,

despite the high incidence of disability due to low back pain (LBP) in the general

population (Anderson, 1999). The lifetime prevalence of LBP has been estimated to

range anywhere from 70% to 85% (Anderson, 1999). In any one year, the incidence of

back pain is reported to affect approximately 5% of the population. In fact, it is estimated

that in Europe and the United States at least 1% to 2% of the gross national product

(GNP) is allocated towards the management of this condition (Norlund & Waddell,

2000). In the United States this equates to approximately 1-2 billion dollars. Studying

the detailed architecture of the back muscles may lead to better biomechanical models

which would ultimately improve our understanding of LBP.

Cholewicki and McGill (1996) and Crisco and Panjabi (1991) have demonstrated

the vital role of the deep, local muscles in controlling spinal stiffness. One such muscle

thought to be pivotal in this regard is the lumbar multifidus (LMT). LMT is “the most

medial of the back muscles and is the largest muscle that spans the lumbosacral junction.

Due to its prominence in this region, it is a preferred target for diagnostic paraspinal

electromyography (EMG), and has been the subject of histochemical studies of patients

with lumbar disorders” (Macintosh et al., 1986). In addition, alteration in neuromuscular

recruitment of this muscle following acute LBP has been postulated to predispose the

lumbar spine to further injury and instability (Hides et al., 1996; Hides et al., 2001).

However, due to the lack of architectural data available on LMT, the precise nature,

distribution and magnitude of forces exerted on the lumbar spine by this muscle is

3

unavailable. Hence the functional role(s) of LMT within the lumbar spine remains

largely unknown.

Fortunately advances in the area of computer graphics and modeling now make it

possible to collect large quantities of morphological data to reconstruct the 3D

architecture of skeletal muscle in situ (Ng-Thow-Hing, 2001; Agur et al., 2003; Kim,

Boynton et al., 2007). Using these techniques of data acquisition and modeling, the

current study examines the morphology and architecture of LMT throughout its entire

volume.The results obtained may help explain, among other things, how the fibers of

LMT work collectively to provide multidirectional movement to the lumbar spine while

also providing stability.

Making matters even more complex, is the notion that certain muscles (e.g.

supraspinatus) can be divided into architecturally distinct regions defined by fiber bundle

orientation i.e. fiber bundle length, pennation angle (or fiber bundle angle) and tendinous

attachments (Kim, Boynton et al., 2007) The unique arrangement of fiber bundles

suggests that some muscles like supraspinatus may be composed of “neuromuscular

compartments” which have specific, task-oriented roles. Neuromuscular compartments

are defined as architecturally distinct regions within the muscle which are independently

innervated by an individual nerve branch. Each compartment contains motor unit

territories with a unique array of physiological attributes (English & Letbetter, 1982a). In

other words, the intensity and timing of motor unit activation of each compartment can be

independently controlled and can vary between regions. English & Letbetter (1982b)

showed that the distribution of fiber types (Type I and Type II) within a muscle can vary

4

between compartments. Thus, the endurance and force producing abilities of individual

muscle compartments may differ.

Evidence suggests that LMT provides stability to the lumbar spine but also

mobilizes and produces movement. This functional duality suggests that LMT like

supraspinatus is made up of neuromuscular compartments, with each compartment sub-

serving a particular function. To substantiate the presence of neuromuscular

compartments within LMT requires a thorough investigation of the innervation and fiber

type distribution pattern throughout the volume of the muscle. To date however neither

has not been investigated fully. Hence, there is insufficient morphological and

histochemical evidence to support the hypothesis that LMT consists of architecturally

distinct regions which are independently innervated by a single motor branch. Nor do we

know if the fiber type distribution through the volume of LMT differs in any way from

superficial to deep or from superior to inferior.

In this study, the muscle architecture, innervation and fiber type distribution of the

human LMT has been documented throughout its entire volume. Ultimately, these data

may lead to the development of more robust, biomechanical models to help elucidate the

relationship between structure and function as it applies to the human LMT. With a

better understanding of the 3D architecture of the LMT, including its nerve and fiber type

composition, new insights into the way in which this muscle helps to protect and stabilize

the spine during activities of daily living can be developed

5

1.1 Contents of Thesis The present thesis consists of eight chapters presented in the following sequence.

Chapter 1 provides introductory background information on the importance of

skeletal muscle architecture, nerve supply and fiber type distribution to muscle function.

Chapter 2 is the literature survey which is intended to provide background

information on the structure and function of skeletal muscle, muscle modelling and

imaging modalities including ultrasound and magnetic resonance imaging used to study

human muscle in vivo. The existing literature of the architecture and functions of the

cadaveric and in vivo human LMT is reviewed in detail.

Chapter 3 includes the hypotheses, objectives and significance of the study. The

anatomy and terminology used to describe LMT in the present thesis are also defined.

Chapter 4 outlines the methods that are used to address the hypotheses and

objectives of this thesis.

Chapter 5 is a summary of the results. This section is divided into three parts.

The first section reports both morphological and architectural data on LMT including

visualization of the complex fiber bundle arrangement of this muscle in situ using 3D

computer modelling. The next section documents the detailed course of the medial branch

of the posterior ramus through the volume of LMT. The final section reports on the

distribution of Type I and II muscle fibers throughout the volume of the muscle.

Chapter 6 is a discussion of the results and innovations of this thesis.

Chapters 7 and 8 consist of the conclusions and future directions of this work.

6

Chapter 2 Literature Survey

This literature survey provides background information for this comprehensive

study of the muscle architecture, nerve and fiber type distribution of LMT. Section 2.1

provides the reader with a general overview of skeletal muscle architecture, including

discussion of architectural parameters important in defining and understanding muscle

function. Section 2.2 discusses briefly the importance of studying the morphology and

architecture of lumbar multifidus. Section 2.3 provides an overview of the morphology,

nerve supply, fiber type distribution and action(s) of LMT. A summary of previous

descriptive and quantitative studies of LMT architecture, including previous data on FBL,

FBA and muscle volume (V) are also included in this chapter. Lastly, in Section 2.4,

studies which have investigated the function(s) of lumbar LMT are discussed.

2.1 Muscle Architecture 2.1.1 Overview of Architectural Parameters

The arrangement of muscle fibers (i.e. muscle architecture), is a primary

determinant of muscle function (Lieber & Friden, 2000). Hence, understanding how

muscle structure influences muscle function is of significant scientific and clinical

importance. This structure-function relationship is essential for the following reasons:

“Clarifies the physiological basis of force production, movement” (Lieber &

Friden, 2000) and stability (Panjabi et al., 1989; Panjabi, 1992b, 1992a; Wilke

et al., 1995; Cholewicki & McGill, 1996).

7

Provides data for the proper placement of electromyographic surface

electrodes with respect to the muscle fiber direction which is critical for

obtaining valid measures of muscle activity (De Foa et al., 1989),

Helps infer the “mechanical basis of muscle injury during normal movement”

(Lieber & Friden, 2002), and

Assists in the “interpretation of histological makeup of specimens obtained

from muscle biopsies” (Roy et al., 1991).

Despite the importance of muscle architecture, little attention has been directed

towards quantifying many of its associated parameters. These include measures of: fiber

bundle length (FBL), muscle length (ML), fiber bundle angle (FBA) (also used

interchangeably with pennation angle), muscle volume, density, physiological cross

sectional area (PCSA) and fiber type distribution. Most anatomical text books typically

depict the macroscopic structure of skeletal muscle as collections of muscle fiber bundles

(i.e. fascicles) projecting from a point of “origin”, to a point of “insertion”. This

oversimplification of skeletal muscle architecture does not adequately capture the in situ,

complex, three dimensional nature of this highly organized tissue. The architecture of a

given muscle has been shown to be relatively consistent between members of the same

species (Lieber & Friden, 2000). This being said, the function of a particular muscle may

be altered by pathology, injury or disease processes, particularly if the macroscopic

arrangement of its muscle fibers are affected. Therefore, it is essential to clearly establish

the normal structure and architectural parameters of skeletal muscle tissue in order

compare and contrast the same parameters examined in abnormal skeletal muscle tissue.

8

It has been argued that the architectural properties of FBL, muscle length, and

FBA are the “most structurally significant parameters, whereas FBL, muscle length, and

fiber type distribution are the most functionally determining” (Burkholder et al., 1994).

In addition, architectural differences between muscles have been shown to result in

significant effects on tension and contractile speed among muscles despite having the

similar fiber type (Gans, 1982; Sacks & Roy, 1982; Roy et al., 1984). In this way, the

contractile properties of muscle can be modulated by changing the muscle’s architectural

properties. For example, Mohagheghi et al (2007) used ultrasonography to assess, in

vivo, the gastrocnemius muscle architecture in the paretic and non-paretic legs of eight

children with cerebral palsy. They found that fiber bundle/fascicle length and muscle

thickness were reduced by up to 18% and 20% in the paretic compared to the non-paretic

legs respectively. The authors concluded that paresis in hemiplegic cerebral palsy may

affect the geometry of skeletal muscle which in turn will alter its function.

Ward et al (2006) examined the architectural properties of the rotator cuff muscles

in 10 cadaveric specimens and concluded these muscles are important in maintaining

glenohumeral stability both at rest and in end range positions. These authors also

suggested small changes in rotator cuff muscle length that can occur as a result of

surgery, may result in relatively large changes in shoulder function.

Kim, Boynton et al (Kim, Boynton et al., 2007) studied the muscle architecture of

the supraspinatus muscle in 10 cadaveric specimens. The authors found that the muscle

belly of supraspinatus could be divided into anterior and posterior regions. Each region,

in turn, could be further subdivided into superficial, middle, and deep parts. The

significantly larger muscle volumes associated with the anterior region when compared

9

with the posterior region were suggested to influence the amount of force generated by

each region, with the anterior region generating the majority of the muscle’s tensile force.

The significant differences in fiber bundle angle found among different parts of the

anterior region of the muscle was postulated to result in a heterogeneous distribution of

forces and hence influence the higher occurrence of deep, articular tears of the anterior

tendon with rotator cuff pathology.

Based on the aforementioned evidence, muscle architectural parameters such as

FBL, FBA, ML, PCSA, etc…directly influence muscle function and pathology.

However, before proceeding any further, it is important to define these variables.

2.1.1.1 Fiber Bundle Length (FBL) FBL is the length of a muscle fiber bundle from its most proximal end to its most

distal end (Figure 2.1). The length of a muscle fiber is determined by the number of

sarcomeres arranged end to end. Within one muscle fiber bundle the sarcomere length

remains quite consistent (Wickiewicz et al., 1983). When a muscle contracts, each

sarcomere shortens proportionately (Alberts et al., 1989), resulting in shortening of the

muscle fiber by approximately one third of its length (Enoka, 1988). This relationship

can be expressed as follows:

( )L n lδΔ =

ΔL is the change in length of the muscle fiber

n is the number of sarcomeres in series

lδ is the change in length of a sarcomere

10

Therefore, a muscle with more sarcomeres in series undergoes the greatest

absolute change in length thereby resulting in greater muscle excursion or degree of

shortening.

Muscle length (ML) in comparison to FBL is defined as “the distance from the

origin of the most superior muscle fibers to the insertion of the most inferior fibers”

(Lieber, 2002). Under most circumstances, ML is greater than FBL (Table 2.1);

however, as muscle fibers become oriented more parallel to the force generating axis of

the muscle, these two values approach one another (Table 2.1). For example the muscle

fibers of the brachioradialis muscle are oriented almost parallel to the force generating

axis of the muscle and its ML (175 mm ± 8.3mm) is approximately equal to its FBL

121mm ± 8.3mm.

Muscle Muscle Length (mm) Fiber Bundle Length Brachioradialis (BR) 175 ± 8.3 121 ± 8.3 Biceps Femoris (BF) 271 ± 11 139 ± 3.5 Flexor Pollicis Longus (FPL) 168 ± 10.0 45.1 ± 2.1 Medial Gastrocnemius (MG) 248 ± 9.9 35.3 ± 2.0 Popliteus (POP) 108 ± 7.0 29.0 ± 7.0 Pronator Quadratus (PQ) 39.3 ± 2.3 23.3 ± 2.0 Pronator Teres (PT) 130 ± 4.7 36.4 ± 1.3 Rectus Femoris (RF) 316 ± 5.7 66.0 ± 1.5 Sartorius (SAR) 503 ± 27 455 ± 19 Semimembranosus (SM) 262 ± 1.5 62.7 ± 4.7 Soleus (SOL) 310 ± 1.5 19.5 ± 0.5 Vastus Lateralis (VL) 324 ± 14 65.7 ± 0.88 Vastus Medialis (VM) 355 ± 15 70.3 ± 3.3

Table 2.1. Muscle Length and fiber bundle length of selected upper and lower limb muscles (Lieber, 2002).

11

2.1.1.2 Fiber Bundle Angle (FBA) In general for skeletal muscle, FBA is defined as the angle subtended between a

muscle fiber and its force-generating axis. It is usually measured by determining the

average angle of fibers on the superficial surface of the muscle. When the fibers run

essentially parallel to the line of force, the muscle is said to be longitudinal. If the fibers

are oriented at a single angle relative to the line of force then they are said to be

unipennate. While, muscles that are made up of fibers that are oriented at several angles

relative to the axis of force generation are termed multipennate (Figure 2.1). Generally,

the greater the FBA, the smaller the amount of effective force transmitted to the tendon.

This relationship is represented using the following equation:

Fnet= F(muscle fiber bundle) cos θ

(Enoka, 1988)

As the fiber angle θincreases, the force transmitted to the attachment site will

consequently decrease. Hence, using the cosine law, as θ approaches 90 degrees, the net

force generated by the muscle fiber approaches zero.

2.1.1.3 Muscle Volume and Mass Volume and mass can also be measured. Volume is usually measured using water

displacement techniques and mass is measured as the wet weight of the muscle

(Friederich & Brand, 1990). If the volume and mass of a muscle are known, the density

can be calculated using the following formula:

F represents force θ represents the FBA

12

Figure 2.1: Muscle architectural parameters. (A) Longitudinal arrangement of muscle fiber bundles running parallel to the muscles’ force-generating axis. Note: Since muscle fibers run parallel to the axis with which force is generated (i.e. tendon), then the fiber bundle angle (FBA) is zero; (B) Unipennate arrangement of muscle fiber bundles. Fibers run at a fixed angle relative to the muscles’ force-generating axis. (C) Multipennate architecture in which muscle fibers run at several angles relative to the muscles’ force-generating axis. ML, muscle length; FBL, fiber bundle length; FBA, fiber bundle angle. (Adapted from Lieber and Friden, 2000, Figure 1 using Figures 5.23B, 5.23C, and 6.32 from Grant’s Atlas of Anatomy, 2005).

2.1.1.4 Physiological Cross Sectional Area (PCSA)

Perhaps one of the most important architectural parameters to quantify is PCSA.

It is usually derived from the more common architectural parameters discussed

previously. This measure is important as it is directly proportional to the amount of force

a muscle can generate. The PCSA of a muscle is represented by the formula:

ρ (g/cm3)* Fiber length (cm)

PCSA (cm2) = Muscle Mass (g) * cosine

θ = fiber bundle angle ρ = muscle density

13

Estimations of muscle force are sometimes based on muscle volume, without

taking into consideration the arrangement of muscle fibers (i.e. muscle architecture).

Using cross sectional areas of muscles obtained from Computed tomography (CT) or

Magnetic resonance imaging (MRI) images has typically led to erroneous estimations of

force production and false extrapolations of muscle function (McGill et al., 1988).

Transverse sections taken of a muscle with CT or MRI do not capture the large number of

muscle fibers that are potentially present in pennated muscles. As a result, muscle forces

are commonly underestimated. Hence, areas obtained from CT or MRI must take into

consideration the architecture of a muscle (McGill et al., 1993).

2.1.1.5 Fiber Type Distribution The fiber type composition of a skeletal muscle is an important parameter to

consider when determining the function of a muscle. There are two main fiber types:

Type I (slow twitch) and Type II (fast twitch). Although muscles typically have a

mixture of both these fiber types, there is usually a predominance of one that influences

the contractile properties of the entire muscle (Lieber, 2002). Type I fibers are found to

predominate in postural muscles that are able to maintain repetitive contractions for long

periods of time before fatigue occurs. Type II fibers fatigue more easily, but can contract

about three times as fast as Type I fibers and are suited for generating high forces for

short periods of time. Each fiber type has a different protein composition which can be

altered by hormonal and neuronal factors as well as specific exercise training. These

changes can result in subtle changes in the contractility of a muscle (Lieber, 2002).

14

2.1.2 Functional Significance of Muscle Architecture Examination of mammalian muscles tissue reveals a complex internal

architecture. Lieber and Blevins (1989) pointed out, “skeletal muscle contractile

properties are a function of both the intrinsic muscle fiber properties and the fiber

arrangement within the muscle (i.e., the architectural design)”. Hence, the determination

of muscle function is more intricate than just considering ‘origin and insertion’, or

assessing activity profiles with EMG. As Liber & Friden (2000) point out, muscle

architecture has a significant impact on the functional properties of a muscle. In fact,

architectural differences between muscles are excellent predictors of force generation

(Lieber & Friden, 2000).

The functional effect of muscle architecture can be simply stated as: “muscle force

is proportional to the physiological cross sectional area PCSA, and muscle velocity and

excursion are proportional to the fiber length” (van Eijden et al., 1995). Neither fiber

length nor PCSA can easily be measured based on gross muscle inspection. Instead,

detailed dissections of cadaveric muscles are required for architectural determination

(Lieber, 2002). Upon determining architectural properties, it is possible to understand

how much force the muscle generates and how fast it contracts.

Take for example the human pterygoid muscles which have a complex architectural

design. The lateral pterygoid is fan shaped with relatively long muscle fibers, while its

counterpart, the medial pterygoid, is multipennated with short muscle fibers (Williams,

1995). Furthermore, the lateral head is composed of two separate heads or regions, a

superior head and an inferior head. The superior head is thought to be activated during

jaw closing, while the inferior head is activated during opening (Juniper, 1981; Wood et

15

al., 1986). The medial pterygoid is a primary elevator of the mandible (Friedman, 1988).

In order to determine the magnitude and degree of excursion associated with the lateral

and medial pterygoid muscles architectural parameters such as FBL and PCSA must first

be quantified experimentally. Using the results obtained from eight cadavers, van Eijden

et al, 1995 showed that these parameters vary between the pterygoids. The lateral

pterygoid is characterized by relatively long fibers and a small PCSA, whereas the medial

pterygoid has relatively short fibers and a large PCSA. The mechanical consequence is

that the lateral pterygoid can produce displacements and velocities that are 1.7 times

larger than the medial pterygoid, whereas the medial pterygoid can produce forces that

are about 1.6 times greater than the lateral pterygoid. Similar results have been found in

other human skeletal muscles in both the upper (Lieber et al., 1990) and lower extremities

(Wickiewicz et al., 1983); (Friederich & Brand, 1990).

Two specific architectural examples and their impact on the length-tension and

force-velocity relationships are illustrated below.

Assume that two muscles had identical FBL and FBA, but one muscle had twice the

PSCA. What effect would this have on the functional properties of the muscle? Figure

2.2 below demonstrates the only functional effect that would occur is an increase in

maximum tension so that the length-tension curve would be the same basic shape but

simply amplified upward in the case of the stronger muscle (Lieber, 2002).

16

Figure 2.2. Length-force curve of two muscles with different PCSAs but equal FBL. The muscle with the larger PCSA produces the greatest force output. (Adapted from Lieber and Friden, 2000, Figure 8).

On the other hand, if two muscles with identical PCSAs and fiber bundle angles had

different fiber lengths the effect would be an increase in the muscle velocity (i.e. increase

muscle excursion). The peak absolute force of the length-tension curves would be

identical, but the absolute muscle active range would be different (Figure 2.3).

17

Figure 2.3. Length-force curve of two muscles with different fiber lengths but equal FBA and PCSAs. The effect is to increase the range through which a muscle can generate a contraction but with retention of the same peak or maximal force. (Adapted from Lieber and Friden, 2000, Figure 9). 2.2 Why Study Human Lumbar Multifidus Architecture? The isolated osseoligamentous lumbar portion of the spine is inherently unstable

and has been shown to buckle under compressive loads of only 90N. In marked contrast,

competitive weightlifters have safely and routinely exceeded 20,000 N of compressive

force without their spines showing signs of mechanical failure (McGill, 2007). The

human spine can obviously tolerate a great deal of stress, but the question remains how?

Panjabi (1992b, 1992a) proposed a model of lumbar spinal stability which seemed to

18

provide a plausible explanation to this apparent paradox as well as an impetus for more

research. In his model, he proposed that spinal stabilization is a function of the

interaction of three systems: the osseoligamentous system, the muscular system and the

neural control system. Unlike earlier models of the spine which focused only on the role

of passive structures (e.g. ligaments and joints) in stabilizing the spine, Panjabi’s model

highlighted the important role of muscles, especially the deep (local) muscles (e.g.

lumbar multifidus) in controlling spinal stiffness.

Human muscle architectural data, particularly those pertaining to the lumbar back

muscles, are incomplete and based on relatively small sample sizes (McGill et al, 1986).

In addition, the sites of data collection have not been identified precisely within the

muscle (Yamaguchi et al., 1990). As a consequence, modeling studies which included

skeletal muscle did so in a “nominal or abbreviated way, such as including only some of

the back muscles or approximating the action of several muscles into a single force-

equivalent” (Bogduk et al 1992). Although the vital role of the local muscles in spinal

stabilization has been recognized (Crisco & Panjabi, 1991; Cholewicki & McGill, 1996),

“many anatomic features accepted in the modeling literature were found to be highly

inaccurate…” (McGill & Norman, 1986). The greatest oversimplification appears to have

been made in the representation of the trunk extensor muscles (e.g. lumbar multifidus and

erector spinae). Although Hides et al (1996; 2001) found that the LMT is a key muscle

which provides stability to the lumbar spine, 3D morphology and architecture of the

muscle are still not well represented in the literature. By studying the structure and

architecture of LMT, that is, the size, arrangement, and distribution of its muscle fibers,

19

we may better understand the extent to which this muscle helps control stability, generate

movement, and/or provide proprioceptive feedback.

2.3 Previous Studies on the Morphology, Architecture, Innervation and FiberType distribution of LMT

2.3.1 Morphology of LMT Lumbar LMT is the most medial of the back muscles and is the largest muscle

that spans the lumbosacral junction (Macintosh et al., 1986). The LMT is described in

anatomical textbooks (e.g. Williams, 1995)as being made up of a “number of fleshy and

tendinous fasciculi1 which fill up the groove on either side of the lumbar spinous

processes of the vertebrae”. These fasciculi extend from the posterior surface of the

sacrum, as far caudally as the “fourth sacral foramen, from the aponeurosis of origin of

the sacrospinalis from the medial surface of the posterior superior iliac spine, and from

the posterior sacro-iliac ligaments” (Williams, 1995). Muscle fibers of LMT are

described as having an oblique orientation, traveling superior-medial direction to insert

onto the spinous process of one of the vertebrae above. These fasciculi vary in length:

the most superficial and longest pass from either the sacrum or “one of the lumbar

vertebrae to the third or fourth above; those medial to or next in order run from one

vertebra to the second or third above; while the deepest connect two adjacent vertebrae”

(Williams, 1995).

The LMT is supplied by the medial branches of the posterior rami of L1 to L5

(Macintosh et al., 1986). Each medial branch crosses the vertebral lamina, deep to the

muscle, embedded in a layer of fat that separates LMT from bone. Each medial branch

1 Fascicle = a collection of muscle fiber bundles.

20

further divides into several branches to supply separate fascicles of LMT. Each nerve

innervates only the fascicles that arise from the spinous process or lamina of the vertebrae

with the same segmental number as the nerve (Macintosh et al., 1986).

2.3.2 Architecture of LMT 2.3.2.1 Qualitative/Descriptive Studies Leonardo da Vinci (1452-1519) was the first “anatomist/biomechanist” to

recognize the importance of understanding the relationship between muscle structure and

function. He was the first to hypothesize that stability of the cervical spine was imparted,

at least to some degree, by the unique architectural design of the cervical musculature

(Figure 2.4). Similar to the way guywires help to stabilize the mast of a ship, the cervical

musculature helps to prevent the cervical spine from buckling beneath the weight of the

head.

Figure 2.4. Drawing of the cervical vertebrae showing how the cervical musculature stabilizes the cervical spine similar to guywires stabilizing the mast of a ship. This is a faithful photographic reproduction of an original two-dimensional work of art. The work of art itself is public domain.

21

Following da Vinci’s line of reasoning, Bergmark (1989), suggested that the trunk

muscles can be divided in two subsystems: the global-mobilizing-system and the local-

stabilizing-system. Muscles belonging to the local muscle system are thought to be deep,

have a small moment arm, span only a few vertebral levels, and are positioned close to

the spines’ central axis. On the other hand, muscles belonging to the global system are

thought to be more superficial, have much larger moment arms which are capable of

producing larger forces, and are further away from the spinal axis of rotation

(Richardson, 1999). At present the LMT muscle is commonly assigned to the local

system whereas the longissimus and iliocostalis muscles, which form the erector spinae,

are assigned to the global system (Richardson, 1999).

These and other earlyaccounts of muscle architecture are purely descriptive in

nature. In addition, while anatomical text books are invaluable as educational tools to

assist with muscle localization, they are limited to the use of two dimensional images or

photographs to represent complex three dimensional skeletal muscle structures or

architecture. In the case of LMT, many anatomy texts books report this muscle as being

part of the transversospinalis group of deep back muscles (Agur & Grant, 2005). They

describe its structure by showing that its muscle fiber bundles as passing obliquely from

the sacrum, mammillary processes of the lumbar vertebrae, and aponeurosis of the erector

spinae, to insert into the spinous processes approximately three segments higher (Agur &

Grant, 2005).

These descriptions of LMT, however, do not provide the necessary anatomical

data with which to accurately and consistently perform electromyography or tissue biopsy

analysis of patients with lumbar disorders. As a consequence of this, Macintosh et al

22

(1986) performed an investigation to determine the morphology of LMT in order to

provide an accurate anatomical foundation for future electromyographic and pathological

studies of this muscle. These researchers studied 12 cadavers by gross dissection

identifying small collections of muscle fiber bundles at either the superior or inferior ends

and then stripping these away carefully from the remaining muscle mass while taking

note of their specific attachment patterns.

These researchers found that LMT consisted of five separate bands, with each

band consisting of a series of fascicles (collections of muscle fiber bundles) which stem

from the spinous processes and laminae of each lumbar vertebra. Macintosh et al (1986)

reported that in each band, the deepest and shortest fiber bundles arise from the vertebral

lamina, while the other fiber bundles arise from the spinous process. The fibers

originating from the L1 to L4 vertebral lamina insert onto the mammillary processes of

the vertebra two levels caudally (Figure 2.5A), while the L5 fibers insert onto the dorsal

surface of the sacrum just above the first sacral foramen. The fibers which originate from

the spinous processes are longer than the laminar fibers. Fiber bundles from a given

spinous process insert onto mammillary process (when present) three, four and five levels

inferiorly. The longest fibers from L1 insert onto the posterior superior iliac spine

(Figure 2.5B). The fibers from L2 insert onto the posterior superior iliac spine and an

area of the iliac crest just inferior to the posterior superior iliac spine (Figure 2.5C).

Similarly, the longer fibers from L3 insert onto an area from the posterior superior iliac

spine to the lateral edge of the third sacral vertebra (Figure 2.5 D). The fibers from L4

insert onto the sacrum in an area medial to the L3 area of insertion (Figure 2.5E), while

the fibers from L5 insert into an area medial to the dorsal sacral foramina (Figure 2.5F).

23

The schematic illustrations and descriptive accounts of LMT provided by

Macintosh et al (1986) while important to our overall understanding of LMT,

unfortunately, do not provide a three dimensional perspective of the muscle as it appears

in situ. Furthermore, data important to elucidating muscle architecture such as FBL,

FBA, muscle volume, PCSA and so forth cannot be extracted or determined by means of

these linear models. Indeed, few studies have been performed which have attempted to

fill this void.

Figure 2.5. Illustrations of the fascicles of lumbar LMT as seen in a posterior-anterior view. A illustrates laminar fibers from L1 to L5. B-F illustrates the longer, more superficial fibers attaching to the L1-L5 spinous processes. (from Macintosh et al, 1986, Figure 2).

24

2.3.2.2 Quantitative Cadaveric Studies and their Results

LMT is structurally and architecturally complex. Despite its complexity, LMT

and the other back muscles are commonly incorporated into biomechanical models in an

“abridged manner, reducing their actions more or less to a single force equivalent”

(Hansen et al., 2006). A great deal of architectural data is lost in the process; data which

are inherently important in determining the nature, distribution and types of forces acting

through the lumbar spine. The lost architectural data include measurements of FBA,

FBL and muscle volume. Hence, the ability to extrapolate the actions of the back

muscles from these models is dependent on the accuracy of the architectural data used as

input parameters (Hansen et al., 2006). The sections which follow summarize the

architectural data currently available for LMT.

2.3.2.2.1 Quantification of FBL In their paper entitled “A Universal Model of the Lumbar Back Muscles in the

Upright Position” Bogduk et al (1992) constructed a model of the lumbar back muscles

incorporating 49 fascicles of the lumbar erector spinae and LMT. These authors were

primarily concerned with developing a model of the back muscles incorporating every

fascicle in order to represent the actions of each of these muscles and all their fascicles on

the lumbar spine.

In theory, the maximum force that can be exerted by a muscle is proportional to its

PCSA. Thus, in the case of the back muscles, in order to determine the PSCA of each

component fascicle of the erector spinae and LMT, one needs to have measured both

volume and FBL (recall: PCSA = volume/length). Bogduk et al (1992) measured these

25

architectural parameters during the course of previous morphological studies of the

muscles (Macintosh et al., 1986; Macintosh & Bogduk, 1991). The authors report that as

they resected away intact fascicles belonging to LMT they would record the length of the

muscle belly (to the nearest 5mm), and its volume (to the nearest 1 ml) by inserting it into

a volumetric cylinder containing water and recording the volume of water displaced.

Accordingly, physiologic cross-sectional areas were calculated by dividing the volume of

each fascicle by its length (Bogduk et al., 1992).

Values for FBL (Table 2.2) were reported by Bogduk et al (1992), but these results

do not represent the true length of each fiber bundle or fascicle. Rather, they are the

length of the fascicle as projected in lateral radiographs of the spine (Bogduk et al.,

1992).

No other evidence on FBL for LMT was found in the literature.

2.3.2.2.2 Quantification of FBA Macintosh and Bogduk (1986) studied the orientation of the fascicles of the LMT

in five cadaveric specimens and summarized the descriptive data by plotting the

orientation of a total of eleven fascicles with respect to their vertebrae of origin. These

authors then measured the angle of each fascicle with respect to a standard reference line

through each vertebra in both anteroposterior and lateral radiographs of the lumbar spine.

In the anteroposterior view, the reference line was a vertical line through the lumbar

spinous processes on each outline of the lumbar spinous processes on each radiograph

tracing (Figures 2.6). On the lateral radiographs the reference line was drawn

26

perpendicular to the posterior surface of the vertebral body through the superior-inferior

midpoint of the vertebral pedicle (Figure 2.7).

Figure 2.6. Posteroanterior view of the mean FBAs of the various fascicles of multifidus from the Ll to L5 spinous processes. Each figure is labeled with the angle of every fascicle at each segmental level (with respect to the sagittal plane). Four fascicles are shown at Ll, with fewer fascicles represented at lower levels (from Macintosh and Bogduk, 1986, Figure 3).

27

Figure 2.7. Lateral view of the mean orientation of the fascicles of the multifidus from the Ll to L5 spinous processes. The dotted line represents the reference line through the pedicle against which the angle of orientation was measured. The table summarizes the orientations of the shortest to the longest fascicles at each segmental level. (From Macintosh and Bogduk, 1986, Figure 4).

28

In two later studies by De Foa et al (1989) and Biedermann et al (1991) the fiber

direction of longissimus, iliocostalis and multifidus were studied to establish reference

data in order to more reliably place surface electrodes for the purpose of

electromyography of muscle activity. Using photographs taken from the posterior view

of the back of both male and female cadaveric specimens they reported the “muscle fiber

direction”, that is, fiber bundle angle for the dorsal back muscles relative to an anatomical

reference line and the spine. For LMT, the FBAs reported for male and female cadaveric

specimens were 15.1° (range: 13.5°-18.0°) and 23.5° (17.5°-28.5°) respectively. The FBA

results from previous studies are summarized in table 2.2 below.

Level Ant-Post Angle (°) Macintosh et al, 1986

Lateral Angle (°)

Macintosh et al, 1986

Muscle Fiber Angulation (°) De Foa et al,

1989

Muscle Fiber Angulation (°) Biedermann et

al, 1991

Fascicle Length (cm)

Bogduk et al, 1992

L1-L4 14.8 ± 0.8 86.8 ± 1.5 11.1 L1-L5 15.0 ± 0.7 85.4 ± 0.6 14.6 L1-S1 12.6 ± 0.6 90.0 ± 0.7 17.7

L1-Sacrum 16.6 ± 0.9 101.2 ± 1.1 19.0

L2-L5 18.8 ± 1.1 85.8 ± 1.6 9.8 L2-S1 18.0 ± 1.0 86.2 ± 0.4 12.4

L2-Sacrum 20.0 ± 1.6 103.0 ± 0.7 15.4

L3-S1 23.2 ± 1.1 88.4 ± 2.3 8.0 L3-

Sacrum 19.6 ± 0.9 102.0 ± 1.4 11.9

L4-Sacrum 15.6 ± 0.9 93.6 ± 2.3 7.3

L5-Sacrum 5.4 ± 1.5 93.8 ± 0.8

Male 15.1± 1.43 (13.5-18.0)

Female 23.5 ± 4.5 (17.5-28.5)

4.1

Table 2.2. Architectural Data from Previous Studies

29

2.3.2.2.3 Quantification of Volume Bogduk et al (1992) reported having calculating muscle volume for LMT in a

previous study using water displacement (Macintosh & Bogduk, 1986) however the

authors have not reported the original data.

2.3.3 Innervation of LMT

Whatever structure may be causing pain, it must invariably involve a

neuroanatomical physiological pathway. The lumbar spine is certainly no exception to

this rule. Back pain is the second most common cause of sick leave in the United States,

next only to the common cold (Guo et al., 1999). In 1990, it was estimated that 50-100

billion dollars were allocated to the treatment of this medical enigma (Guo et al., 1999).

Knowing the innervation of the lumbar spine is particularly important to surgeons

performing minimal invasive surgery given their goal of preserving as much of the nerve

supply as possible and to improving surgical outcomes.

Historically, the study of the nerve supply to the lumbar spine focused on

descriptions of the sinuvertebral nerve, the lumbar dorsal rami and nerve supply of the

lumbar zygapophyseal joints (Bogduk, 1983). Little attention has been given to studying

the specific innervation pattern of the lumbar muscles and in particular LMT.

Recognizing that discrepancies existed in regards to the innervation of this muscle in the

literature and many standard anatomical textbooks, Macintosh et al (1986) investigated

its morphology and innervation pattern. Macintosh et al (1986) examined eight cadavers

using microdissection of each medial branch from L1 to L5. Removal of sequential

multifidus muscle fibers exposed deeper portions of each nerve and their course was

30

subsequently recorded. Individual branches were traced as far as possible through the

muscle. Any intercommunicating branches were also recorded. The medial branch of

each posterior ramus was found to cross the vertebral lamina deep to lumbar multifidus

before dividing into several branches. One branch passed medially to supply the

interspinous ligament, while the remaining branches supply separate muscle fascicles of

lumbar multifidus. Interestingly and clinically relevant was the finding that “each medial

branch innervates only those fascicles that arise from the spinous process or lamina of the

vertebra with the same segmental number as the nerve. Conversely, this relationship can

be expressed as, “the fascicles arising from a given vertebra are innervated by the nerve

that issuesbelow that vertebra” (Macintosh et al., 1986).

Despite their pioneering work and subsequent evidence implicating LMT in the

precise control of intersegmental movement, little has been done since to further unravel

the detailed intramuscular nerve innervation pattern of LMT.

2.3.3.1 Motor Control of Lumbar Stability The oseoligamentous spine is inherently unstable and has been shown to buckle

under minimal loading conditions (Crisco & Panjabi, 1991). Panjabi (1992a)

demonstrated the vital role of muscle to the control of spinal stability in addition to its

passive elements. Although the trunk muscles are capable of providing ample stability to

the spine under most day-to-day activities, control over these muscles is a function of the

central nervous system (CNS) (Panjabi, 1992a). The CNS is continuously monitoring the

current state of stability of the spine. It pre-plans muscle activation strategies to

overcome known challenges to stability and must react quickly to unpredictable

31

challenges using information received from various sensory systems, including muscle,

ligament and joint proprioceptors. To make matters even more complex, muscle

activation patterns must be coordinated such that intersegmental movements such as

shear and torsion between vertebrae is controlled while simultaneously allowing the spine

to move.

The two primary strategies used by the CNS to control movement and stability of

the trunk are: feedforward (open-loop) strategies and feedback (closed-loop) strategies.

Feedforward strategies are implemented when the CNS can extrapolate the outcome of a

predictable perturbation and responds appropriately using pre-planned motor strategies.

Movements which are considered representative of feedforward movements include

ballistic or repetitive movements as well as predictable challenges to spinal stability such

as lifting an arm or leg away from the body. Feedback strategies cannot be pre-planned.

Movements patterns included in this category are generated and modulated by sensory

inputs from various sources including the visual, vestibular and proprioceptive systems.

In reality, control over trunk stability likely occurs as the result of interplay between both

these control systems.

2.3.4 Fiber Typing of LMT The muscles of the backhave been shown to counter-act the force of gravity with

nearly continuousactivity in the erect human spine (Asmussen & Klausen, 1962). To

determine if Type I muscle fibers predominate in the back muscles several histochemical

studies have been conducted.

32

2.3.4.1 Cadaveric Investigations Previous studies using autopsy specimens have found that the LMT as well as the

lumbar and thoracic components of the erector spinae muscles have a higher percentage

of Type I muscle fibers compared with Type II muscle fibers (Johnson et al., 1973; Fidler

et al., 1975; Jowett et al., 1975; Sirca & Kostevc, 1985; Thorstensson & Carlson, 1987;

Jorgensen et al., 1993; Rantanen et al., 1993; Mannion, Dumas et al., 1997; Mannion,

Weber et al., 1997). Furthermore, the proportion of Type I fibers in the thoracic erector

spinae muscles has been reported to be as high as 75% (Sirca & Kostevc, 1985). A lower

percentage of Type I fibers has been found in the lumbar erector spinae muscles, with

reported percentages varying from 58-67% (Fidler et al., 1975; Jorgensen et al., 1993;

Mattila et al., 1986; Sirca and Kostevc, 1985).

Relatively few studies have compared the composition of the LMT to the lumbar

erector spinae muscles. Those that do are insufficient in one or more respects. Sirca and

Kostevc (1985) reported a higher percentage of Type I fibers in the LMT (63%)

compared to the lumbar longissimus (57%). Although a relatively large number of

autopsy specimens were used (21 male subjects), muscle biopsies were harvested at only

one site, that is, at the level of the second lumbar spinous process. Similarly, Verbout et

al (1989) reported 13% more slow twitch fibers in medial column muscles (i.e. LMT)

than in lateral column muscles (i.e. the lumbar erector spinae). Once again although a

large number of specimens were used (30 cadavers), only a small number of muscle

biopsies were taken from the lumbar column per specimen. In addition, the precise

location and level of the spine from which these muscle biopsies were harvested is not

clear in this study.

33

In contrast to these previous findings, Jorgensen et al (1993) evaluated the

histochemical composition of the LMT and lumbar erector spinae musclesin six male

cadavers with no history of LBP (age 17-29). Superficial and central portions of LMT,

longissimus, and iliocostalis were sampled bilaterally at the level of L3 vertebra. When

the results for superficial and central portions were pooled, it was found that longissimus

had a significantly greater percentage of Type I fibers (70.5%) compared with LMT

(54%) and iliocostalis (55%). Similar to the study by Sirca and Kostevc (1985), the study

by Jorgensen et al (1993) is limited in that these authors sampled paraspinal muscle tissue

from only one level within the lumbar column.

In summary, there is conflicting data on the fiber type composition between LMT

and the erector spinae muscles. The studies described above use a relatively large

number of specimens but have arrived at their conclusions using muscle biopsies taken

from only a few sites within each specimen. Hence, one cannot say for certain if or how

the distribution of Type I and II fibers changes throughout the extent or “volume” of each

of these paraspinal muscles.

2.3.4.2 In Vivo Investigation Thorstensson and Carlson (1987) studied the distribution of Type I and II fibers in

9 male and 7 female subjects with no history of LBP aged 20-30 years. Muscle samples

were taken from superficial longissimus and LMT on the left side of the L3 spinal level.

Although a higher percentage of Type I fibers was present in both, no significant

difference in the relative number of fiber types between LMT (62%) and longissimus

(57%) were observed. The authors concluded that these results did not support the

34

hypothesis of functional differentiation between these two anatomically different regions

of the lumbar erector spinae.

Mannion, Weber et al (1997) obtained superficial samples of LMT at the level of

L3 or L4 during spinal surgery from 21 patients with low back pain and from the belly of

the lateral tract (iliocostalis/longissimus muscles) in 21 control volunteers matched for

gender, age and body mass. The biopsies from each group were subjected to routine

histochemical analysis to determine characteristics of muscle fiber types. Their results

showed that the proportion of Type I (51%-66%) versus Type II (7.9%-24.4%) fibers was

significantly higher in both the patient and the control groups. In addition Mannion,

Weber et al (1997) were able to demonstrate that the proportion of Type I fibers was

significantly higher in the muscles of the controls than in patients with LBP (p=0.0001).

Since muscle biopsies taken from LMT in this study were those belonging to patients

with LBP, one cannot draw any conclusions on the possible percentages of Type I/II

fibers in normal individuals. Indeed much of the in vivo data on the fiber type

composition of LMT has been drawn from studies on patients with a history of LBP or

spinal pathology (Mattila et al., 1986; Meier et al., 1997; Bajek et al., 2000) that generally

report a high percentage of Type I muscle fibers.

2.3.4.3 Comparison of Cadaveric and In Vivo Measurements Very few studies compare cadaveric measures to in vivo measures of muscle fiber

type, diameter and volume/CSA. In so far as changes in muscle fiber diameters are

concerned, it has been found that fiber diameters decrease when skeletal muscle enters a

state of rigor mortis (Levine & Hegarty, 1977). This decrease in fiber diameters is

35

attributed to the movement of cytoplasmic fluid into the extraxellular space. The degree

to which this occurs however has not been quantified. Hence, direct comparison between

cadaveric and in vivo muscle fiber diameters is not possible at this time. One factor

complicating such a comparison is that most in vivo muscle biopsies are routinely fixated

with different chemical agents which in turn can also influence the diameter and

size/volume of muscle fibers. In one study by Stickland (1975), different fixatives were

demonstrated to decrease the diameters and volumes of muscle tissue by as much as

62%-83% and 24%-44% respectively.

Unfortunately, there are no studies which have examined differences in fiber type

distribution between cadaveric and in vivo skeletal muscle tissue making comparison

between these studies impossible.

2.4 Functions of LMT based of Morphological, Biomechanical, Electromyographic and Clinical Evidence

The structure of LMT is extremely complex allowing it to assume several possible

roles. Of these roles, considerable evidence exists to support its’ proposed function in

supporting and stabilizing the lumbar spine.

2.4.1 Morphological Evidence Based solely on morphological evidence, LMT is likely to provide lumbar spinal

stability. Its fiber bundles have not one, but multiple points of attachments onto the

lumbar spine, sacrum and pelvis, thereby making it possible to control the intersegmental

movement that occurs between adjacent vertebrae (Macintosh & Bogduk, 1986; Crisco &

Panjabi, 1991). The segmental arrangement of its fascicles coupled with its segmental

36

innervation makes LMT well suited for the precise control of movement in the lumbar

spine. However, because of its overall small size as compared with other trunk muscles

and its proximity to the centre of rotation of the intervertebral joints, the LMT muscle has

little mechanical effect. Nevertheless, the arrangement of its fascicles suggests that LMT

can also produce some movement in the sagittal plane. A posterior view of the muscle

shows that its fascicles are aligned primarily in a vertical plane with a slight horizontal

deviation (Figure 2.8). Hence, LMT is considered to be an extensor muscle along with

the erector spinae muscles of the lumbar spine (Macintosh & Bogduk, 1986). Only slight

movement is possible in the horizontal plane making LMT a poor rotator of the lumbar

spine (Macintosh & Bogduk, 1986).

Because of its polysegmental nature, LMT also exerts indirect effects on

interposed vertebrae. Since the line of action of the fiber bundles of LMT lie behind the

lumbar lordosis, contraction of LMT would result in something of a “bowstring” effect

on vertebrae situated between those segments to which LMT attaches (Adams, 2002).

This bowstring effect would result in compression of posterior column structures and

tensioning of anterior column structures. Hence, a secondary effect of LMT on the

lumbar spine might be to maintain and accentuate the lumbar lordosis.

Lastly, the deepest, laminar fibers of LMT attach onto the joint capsules of each

zygapophyseal joint and hence contraction prevents entrapment of soft tissue structures

during spinal movements, particularly extension (Bogduk & Endres, 2005).

In summary, based primarily on morphological data, LMT is more likely to

function as a stabilizer of the lumbar spine than as a prime mover. In many ways, the

LMT is to the lumbar spine as the rotator cuff muscles are to the shoulder joint.

37

Fig. 2.8. Posterior view of lumbo-sacral spine showing typical orientation of fascicle of LMT. The line of action of each fascicle of LMT (bold arrow) can be resolved into a major vertical vector (V) and a smaller horizontal vector (H). 2.4.2 Biomechanical Evidence Biomechanical studies have provided some of the most convincing evidence of

the roles of LMT. The majority of these investigations, especially those referring to the

deep laminar fibers of LMT, are linked to its primary stabilizing function. Biomechanical

models of LMT function have confirmed the extensive role played by this muscle in

producing torque and augmenting spinal stability.

2.4.2.1 Role of LMT in Torque Production Although two or more muscles may perform similar functions, it has been

proposed that certain muscles may exhibit functional differentiation. Bergmark (1989)

suggested that the trunk muscles could be divided into one of two categories: ‘global’ or

‘local’ muscles. Global muscles cross several segments (i.e. vertebrae) and have a large

H

V

38

moment arm which is capable of generating substantial torque. Local muscles on the

other hand cross only one or at most a few vertebral segments. They have a relatively

small moment arm and are better suited to maintain joint and hence spinal stability.

Bergmark (1989) proposed that the muscles belonging to the global muscle system (e.g.

thoracic divisions of the erector spinae muscles) are the main torque producing muscles

of the lumbar spine. In contrast, muscles such as the LMT are included in the local

muscle system. These later muscles generally lie deeper, attach directly to the lumbar

vertebrae, and function primarily to stabilize the lumbar motion segments. While this

classification is useful, it should not be used dogmatically, particularly when dealing with

complex muscles such as LMT. Hence, although LMT is apparently well designed to

provide stability of the lumbar vertebrae, it may also play a part in torque production.

The fascicles of LMT run obliquely when viewed from its posterior aspect.

Hence, their line of action can be divided into two vectors (Figure 2.8), a large vertical

vector and a smaller horizontal vector (Macintosh et al., 1986). Since the magnitude of

the vertical vector greatly exceeds that of the horizontal vector, the principle action of

LMT is lumbar extension. LMT thus contributes to the total extensor moment and is

estimated to provide approximately 20% to the overall extensor torque at the L4 and L5

vertebral levels (Bogduk et al., 1992). Even though LMT has the distinction of being the

largest muscle at the lumbosacral junction, it is at a mechanical disadvantage in

producing extension of the thoracic cage on the pelvis. The majority of the torque is

produced by the biomechanically well-positioned thoracic components of the erector

spinae muscles with their long lever arm (Bogduk et al., 1992).

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2.4.2.2 Role of LMT in Spinal Stability The stabilizing role of LMT has been elucidated largely through biomechanical

investigations addressing its possible roles in maintaining the lumbar lordosis and controlling

shear forces.

2.4.2.2.1 Role of LMT in Maintaining the Lumbar Lordosis

LMT consists of many fiber bundles arranged into fascicles, the longest of which

can accentuate the lumbar lordosis (Bogduk & Endres, 2005). In addition, the fact that

each band of LMT has its own nerve supply (Section 2.3.3) implies that each vertebra is

independently controlled and therefore “the curvature of the lumbar spine can be adjusted

very precisely to match the loading being imposed. It is the overall curvature, or posture,

of the spine which determines its stability” (Aspden, 1992).LMT can anteriorly tilt the

pelvis relative to the lower lumbar segments (Bogduk & Endres, 2005). This allows LMT to

maintain control over the lumbar lordosis while the erector spinae simultaneously moves the

trunk, which may offer the spine some safeguard from injury, particularly during lifting

activities.

Aspden (1992) suggested another mechanism whereby LMT may enhance lumbar

stability by controlling the lumbar lordosis. Contraction of LMT increases the lumbar

lordosis, thereby generating compressive forces on the interposed vertebrae. Compression

increases the resistance of the lumbar spine to torsional forces thereby augmenting lumbar

stability.

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2.4.2.2.2 Role of LMT in Controlling Shear Forces Anterior/posterior shear relates to the movement of one vertebra forward/backwards

relative to the sub-adjacent vertebra. Bogduk et al (1992) found that contraction of LMT led

to posterior shear forces at the L1 to L4 vertebral levels, and anterior shear forces at the L5

level. To explain the generation of these forces,the authors proposed that shear is generated

not only by muscle fiber bundles attached to their vertebrae of origin, but also on all

interposed vertebrae between their origins and insertions. For this reason, shear forces

generated by lumbar fiber bundles reach a maximum at intermediate segments. Secondly,

although each fiber bundle or fascicle of LMT may contribute minimally to shear forces on

lumbar vertebrae in the upright position, when their individual contributions are summated

the resultant shear force generated is substantial.

LMT is also theorized of being capable of controlling shear forces in standing and

forward flexion. Cholewicki et al (1991) proposed that LMT opposes the anterior shear

caused by bending and lifting due to production of posterior shear by LMT.

2.4.2.2.3 Biomechanical Models for the Stability Role It has been proposed that the spinal muscles may impart stabilization via control

of the spinal segments’ neutral zone (NZ) (Panjabi 1992b). The neutral zone is a region

of intervertebral motion around the neutral posture where little resistance is offered by

the passive spinal column. The NZ appears to be a clinically important measure of

spinal stability function. Its size may increase with injury to the spinal column, which in

turn may result in spinal instability or low-back pain. Panjabi et al (1989) looked at the

effect of intersegmental muscle forces on spinal instability in an in vitro study

41

performed on intact and sequentially injured fresh lumbar spinal segments. Simulated

muscle forces representing the intersegmental LMT, interspinales and rotatores muscles

were applied to the specimens. Panjabi et al (1989) concluded that the “intersegmental

nature of the deep muscle group gives a tremendous advantage to the neuromuscular

control system for fine tuning the stability of the spine”. LMT fibers are placed close to

the centers of rotation of spinal movements and connect adjacent vertebrae at appropriate

angles.

In a three dimensional study of the lumbar spine mechanics, McGill (1991)

provided supporting evidence of the stabilizing role of LMT. Based on the results of this

study, McGill (1991) concluded that the “unchanging geometry of the LMT through a

range of postures” indicates that “the purpose of LMT was to finely adjust vertebrae with

small movements rather than to function as a prime mover. The results of this study

showed that LMT could function in this way at any physiological posture”.

Goel et al (1993) provided further evidence tosupport the stabilizing function of

LMT using a combined finite element and optimization approachto study the effects of

muscles on the of biomechanicsof the lumbar spine. Briefly, two finite element models

(ligament and muscle) of an L3-4 motion segment were created and subsequently

compared. Muscles included in the muscle model were the interspinous and

intertransverse muscles, LMT and the quadratus lumborum. The results of this study

showed that the incorporationof muscular forces led to a decrease in anteroposterior

translation and flexion rotation (displacements in the sagittal plane)of the L3-4 motion

segment. Hence, muscles imparted stability on the ligamentous system. The addition of

muscles also led to a decrease in stresses in the vertebral body and intervertebral disc.

42

However, the load bearing of thefacet joints increased, indicating that these joints play a

significant role in “transmitting loads in a normal intact spine” (Goel et al., 1993). This

study supported the earlier findings of Farfan (1975). Muscle dysfunction (simulated by

decreasing the computed force in the muscles to 90%, 80% and 70% of the original

values) destabilised the motion segment. This led to a shift of loads to the disc and

ligaments and decreased the role of the facet joints in transmitting loads (Goel & Gilbertson,

1995).

The stabilizing function of the LMT was also demonstrated in a biomechanical in

vitro study performed by Wilke et al (1995). This study investigated the influence of five

different muscle groups on the monosegmental motion of the L4-L5 segment during the

movements of flexion/extension, lateral flexion and axial rotation. The muscles examined

were the LMT, lumbar erector spinae and psoas major. Seven human lumbosacral spines

were tested on a spine tester that allowed simulation of muscle forces. The combined

muscle action of the muscles tested was found to decrease the total range of motion and

neutral zone motion of the L4-L5 segment. Total range of motion was decreased by 93%

in flexion, and 85 % in extension. The total neutral zone motion in flexion and extension

was decreased by 83 %.

This supported the findings of Steffen et al (1994), who also used a biomechanical

in vitro experimental design for assessing lumbar instability, and found that the influence

of the LMT decreased the neutral zone in flexion and extension. In lateral flexion, Wilke

et al (1995) showed that the total range of motion was decreased by 55 % and the neutral

zone motion was decreased by 76%. In axial rotation, total range of motion was

decreased by 35%, but the neutral zone did not change significantly.

43

The studies of Crisco and Panjabi (1991), Goel and Gilbertson (1995), Panjabi et

al (1989), Steffen et al (1994) and Wilke et al (1995) exemplify the stabilizing role of the

LMT on lumbar spinal segment in the sagittal and frontal planes. Unfortunately, the role

of the LMT in stabilizing axial rotation is not as clearly defined (Wilke et al., 1995).

By modeling the actions of LMT and the lumbar erector spinae, Macintosh et al

(1993) concluded these muscles were incapable of providing axial stability of the lumbar

spine due to insufficient torque production by these muscles in trunk rotation. These

authors however, did not consider that muscle stiffness, which is a critical element in

joint stabilization, can be produced at low levels of maximal voluntary contraction

(MVC) (Hoffer & Andreassen, 1981).

The role of the LMT in axial rotation has also been considered in relation to the

abdominal muscles. Joint stiffness can be improved by co-contraction of agonist and

antagonist muscles (Lee et al., 2006). Therefore, although LMT may not be a strong

torque producer in rotation, this does not preclude it from contracting in association with

other muscles such as the oblique abdominals in stabilizing trunk movement in axial

rotation (Bogduk & Endres, 2005).

A study by Kaigle et al (1995) took the research of the previous authors [Crisco

and Panjabi (1991), Goel et al (1993), Goel and Gilbertson (1995), Panjabi et al (1989),

Steffen et al (1994) and Wilke et al (1995)] one step further. While these researchers

have used biomechanical in vitro experimental designs, Kaigle et al (1995) developed an

in vivo animal model of lumbar segmental instability. In this investigation, passive

stabilizing structures (disc, facet joints and ligaments) were transected, and the effects of

active musculature on spinal kinematics were examined in 33 pigs. Muscles surrounding

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the spine (LMT, lumbar erector spinae, quadratus lumborum and psoas major and minor)

were subjected to muscle stimulation using wire electrodes. Kaigle et al (1995) showed

that increased combined muscular activation stabilized the injured motion segment by

reducing aberrant patterns of motion in the neutral zone. However, these results must be

interpreted with caution as experiments on quadrupeds may not be directly applicable to

bipeds because of the different directions of external forces relative to the structures of

the vertebral column. Overall, Kaigle et al (1995) concluded that rehabilitation for

patients with segmental spinal instability should focus on the stabilizing influence of the

surrounding spinal musculature.

2.4.2.2.4 Role in Providing Stiffness to the Spine LMT has been shown to decrease the available movement of the lumbar spine and

neutral zone motion of individual lumbar motion segments, thereby stiffening the spine

(Wilke et al., 1995). Stiffness is defined as resistance to deformation. If a structure has

increased stiffness, greater outside force would be required to deform the structure to the

same amount (Porterfield & DeRosa, 1998), and consequently, it is considered more

stable.

Muscle stiffness is the ratio of force change to length change and consists of two

components: reflex mediated muscle stiffness and intrinsic muscle stiffness (Johansson et

al., 1991). The neuromuscular system regulates muscle stiffness in postural control

(Crisco and Panjabi 1991), and the stiffness of a muscle can be increased by increasing

the neural outflow to it and thus increasing muscle tone (Porterfield & DeRosa, 1998).

This neural outflow affects both the alpha and gamma motor systems. Peripheral

45

feedback from the joint and ligament afferents may also regulate muscle stiffness via

effects on the gamma spindle system (Johansson et al., 1991). The gamma system

sensitizes the spindle to movement (Porterfield & DeRosa, 1998).

The bending stiffness of the spine will also be influenced byother factors such as

the thoracolumbar fascia (Porterfield & DeRosa, 1998), whichlimits the radial expansion

of the backmuscles (Aspden, 1992). It has been sugested that contraction of the back

muscles, including the LMT and the lumbar erector spinae muscles, exerts a pushing

force on the fascia (Farfan, 1973). The influence of the LMT and the lumbar erector

spinae muscles on the thoracolumbar fascia was investigated by Gracovetsky et at (1977)

using a mathematical model. It was proposed that since the thoracolumbar fascia

surrounded the back muscles, it could serve to brace these muscles. The authors called

this the hydraulic amplifier mechanism. These forces may result in increased lumbar

spine stiffness and contribute to lumbar stabilization.

2.4.3 Electromyographic Evidence Electromyographic analysis has allowed evaluation of the functionof the LMT

through determination of invivo muscle activation. Many classic studies have been

performed using indwelling electrodes. A tonic or almost continuous level of activation

of the LMT has been demonstrated in many of these studies, which have examined the

role of the LMT in upright postures and during active movements.

46

2.4.3.1 LMT Activity Involved in the Maintenance of Posture LMT has been shown to be continuously active in upright postures, compared

with relaxed recumbent positions. Along with the other paravertebral muscles, LMT

provides antigravity support to the spine with essentially continuous activity (Asmussen

and Klausen 1962). In fact, the LMT is probably active in all antigravity activity

(Donisch & Basmajian, 1972; Valencia & Munro, 1985).

In the standing position, slight to moderate EMG activity has been demonstrated

in LMT (Donisch and Basmajian 1972, Valencia and Munro 1985). The explanation for

this activity lies in the location of the line of gravity in relation to the lumbar spine

(Valencia & Munro, 1985). In approximately 75 % of people, the line of gravity passes in

front of the centre of the L4 vertebra (Asmussen & Klausen, 1962). Gravity creates a

force ventral to the spine which tends to pull the thorax and lumbar spine into flexion.

The LMT, which is a posterior sagittal rotator of the lumbar spine, is therefore constantly

active when standing to maintain an upright posture and opposes the tendency to flexion

(Bogduk & Endres, 2005). This is an example of the tonic postural role of the LMT

muscle. During walking the LMT is tonically active (Morris et al., 1962).

2.4.3.2 LMT Activity in Active Lumbar Movements Activation of the LMT has been examined using EMG during several movements

including forward flexion and extension from the flexed position, trunk extension in the

prone position and trunk rotation. In all cases the function of LMT appears to be primarily

one of stabilization.

As the spine bends forward from the erect standing posture, there is an increase in

47

LMT EMG activity (Morris et al., 1962; Valencia & Munro, 1985). At a certain point during

flexion known as the ‘critical point’ the activity of the back muscles ceases (Morris et al.,

1962; Kippers & Parker, 1984). Kippers and Parker (1984) demonstrated that the EMG

activity of the erector spinae ceased at about 90% of lumbar spine flexion. The critical point

for LMT is not as characteristic a feature as it is for the erector spinae muscles; however a

decrease in activity is evident, with EMG silence of the LMT occurring infrequently

(Valencia & Munro, 1985). Valencia and Munro (1985) proposed on the basis of these results

that the LMT has a stabilizing role in flexion, providing localized control of lumbar vertebrae

motion during this activity. An alternative explanation could relate to the small length

changes in the LMT related to sagittal movements (McGill, 1991).

Extension of the trunk from the flexed position predictably evokes high levels of

LMT activity (Morris et al., 1962; Donisch & Basmajian, 1972). Significant activity of the

LMT also occurs when the trunk is extended or hyperextended in the prone position

(Jonsson, 1970; Donisch & Basmajian, 1972; Valencia & Munro, 1985). Valencia and

Munro (Valencia & Munro, 1985) also found that unilateral hip extension in the prone

position evoked high levels of LMT activity on both sides of the spine. During these highly

loaded activities, similar to extension of the trunk from a flexed position in standing, LMT

activity causes posterior rotation of the lumbar vertebrae (Bogduk & Endres, 2005), and

controls the lumbar lordosis (Aspden, 1992). Although activity in LMT is marked in

extension, the majority of the actual trunk extension torque (80% at the L4 and L5 vertebral

levels) is provided by the thoracic components of the erector spinae muscles (Bogduk et al.,

1992).

During trunk rotation, the LMT has been shown to be active bilaterally in both

48

ipsilateral and contralateral rotation of the trunk in sitting and standing (Morris et al.,

1962; Jonsson, 1970; Donisch & Basmajian, 1972). For this reason, it has been

suggested that in rotation, the LMT acts as a stabilizer rather than a prime mover

(Valencia & Munro, 1985).

2.4.3.3 LMT Activity During Internal and External Perturbations of the Trunk

Studies used to elucidate the role of LMT in the control of movement and/or

stability have evaluated the recruitment of this muscle when spine stability is challenged

due to both predictable and unpredictable challenges to trunk stability (Moseley et al.,

2002, 2003). Electromyographic activity was recorded from the superficial and deep

fibers of LMT, erector spinae, transversus abdominis, and deltoid muscles using fine wire

electrodes during single and repetitive arm movements. The activation patterns of these

muscles in response to elevating the arm were compared to one another. During single

arm movements, the onset of electromyographic activity in deep fibers of LMT was

shown to occur irregardless of the direction of arm movement while the superficial fibers

of LMT were activated in a direction specific manner. These researchers postulated that

the deep fibers of LMT controlling intersegmental motion while the superficial fibers

control spinal orientation (Moseley et al., 2002). To explore the activation pattern of

LMT when the spine cannot make any predictions to stability and hence pre-plan a motor

response, Moseley et al (2003) recorded the electromyographic activity from the

superficial and deep fibers of LMT as a result of unexpectedly dropping a weight into a

bucket that was held by a standing subject with the eyes closed. These researchers did

not observe differential electromyographic activity in the deep and superficial fibers of

49

LMT (Moseley et al., 2003). Hence, it was suggested that the deep fibers of LMT may

help to stabilize the lumbar spine but only when the CNS can anticipate the timing of the

perturbation.

2.4.4 Clinical Evidence Therapeutic exercise programs are an important component in the treatment and

management of clients with LBP. These exercise programs are directed towards recruiting

one or a combination of different trunk muscles with the ultimate goal being to improve

“stability” of the lumbar spine. In doing so it is generally felt that both a reduction in

symptoms and reoccurrence rates of LBP would naturally ensue. Despite this, there is

controversy over which muscles are most important in attaining optimum function and

performance following or during an episode of LBP. While some authors favor activation of

the paraspinal muscle group as a whole to achieve control over spinal motion (Porterfield &

DeRosa, 1991, 1998; McGill, 2001), others (Richardson et al., 2004) propose the initial

activation and rehabilitation of specific muscles which are preferentially suited to stabilizing

the lumbar spine. For the later group, specific exercises have been designed to target LMT in

such a precise manner that normal function and hence stability to the spine would naturally

be restored (O'Sullivan et al., 1997; Richardson, 1999; Richardson et al., 2004). Hides et al

(2001) demonstrated that by using specific exercise therapy to activate LMT it was possible

to dramatically reduce the reoccurrence rate of acute LBP while O’Sullivan et al (O'Sullivan

et al., 1997) provided convincing evidence of reduced pain and disability in patients with

chronic LBP. Furthermore, patients who suffered from poor outcome following back

surgery, also exhibited local denervation of LMT (Zoidl et al., 2003).

50

2.4.5 Summary Despite evidence to support the role of LMT in maintaining and facilitating

movement, providing stability, controlling shear forces and maintaining the lumbar lordosis,

there is lack of consensus as to how LMT is capable of supporting these multidimensional

task oriented roles within the in vivo lumbar spine. Even though the morphology/anatomy of

this muscle has been looked at previously, it has not provided the necessary substrate data

with which to ‘piece together’ and thereby formulate a functional paradigm of this muscle.

What is needed is a robust, anatomically accurate, 3D representation of LMT which

incorporates all of its fiber bundles, nerve supply and fiber type distribution throughout its

volume! Architectural data, innervation patterns and fiber type constitution may then be

explored and analyzed to draw meaningful conclusions as to the relationship between

structure and function. In the past the ability to render such a complete model of LMT has

been impossible, however, with the advent of novel microdissection and digitization

techniques pioneered predominately in this laboratory a virtual model of the entire back

musculature is now possible.

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Chapter 3 Hypothesis and Objectives

3.1 Hypotheses

The human lumbar multifidus is functionally subdivided based on regional

differences in muscle architecture, innervation pattern, and fiber type distribution.

3.2 Objectives

The objectives of this study are as follows:

1. (a) To serially dissect and digitize lumbar multifidus in situ throughout its entire

volume from L1 to L5;

(b) To construct a 3-D model of each specimen using MAYA™ (Alias Systems

Corporation, Toronto, ON) in conjunction with additional software developed in

the laboratory.

(c)To model and quantify the architecture of LMT throughout its volume.

2. To determine if the lumbar multifidus is architecturally divided based on gross

morphology and architectural parameters (fiber bundle length, fiber bundle angle,

and volume).

3. Digitize and model the innervation of lumbar multifidus in 3D throughout its

volume.

4. Catalogue the distribution of Type I and Type II muscle fibers throughout the

volume of lumbar multifidus from L1 to L5.

5. Use the architectural, nerve and fiber type data to establish a functional paradigm

which explains the role of lumbar multifidus within the lumbar spine.

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3.3 Significance This study provides a unique three-dimensional model of the detailed architecture,

innervation, and fiber type distribution of lumbar multifidus. The results will provide

new insights into the architectural infrastructure of lumbar multifidus and assist in

clarifying whether different regions serve different functions. It is known that lumbar

multifidus is innervated segmentally from the medial branch of the posterior ramus, but

what is not known is whether the medial branch divides further to supply architecturally

distinct regions. An architecturally distinct region which is independently innervated is

termed a neuromuscular compartment which may have functional or task-oriented roles;

that is, different portions of the same muscle may be utilized proportionally depending on

the task demands of the situation (English et al., 1993).

The fiber type distribution, innervation pattern and fiber architecture within each

compartment could be used to predict functional properties and to elucidate how

compartments coordinate with each other during movement.A functional paradigm can

then be formulated which allows analysis of normal and abnormal movement, which is

the basis for understanding and managing dysfunctions of the musculoskeletal system

(English et al., 1993)

53

Chapter 4 Methods

The methods are divided into three sections. Section 4.1 outlines the procedure

used to dissect, digitize, model and quantify the architectural parameters of LMT. A

flowchart which outlines this process is provided in Figure 4.1. In section 4.2 the method

used to delineate the nerve innervation pattern of LMT throughout its volume is

discussed. The methods used to determine the proportion of Type I/II fibers throughout

the volume of LMT as well as the least mean diameter (LMD) for each fiber Type is

described in section 4.3.

4.1 Digitization, modeling and quantification of architectural parameters of LMT 4.1.1 Specimens

Ten formalin embalmed human cadaveric specimens (9M/1F) with a mean age of

80±11 years were studied. Specimens with visible evidence of musculoskeletal

deformity, muscle pathology, indication of previous surgery or trauma were excluded.

Ethics approval was received from the University of Toronto Research Ethics Board

(Protocol Reference # 20830).

4.1.2 Serial dissection and digitization of muscle fiber bundles, tendons and spinal column

The LMT was exposed by removing the skin, fascia, superficial muscles and

aponeuroses. Each specimen was securely bound to a metal tray and three reference

points (bilateral posterior superior iliac spines, and sacral apex) were demarcated clearly

by drilling in screws. These reference points were necessary in order to later reconstruct

54

Figure 4.1 Digitization of human LMT.

55

the 3D model of LMT from the digitized data. Using a 1.75x magnifier, muscle fiber

bundles of LMT were identified and traced along their full length craniocaudally starting

at the L1 spinous process. The course of each fiber bundle was delineated by marking

its superior and inferior attachment sites and 5-20 intervening points using a fine paint

pen (Figure 4.2).

Figure 4.2 Delineation of muscle fiber bundle of LMT (left, lateral view of lumbosacral spine). Small black markings (indicted with arrows) represent points used to outline the course of a muscle fiber bundle. Points were subsequently digitized. Tendons were digitized in a manner similar to muscle fiber bundles (small white markings on specimen). The x, y, and z coordinates of each point were then obtained using a Microscribe®

G2 Digitizer (Immersion Corporation, San Jose, CA, USA). The removal of individual

fiber bundles permitted the identification, marking, and digitization of successively

deeper segments of the muscle. The dissection and digitization process continued

56

sequentially from cranial to caudal and from superficial to deep until the entire LMT had

been resected from the lumbar spine, ilium and sacrum.

During the dissection process, the surfaces of all tendons forming part of the

structure of LMT were demarcated using small points. Points were placed on the surface

of each tendon from its superior to inferior ends across its entire width/girth. These

points were then digitized in a linear and sequential manner (Figure 4.2 and 4.3).

Figure 4.3. Close up, lateral view of LMT originating from the L1 spinous process showing a small segment of tendon (left, lateral view of spine). The tendon was marked using small points (black dots) which were digitized in a sequential manner (e.g the points in row 1 followed by those in row 2, etc…) from superior to inferior. Solid blue arrows demonstrate direction of digitization procedure.

Lastly, the surface and peripheral outline of each lumbar vertebra, the sacrum and

ilium were digitized using a grid that was marked or etched directly onto each of these

structures using a fine marker (Figure 4.4).

57

Figure 4.4. Right, lateral view of digitized lumbar spine and sacrum as viewed in Autodesk® Maya®. Each small red dot represents a digitized point.

4.1.3 Microscribe® 3G2 Digitizer The Microscribe® 3G2 Digitizer consists of a small, sensorized mechanical arm

that sits on the support base. The probe has six joints (joints 0-5), as illustrated in Figure

4.5.

Figure 4.5 The Immersion Company Microscopic 3-G2 Digitizer.

58

Each rotary joint represents one degree of freedom, and thus the probe has six

degrees of freedom, allowing simultaneous positioning and orientation of its tip. A

counterbalance is placed close to the base to minimize the user’s fatigue. The tip position

relative to the base is obtained through direct kinematics calculations, based on sensor

values and the length of the links. Software on the host computer reads the joint sensors

and uses its kinematic model to determine where the tip is. A binary switch on a

footpedal or hand switch is used to select/deselect virtual objects, navigate, or mark

points on the real objects surface for digitization purposes.

Calibration is done by placing the probe tip in a housing close to the base and

reading the rotary sensor data. These readings are then compared with the stored values

corresponding to the joint angles when the probe arm is in this known position. The G2

model, which is the digitizer used in this study uses high-resolution joint sensors with a

tip accuracy of 0.23 mm (0.009 in.).

4.1.4 3D reconstruction and modeling of LMT The digitized data were used to reconstruct the structure of the LMT as it

appeared in situ using Alias® Maya® in conjunction with customized software developed

in our laboratory. The virtual model created for each specimen was then used to

visualize and document the attachment sites, spatial orientation and distribution of fiber

bundles and tendons in relation to the bony structures. Morphological differences

observed within the LMT were used to demarcate architecturally distinct regions.

59

4.1.5 Quantification of architectural parameters of LMT The architectural parameters of LMT investigated in this study were FBL,

extramuscular tendon length, FBA, and volume.

4.1.5.1 Fiber Bundle Length (FBL) FBL (cm) was calculated as the sum of the distances between each of the digitized

points along the length of a fiber bundle from superior to inferior. For example, Figure

4.6 demonstrates the length of a fiber bundle measured from its superior attachment to the

tip of the L1 spinous process to its inferior attachment to the S1 mammillary process.

Figure 4.6. Measurement of muscle fiber bundle length and tendon length of LMT (right, lateral view of lumbosacral spine). Length of muscle fiber bundle (red line) delineated using marked points (yellow dots) Distances between marked points were summed to yield the total length for a particular fiber bundle. Fiber bundle length (FBL); Tendon length (TL); Lumbar multifidus (LMT); Lumbar (L). 4.1.5.2 Tendon Length As described in section 4.1.2, the span of each tendon associated with LMT was

recorded using the digitizer. The extra-muscular tendon length (Figure 4.6) was then

60

computed using a plug-in algorithm developed in our laboratory for Alias® Maya®. The

algorithm calculated the length of each line traced using the digitizer by summing the

distances between digitized points from superior to inferior (Figure 4.3). Average tendon

length was derived by summing the lengths of each line and dividing by the total number

of lines digitized.

4.1.5.3 Fiber bundle angle (FBA) Similar to FBL and tendon length, FBA was computed using a plug-in algorithm

developed for Alias® Maya®. FBA was defined as the angle formed in the sagittal plane

between a tangent line drawn through the centre of each lumbar spinous process (L1-L5)

and the fiber bundles attaching to these vertebrae (see Figure 4.7).

Figure 4.7. Calculation of muscle fiber bundle angle (right, lateral view of lumbosacral spine). Each fiber bundle produces a unique FBA with respect to its vertebrae of origin. Each astrics (*) therefore represents a FBA calculated using a customized plugin in Autodesk® Maya®. Fiber bundle angle (FBA); Lumbar (L).

61

4.1.5.4 Volume Inspection of the morphology and architecture of LMT showed that it consisted of

several bands or fascicles. Each fascicle was comprised of several muscle fiber bundles

with unique superior/inferior points of attachment onto the lumbosacral spine.

Fiber bundles constituting each fascicle of LMT were stripped away carefully.

Fat and other non-skeletal muscle tissue was removed prior to adding the tissue to a

graduated cylinder containing distilled water. Once all fiber bundles constituting a given

fascicle had been added the amount of water displaced was recorded. This process was

repeated for each fascicle of LMT.

+ = Muscle fiber bundle

Figure 4.8. Measurement of muscle volume. 4.1.6 Statistical analysis of architectural parameters Fiber bundle length, fiber bundle angle, and volume of architecturally distinct

parts of the muscle were characterized with descriptive statistics (mean, standard

deviation). ANOVA followed by Tukey’s post-hoc test were carried out to compare

means. Statistically significant difference was declared at p-value = 0.05 level with two

water displacement

62

tails. K-means clustering of the architectural data was performed to determine if the data

could be grouped into distinct clusters. SPSS® version 14.0 under Windows® was used

for all analyses.

4.2 Digitization and modeling of the intramuscular nerve distribution of LMT 4.2.1 Specimens Three formalin embalmed cadaveric specimens with no visible signs of lumbar

muscle pathology and/or deformity were used in this part of the study. All specimens

were obtained from the Division of Anatomy, University of Toronto. Ethics approval

was received from the University of Toronto research ethics board (Protocol Reference

#11761).

4.2.2 Serial dissection and digitization of the medial branch of the posterior rami of L1 to L5.

The LMT was exposed unilaterally by removing the skin, fascia, superficial

muscles and aponeuroses. Medial portions of the erector spinae muscles were retained to

avoid cutting through any nerves during cleaning. Each specimen was secured firmly to a

metal aluminum tray to prevent shifting during the digitization process and three

reference points (bilateral posterior superior iliac spines, and sacral apex) were

demarcated by drilling three Robertson® #2 screws directly into solid bone. The square

head of each screw formed a depression into which the stylus tip of the digitizer could

"sink".

Nerves were dissected with the aid of a x40 dissecting microscope. In order to

ensure preservation of the posterior ramus, its medial branch and LMT, each specimen

63

was initially dissected using an anterior approach. All abdominal viscera and

musculature (e.g. psoas major) were removed and the anterior surface of the lumbar spine

cleaned on order to identify the anterior rami. Anterior rami were then traced back to

their origin from the spinal nerves. The posterior rami and their branches were then

traced inferiorly. The medial branch of each posterior ramus was identified as it exited

the intervertebral foramen and passed over the root of the transverse process. This short

extramuscular portion of the nerve was digitized.

The medial branch was then traced intramuscularly through the volume of lumbar

multifidus as illustrated in figure 4.9 and described below:

1. Individual muscle fiber bundles on the surface of LMT were localized,

digitized and carefully stripped away in the manner outlined in section 4.1.2.

2. Small segments of the medial branch were then exposed, marked and

digitized.

3. Serial dissection and digitization of LMT fiber bundles exposed deeper

segments of the medial branch. These small nerve segments were then

digitized.

4. Dissection and digitization continued until the intramuscular course of the

nerve and all of its branches could no longer be followed using a surgical

microscope.

Once dissection and digitization of the muscle and nerve was complete and each

specimen had been denuded of all ligament and remaining soft tissue structures, the

lumbosacral spine was digitized. Surface reconstruction entailed placing small dots (ie,

formulating a grid) using a fine marker over the entire surface of each lumbar vertebrae,

64

the sacrum and ilium (Figure 4.10). Points on the lumbosacral spines were separated by

2-mm intervals. Each point was then digitized using the Microscribe 3G2 Digitizer to

obtain x, y, and z coordinates.

Figure 4.9. Flowchart outlining the process of serial dissection and digitization of the medial branch of the posterior ramus.

65

Figure 4.10. Digitization of bony skeleton, 3D reconstruction. Small black dots on bony surfaces were digitized for the purpose of reconstructing the spine in 3D. 4.2.3 Reconstruction, modeling and analysis of intramuscular nerve distribution The digitized data were used to generate a fully manipulable model of each

medial branch of the posterior rami (L1 to L5) of each specimen in Autodesk® Maya®. In

turn, the digitized muscle fiber bundles and osseous skeleton data were used to generate a

3D model of the musculoskeletal lumbar spine. Lastly, the 3D model of the LMT,

including its bony correlates was combined with the digitized nerve and its branches.

The 3D model of the medial branch created in Autodesk® Maya® was fully manipulable,

allowing its innervation pattern to be visualized and recorded from several different

perspectives as it entered, divided into branches and supplied different regions of the

muscle. Muscle fiber bundles and portions of the lumbosacral spine could be added to or

subtracted from the model to allow the course of each nerve to be traced through the

66

volume of LMT more readily. Screen images of the medial branch and its subdivisions

were taken with and without the inclusion of muscle fiber bundles and skeletal

components. These images are included in the results section of this thesis.

4.3 Fiber typing of LMT This section of the study was carried out after the results of the muscle

architectural study were known.

4.3.1 Specimen(s) One male cadaveric specimen (age 68 years) with no visible evidence of

musculoskeletal deformity, pathology, or indications of previous surgery/trauma was

used to obtain muscle biopsies. Ethics approval for this portion of the project was

received from the Chief Coroner and General Inspector of Anatomy of the Province of

Ontario and from the University of Toronto (Protocol Reference # 21965).

A total of twenty nine muscle biopsies measuring approximately 1 cm3 were

harvested from different locations (superior, middle or inferior) within the deep,

intermediate and superficial regions of LMT from L1 to L5 unilaterally (Table 4.1 and

Fig. 4.11). Staining of Type I and II fibers was carried out using myosin

immunohistochemistry.

67

Figure 4.11. Lateral view of LMT showing deep (purple), intermediate (yellow) and superficial (red) regions. Small tissue samples were taken from superior (S) and inferior (I) or middle (M) portions of the muscle depending on the region being biopsied.

Superior attachment

(spinal level)

Division of multifidus

Inferior attachment

Biopsy Location

Number of biopsies

Total number of biopsies per level

Deep L3 Middle 1

Intermediate L4 Superior Inferior 2

L1

Superficial L5 S1

Sacrum/ilium

Superior/inferior Superior/Inferior Superior/Inferior

2 2 2

9

Deep L4 Middle 1

Intermediate L5 Superior Inferior 2 L2

Superficial S1 Sacrum/ilium

Superior/Inferior Superior/Inferior

2 2

7

Deep L5 Middle 1

Intermediate S1 Superior Inferior 2 L3

Superficial Sacrum/ilium Superior Inferior 2

5

Deep S1 Middle 1

Intermediate Sacrum/ilium Superior Inferior 2 L4

Superficial Sacrum/ilium Superior Inferior 2

5

Deep Sacrum/ilium Middle 1 L5

Superficial Sacrum/ilium Superior Inferior 2

3

TOTAL 29

Table 4.1: Spatial distribution of muscle biopsies taken from specimen (n=1) used in this study.

68

4.3.2 Sectioning and Immunohistochemistry Specimens were orientated in optimal cutting temperature compound embedding

medium (Tissue Tek; Miles, Elkhart, IN, U.S.A.), snap-frozen in isopentane cooled by

liquid nitrogen, and stored at -80°C. Serial sections, 14 um thick, were cut in a cryostat

at -20°C and then reacted for anti-myosin (slow, Type I) after preincubation with Tris-

HCI (pH 9.0, 115ºC)toexpose antigens which were masked by the tissue fixation process.

Detection of Type I fibers was achieved using MACH3 (Biocare) probe followed by

MACH3 (Biocare) polymer. Specimens were washed in 0.5% BSA and then reacted with

second labeled primary antibody, ALP-anti-myosin (fast, Type II) followed with 2nd

substrate Vector Red (Vecter) and counterstained with Mayer’s Hemotoxylin (Sigma).

Using this protocol Myosin II stains as pink, whereas myosin I stain as dark brown

(Figure 4.12).

Figure 4.12 Typical microscopic view of the transversely sectioned LMT muscle. Type I fibers stain dark, while Type II fibers stain light.

69

4.3.3 Morphometric analyses of Type I/II fibers Muscle fibers (typically 1,000-1,500 per slide) were assessed in order to identify

Type I(slow-twitch) and II (fast-twitch) fiber types. Digital images of each slide were

captured at lowmagnification (10x) with an Olympus DP 70 camera (Olympus, Tokyo,

Japan) attached to a Leica DM 4500 B microscope (Leica N Plan 100 X 1.25, Leica,

Wetzlar, Germany). The camera settings and light level on the microscope were

keptconstant for all of the images captured. Several images were taken from each slide in

a sequential order by motorized stage to avoid selection bias until the entire slide was

captured digitally. Computerized image analysis was performed by using image analysis

software (Visiopharm Integratory System®, VIS, Copenhagen, Denmark).

This software is endowed with an image segmentation routinethat yields distinct

optical density measures for predefinedobjects within the sections (i.e. Type I and II cells;

objects are defined based on their shape and/or relativelevels of staining). Images were

segmentedusing pixel classification. The segmentation routinepermitted the measurement

of the amount of staining specifically associatedwith each fiber type. The Bayesian pixel

classifier withinthe VIS software was initially "trained" to recognize differentcomponents

in the section (i.e., Type I and II fibersand light anddark background), thus creating an

algorithm that was applicableto all sections in the analysis, despite slight variations

instaining intensity and imaging parameters. The classificationof different image

components was thereafter used to quantify morphological parameters included: fiber

diameters cross sectional area (CSA) and total fiber area measured in µm2. Fiber

diameters were calculated using ten representative cells of each fiber type selected at

random from each slide. The total area occupied by each fiber type was used to

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determine the proportion of Type I and II fibers. The proportion of each fiber type by

number was determined taking the total fiber area for a given fiber type and dividing this

by the mean CSA calculated for each fiber type for a given biopsy slide.

4.3.4 Statistical analysis The data were entered into the computer and a univariate distribution of each of

the variables was examined for anomalies and errors were corrected.

The response variables used in the analyses were chosen to be the proportion of

Type I fiber area, the number of Type I fibers (expressed as a percentage), and the

diameter of Type I and Type II fibers. The predictor variables were multifidus region

(deep, intermediate, or superficial), multifidus fascicles (deep: L1 to L5; intermediate: L1

to L4; and superficial: L1-L5, L1-S1, L1-Sa, L2-S1, L2-Sa, L3-Sa, L4-Sa and L5-Sa),

lumbar Level (L1, L2, L3, L4, or L5), and biopsy location (superior, middle, or inferior).

Since the response variable is a continuous variable and the three predictor variables are

discrete variables, analysis of variance is the appropriate method to analyze the

relationship between these variables.

Statistical theory implies that when dealing with a response variable that is a

proportion, it cannot be analyzed directly without violating the assumptions of standard

analysis of variance. However, the theory indicates that a “transformed” version of the

variable can be appropriately analyzed. The appropriate transformation is the arcsine of

the square root of the original proportion values (Winer, 1971, p. 400).

The proportion by number and area of Type I and Type II fibers was calculated

for each architecturally distinct region (superficial, intermediate and deep) and fascicle of

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LMT. Percentage of Type I and Type II fibers within architecturally distinct regions and

fascicle were characterized using descriptive statistics (mean, standard deviation).

Regional differences with LMT were analyzed using analysis of variance (ANOVA).

Post hoc tests were used to explore for a relationship between any of the independent

variables (diameter, fiber type proportion by number, and fiber type proportion by area)

for each region of LMT. Paired t-tests were carried out to compare means. Statistical

significance was set at p<0.05.

72

Chapter 5 Results

5.1 Morphology and Architecture of Lumbar Multifidus The unique modeling technique used in this study allowed both visualization and

quantification of musculotendinous morphology and architecture. The LMT was found

to consist of five bands bilaterally for all ten specimens studied. Each band originated

from a lumbar vertebra (L1-L5). Muscle fiber bundles within each band having similar

superior and inferior points of attachment were used to identify architecturally distinct

regions. The L1-L4 bands consisted of three regions: superficial, intermediate, and deep

(Figure 5.1A) while the L5 band consisted of only two regions: superficial and deep

(Figure 5.1D).

5.1.1 Superficial LMT The fiber bundles of superficial LMT attached superiorly via a common tendon to

the tips of the spinous processes (L1- L5) and passed inferolaterally to the mammillary

processes of L5, S1, the sacrum and ilium. The portion of superficial LMT originating

from:

L1 spinous process had three fascicles2 attaching inferiorly to the L5 and S1

mammillary process and posterior superior iliac spine (PSIS) respectively (Fig. 5.2A).

L2 spinous process had two fascicles, attaching inferiorly to the S1 mammillary

process and PSIS respectively (Fig. 5.2B).

2 Fascicles are collections of muscle fiber bundles with the same origin and insertion (Adams, 2002).

73

L3 spinous process had one fascicle which attached inferiorly to the dorsolateral

aspect of the sacrum between the first to third sacral segments (Fig. 5.2C).

L4 spinous process consisted of one fascicle which attached to the posterior surface of

the sacrum between the second to fourth sacral segments, medial to the inferior

attachment of L3 fascicle (Fig. 5.2D).

L5 spinous process also consisted of one fascicle which attached to the posterior

surface of the sacrum between the level of the third to fourth sacral segments, lateral

to the median crest of the sacrum but medial to the inferior attachment of L4 fascicle

(Fig. 5.2E).

5.1.2 Intermediate LMT Intermediate LMThad a muscular superior attachment to the spinous processes of

L1-L4. Inferiorly, L1, L2 and L3 portions attached via tendons to the L4, L5 and S1

mammillary processes respectively. However, the L4 portion attached onto the sacrum at

the S2 level (Fig. 5.1B). The intermediate LMT was absent at L5 in all ten specimens

(Fig. 5.1D) being replaced by loose fatty tissue.

5.1.3 Deep LMT Deep LMT consisted of five fascicles (L1-L5) which where entirely muscular.

Each fascicle attached superiorly to the lamina of the lumbar vertebra L1-L5, and

inferiorly to the L3- S1 mammillary process two levels inferior to the superior

attachment. The L5 fascicle attached to the sacrum (Fig. 5.1C). Segmental fatty

replacement was observed in three specimens.

74

Figure 5.1. Digitization and three dimensional modeling of superficial, intermediate and deep regions of lumbar multifidus (LMT) of a cadaveric specimen, lateral views. A: Three dimensional reconstructions of the superficial (red), intermediate (yellow) and deep (purple) regions of LMT attaching to the L1 vertebra. B: Fascicles of the intermediate region attaching to the L1-L4 spinous processes. Note that there is no intermediate LMT attaching to the spinous process at L5. C: Fascicles of the deep region attaching to the L1-L5 laminae. D: Regions of LMT attaching to the L5 spinous process. Intermediate LMT is absent at L5. Spinous process (sp); mammillary process (mp); posterior superior iliac spine (PSIS); Lumbar (L).

75

Figure 5.2. Digitization and three dimensional modeling of superficial segments of lumbar multifidus (L1-L5), of a cadaveric specimen, lateral views. A, B, C, D, E: Fascicles of the superficial region attaching to L1-L5 spinous processes. Spinous process (sp); mammillary process (mp); posterior superior iliac spine (PSIS); Lumbar (L).

76

5.1.4 Architectural parameters Mean FBL3 was significantly different between the three regions (p ≤ 0.05).

Deep LMT had the shortest mean FBL (2.9 ± 1.1cm) while superficial had the longest

(5.8 ±1.6cm). Intermediate LMT had a mean FBL of 4.1 ± 1.5cm. Comparison of FBL

among levels L1 through L5 within a given region was shown to be significant for the

superficial region only. Fiber bundles attaching superiorly to the L1 and L2 spinous

processes had the longest FBL, while those attaching to the L4 and L5 spinous processes

were the shortest (Table 5.1).

Mean FBL (cm) LMT (n=10) Fascicle(s) Level Region

Superficial to L5 6.0 ±1.8 to S1 6.9 ±1.7

L1

to PSIS 8.4 ±2.0 7.3 ±1.7a

to S1 5.5 ±1.5 L2 to PSIS 6.8 ±1.2 6.4 ±1.0a,b

L3 to Sa * 5.6 ±1.1b,c L4 to Sa * 4.8 ± 1.2c L5 to Sa * 4.8 ± 1.7c

5.8 ± 1.6a

Intermediate L1 to L4 * 3.9 ±1.7d L2 to L5 * 4.4 ±1.8d L3 to S1 * 3.9 ±1.6d

L4 to Sa * 4.1 ± 0.9d L5 absent absent absent

4.1 ± 1.5b

Deep L1 to L3 * 2.6 ±0.6e L2 to L4 * 2.7 ±0.8e

L3 to L5 * 2.6 ±0.8e L4 to S1 * 3.0 ± 1.3e

L5 to Sa * 3.6 ± 1.4e

2.9 ± 1.1c

Table 5.1. Summary of Mean FBL for LMT. The superscripts letters are used to indicate the presence or absence of statistical significance (analysis of variance) between the three regions or among levels within a region. If the superscripts in a column differ, then the result is statistically significant. If the letter is repeated, there is no statistical significance. PSIS, posterior superior iliac spine; Sa, sacrum; FBL, mean fiber bundle length; *, only one segment present at these levels, hence mean FBL for segment equals mean FBL for level.

3 FBL = Fiber bundle length.

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In contrast to mean FBL, changes in mean FBA4 were not significant (p= 0.0869),

but increased from the superficial to deep regions of LMT (Table 5.2). The average FBA

calculated for the superficial, intermediate and deep regions of LMT for levels L1 –L4

ranged from 11.7º to 17.2º. The largest angle was observed within the deep region at the

L5 level (28.0 ± 11.8º).

Mean FBA (º) LMT (n=10) Fascicle

(s) Level Region

Superficial to L5 14.3 ± 8.0to S1 12.4 ± 8.9

L1

to PSIS 16.4 ± 11.814.5 ±7.7a

to S1 12.5 ± 3.9L2 to PSIS 16.0 ± 6.2 14.4 ±5.1a

L3 to Sa * 11.7 ±5.4a L4 to Sa * 12.6 ± 8.0a L5 to Sa * 15.3 ± 8.4a

13.7 ±6.9a

Intermediate L1 to L4 * 15.9 ±6.7a L2 to L5 * 14.0 ±3.9a L3 to S1 * 16.4 ±8.9a L4 to Sa * 14.8 ±8.6a L5 absent absent absent

15.3 ±7.0a

Deep L1 to L3 * 13.1 ±5.0a

L2 to L4 * 15.9 ±5.6a,b

L3 to L5 * 17.2 ±12.1a,b L4 to S1 * 16.7 ±10.1a,b

L5 to Sa * 28.0 ±11.8b

18.3 ±10.4a

Table 5.2. Summary of Mean FBA for LMT. The superscripts letters are used to indicate the presence or absence of statistical significance (analysis of variance) between the three regions or among levels within a region. If the superscripts in a column differ, then the result is statistically significant. If the letter is repeated, there is no statistical significance. PSIS, posterior superior iliac spine; Sa, sacrum; FBA, fiber bundle angle; *, only one segment present at these levels, hence mean FBA for segment equals mean FBA for level.

Average volumes for the superficial, intermediate and deep region were significantly

different from one another and decreased from superficial to deep (Table 5.3). Within

4 FBA = Fiber bundle angle.

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the superficial region, the mean volume of the L1 to L3 levels was significantly larger

than the L4 and L5 levels. In the deep region, the L5 level had a significantly larger

volume compared with the L1 to L4 levels. No significant difference in mean volumes

was noted within the levels of the intermediate region.

Mean Volume (ml) LMT (n=10) Fascicle

(s) Level Region

Superficial to L5 1.1 ±0.2 to S1 1.6 ±0.6

L1

to PSIS 3.9 ±0.7 6.7 ±0.5a

to S1 1.6 ±0.4 L2 to PSIS 6.3 ±1.6 7.9 ±1.9a

L3 to Sa * 7.0 ±1.7a L4 to Sa * 3.7 ±0.4b L5 to Sa * 2.8 ±0.3 b

5.6 ±2.3a

Intermediate L1 to L4 * 1.7 ±0.4a L2 to L5 * 1.9 ±0.3a L3 to S1 * 1.5 ±0.3a L4 to Sa * 1.8 ±0.5a L5 absent absent absent

1.7 ±0.4b

Deep L1 to L3 * 0.5 ± 0.1a

L2 to L4 * 0.6 ±0.2a L3 to L5 * 0.6 ±0.2a L4 to S1 * 0.5 ±0.1a L5 to Sa * 1.3 ±0.1b

0.7 ±0.3c

Table 5.3. Summary of Mean Volume for LMT. The superscripts letters are used to indicate the presence or absence of statistical significance (analysis of variance) between the three regions or among levels within a region. If the superscripts in a column differ, then the result is statistically significant. If the letter is repeated, there is no statistical significance. PSIS, posterior superior iliac spine; Sa, sacrum; *, only one segment present at these levels, hence mean volume for segment equals mean volume for level.

K-means clustering verified that FBL data could be grouped into three distinct regions

(Fig 5.3).

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Figure 5.3. Histogram of fiber bundle length (FBL). K-means analyses using FBL confirmed the data could be grouped into three distinct regions.

5.1.5 Tendon architecture The superficial and intermediate regions of LMT were both found to have

extramuscular tendons associated with their bony attachments, whereas the deep region

was entirely muscular. Each fascicle of superficial LMT had a common tendon which

attached superiorly to the L1 to L5 spinous processes (Fig 5.2A-5.2E). In contrast to the

superficial region, the tendons associated with the intermediate region were located

inferiorly attaching each fascicle to the mammillary processes of L4, L5, S1 and the

sacrum (Fig 5.1B). The tendons of the superficial region were thick and cylindrical,

while the tendons of the intermediate region were thin and flat. Although the shape of the

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tendinous components of the superficial and intermediate regions differed, the average

tendon lengths were similar, 2.4 ± 0.1cm and 2.3 ± 0.6 cm respectively (Table 5.4).

Region (n=10)

Mean TL (cm)

Mean FBL (cm)

TL + FBL* (cm)

Superficial 2.4 ± 0.1 5.8 ± 1.6 8.2±1.6 L1 2.7 ± 0.8 7.3 ±1.7 10.0 ± 1.9 L2 2.4 ± 0.9 6.4 ±1.0 8.8± 1.4 L3 2.4 ± 0.8 5.6 ±1.1 7.9 ± 1.4 L4 2.4 ± 0.8 4.8 ± 1.2 7.2 ± 1.4 L5 2.4 ± 0.7 4.8 ± 1.7 7.2 ± 1.8

Intermediate 2.3 ± 0.6 4.1 ± 1.5 6.4 ±1.6 L1 2.0 ± 0.9 3.9 ±1.7 5.9 ± 1.9 L2 2.1 ± 0.6 4.4 ±1.8 6.5 ± 1.9 L3 2.0 ± 0.7 3.9 ±1.6 5.9 ± 1.8 L4 3.3 ±0.9 4.1 ± 0.9 7.3 ± 1.3 L5 absent absent absent

Table 5.4. Tendon length, FBL and muscle lengths of superficial and intermediate regions. TL, tendon length; FBL, fiber bundle length; * , muscle length = TL + FBL.

5.2 Innervation of LMT 5.2.1 3D Model A 3D model of the neural distribution of the medial branch to LMT was

reconstructed for each specimen using the digitized data. The model could be

manipulated to view the nerve distribution throughout its course from different

perspectives by rotating (Figure 5.4), magnifying (Figure 5.5), and/or specifying specific

regions to include/exclude (Figure 5.6). Thus, the medial branch and all of its colateral

branches could be studied individually or in groups. In addition color coding of the

individual branches within the different regions of LMT aided in clarifying innervation

patterns.

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Figure 5.4. Views of the nerve supply to lumbar multifidus (LMT) by rotation of model.

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Figure 5.5. Views of the nerve supply to lumbar multifidus (LMT) at different magnifications.

Figure 5.6. Views of the nerve supply to specific regions of lumbar multifidus (LMT).

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5.2.2 Nerve distribution through LMT The innervation pattern described below for LMT is based on meticulous

dissection and digitization of three cadaveric specimens. The distribution of the medial

branch of posterior rami L1-L5 and all of its co-lateral branches was repeatable in all

specimens investigated. Hence, the descriptions of the innervation pattern to LMT which

follow are representative of all specimens used in this study.

The five bands of LMT which originated from each lumbar vertebra (L1-L5) were

supplied by the medial branch of the posterior ramus of its corresponding spinal nerve.

For example, the band of LMT (which contained deep, intermediate and superficial

regions) attaching onto the L1 vertebra was supplied by the medial branch of the

posterior ramus of the L1 spinal nerve which exited from the L1-L2 intervertebral

foramen (Figure 5.7).

From its origin to the posterior ramus, each medial branch traveled inferomedially

crossing over the intersection formed between the root of the next inferior transverse

process and superior articular process (Figure 5.8 & 5.9). For example the L1 medial

branch was observed to cross the root of the inferior transverse process and superior

articular process of L2. The medial branch continued its course inferiorly passing

through a tunnel formed by the mamillary process, the accessory process and mamillo-

accessory ligament (Figure 5.10). In this region the medial branch was tightly bound to

the underlying bone by strong bands of connective tissue. Upon exiting this tunnel, the

medial branch gave off a small collateral branch. This collateral branch traveled in an

inferomedial direction before turning sharply posterior (Figure 5.10) to enter deep LMT.

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This small branch was observed to enter the muscle belly on its deep surface close to its

inferior attachment to the mamillary process two vertebral levels inferior (Figure 5.11).

After giving off the branch to deep LMT, the main trunk of the medial branch

continued its course medially curving around the root of the superior articular process on

route towards the base of the spinous process (Figure 5.12). For example, the L1 medial

branch crossed over the root of the superior articular process of L2 towards the base of

the L2 spinous process. From the main trunk of the medial branch, a small branch was

traced superiorly to supply the zygapophyseal joint (Figure 5.12).

After giving off this articular branch, the main trunk traveled further inferomedially

and divided into three separate branches (Figure 5.13 and 5.14). One branch was traced

posteriorly to supply the interspinous ligament, a second continued inferiorly to enter

intermediate LMT, and a third branch traveled further posteriorly and inferiorly to supply

separate fascicles of superficial LMT. For example, the L1 medial branch supplied the

fascicles of superficial LMT originating from the L1 spinous process and inserting

inferiorly onto the L5 and S1 mamillary processes, and the sacrum (Figure 5.15 and

5.16). In a similar fashion, the L2 medial branch supplied the fascicles of superficial

LMT originating from the L2 spinous process and inserting inferiorly onto the S1

mamillary process and sacrum. Superficial LMT originating from lumbar spinous

processes of L3 and L4 consisted of only one fascicle each; hence only one nerve branch

per fascicle was observed. In all instances, these nerve branches traveled longitudinally

along the undersurface of each fascicle. Smaller branches were observed to radiate

outwards from each main stem to penetrate and supply each fascicle.

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The pattern of innervation outlined above was characteristic of medial branches L1

through L4. The L5 medial branch in contrast arose from the L5 spinal nerve and from

here traveled inferiorly, passing deep to the lumbosacral zygapophyseal joint. The nerve

was traced posteromedially before it bifurcated to give rise to two separate branches.

One branch traveled in a posterior direction to enter deep LMT originating from the

lamina of L5 vertebra (Figure 5.17). The remaining branch coursed in a posterior and

slightly medial direction towards the superficial LMT originating from the spinous

process of L5. This branch traveled on the deep surface of the fascicle giving off

numerous small muscular branches along its course. Since intermediate LMT is absent at

L5, the medial branch which took its origin from the L5 spinal nerve was observed to

innervate only the deep and superficial regions of the LMT.

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Figure 5.7. Lateral view of lumbosacral spine showing medial branches (L1 to L5) which supply the five bands of LMT. L, lumbar; sp, spinous process.

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Figure 5.8. Lateral view of the lumbar spine showing the extramuscular course of the L1 medial branch (solid blue line) traversing the intersection (shaded blue area) formed between the transverse process (tp) and superior articular process (sap) of L2. L, lumbar; iap, inferior articular process; sp, spinous process.

Figure 5.9. Dissection of lumbar multifidus showing extramuscular course of L1 medial branch, right lateral view. The L1 medial branch is shown crossing over the intersection formed between the root of the next inferior transverse process and corresponding superior articular process (shaded blue area). Superficial (red), intermediate (blue), and deep (yellow) regions of LMT attaching to the L1 spinous process and lamina. L, lumbar; LMT, lumbar multifidus; mp, mammillary process; sp, spinous process; t, tendon; tp, transverse process.

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Figure 5.10. Close up lateral view of the L2 lumbar vertebra and L1 medial branch. L1 medial branch is shown passing beneath the mamillo-accesory ligament (denoted with asterisks, “*”). Solid red line (a): branch supplying deep LMT; solid blue line (b): continuation of medial branch. L, lumbar; ap, accessory process; mp, mamillary process; sap, superior articular process; tp, transverse process.

Figure 5.11. Dissection of lumbar multifidus, right lateral view showing main trunk of L1 medial branch (yellow dotted line) giving off a nerve branch to supply deep LMT (a: red dotted line).Continuation of L1 medial branch (b: blue, dotted line).Superficial (red shaded area), intermediate (blue shaded area), and deep (yellow shaded area) regions of LMT attaching to the L1 spinous process and lamina.L, lumbar; LMT, lumbar multifidus; tp, transverse process; sap, superior articular process.

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Figure 5.12. Lateral view of lumbar spine showing L1 medial branch giving off articular branch (solid red line) to supply the superior zygapophyseal joint. Solid blue line denotes continuation of medial branch. L, lumbar; iap, inferior articular process; mp, mamillary process; sap, superior articular process; tp, transverse process.

Figure 5.13. Lateral view of the lumbar spine showing medial branch dividing into three branches. Solid green line (a): branch supplying interspinous ligament; solid red line (b): branch supplying superficial LMT; solid blue line (c): branch supplying intermediate LMT. L, lumbar; iap, inferior articular process; mp, mamillary process; tp, transverse process; sap, superior articular process.

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Figure 5.14. Dissection of lumbar multifidus (LMT), right lateral view showing medial branch (mb) of posterior ramus giving off branches to supply superficial (red), and intermediate (blue) regions of LMT attaching to the L1 spinous process and laminae. Deep LMT has been removed. Ib, branch to intermediate LMT (solid blue line); sb, branch to superficial LMT (dashed red line); branch to interspinous ligament (dashed green line); ab, branch to L1-2 zygapophyseal joint (solid pink line); L, lumbar; sp, spinous process; tp, transverse process; ZJ, zygapophyseal joint.

Figure 5.15. Lateral view of digitized spine showing the L1 medial branch dividing into three branches to supply the three separate fascicle of superficial LMT attaching superiorly to the L1 spinous process. L, lumbar; sp, spinous process; tp, transverse process; iap, inferior articular process; sup, superior articular process.

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Figure 5.16. Dissection of lumbar multifidus (LMT), right lateral view showing medial branch (mb) of posterior ramus giving off branches to supply fascicle of superficial LMT attaching superiorly to the L1 spinous process. Deep LMT has been removed. Intermediate LMT is reflected. sb, branch to superficial LMT; L, lumbar; mb, medial branch; sp, spinous process; tp, transverse process; ZJ, zygapophyseal joint; iap, inferior articular process; sup, superior articular process.

Figure 5.17. Lateral view of lumbosacral spine showing the L5 medial branch innervating deep and superficial regions of LMT. L, lumbar; iap, inferior articular process; sap, superior articular process.

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5.3 Mean characteristics of muscle fiber type for LMT: Pilot Study The results for fiber type distribution and fiber type diameter are based on 29

muscle biopsies taken form one fresh male cadaveric specimen and therefore cannot be

generalized to the population.

5.3.1 Fiber type distribution For the muscle as a whole, the average number Type I muscle fibers in LMT

expressed as a percentage was 60%. Similarly, the average area occupied by Type I

muscle fibers expressed as a percentage was 70% (Table 5.5).

Number of Type I fibers expressed as a proportion

Area of Type I fibers expressed as a proportion

LMT

(n=1) Fascicle(s) Spinal Level Region Total Fascicle(s) Spinal Level Region Total

Superficial to L5 0.71 ± 0.13 0.73 ± 0.09 to S1 0.61 ± 0.61 0.66 ± 0.06

L1

to PSIS 0.43 ± 0.02 0.58 ± 0.14

0.54 ± 0.03 0.64 ±0.10

to S1 0.69 ± 0.13 0.80 ± 0.001 L2 to PSIS 0.67 ± 0.06 0.68 ± 0.09 0.81 ± 0.006 0.81 ± 0.01

L3 to Sa * 0.73 ± 0.06 * 0.86 ± 0.02 L4 to Sa * 0.65 ± 0.28 * 0.82 ± 0.13 L5 to Sa * 0.49 ± 0.10

0.62 ±

0.14

* 0.74 ± 0.04

0.75

± 0.11

Intermediate L1 to L4 * 0.66 ± 0.06 * 0.75 ± 0.04 L2 to L5 * 0.61 ± 0.19 * 0.68 ± 0.11 L3 to S1 * 0.58 ± 0.04 * 0.69 ± 0.05 L4 to Sa * 0.69 ± 0.02 * 0.67 ± 0.02 L5 - - -

0.63 ±

0.09

- -

0.70 ±

0.06

Deep L1 to L3 * 0.52† * 0.52† L2 to L4 * 0.47† * 0.53† L3 to L5 * 0.51† * 0.54† L4 to S1 * 0.45† * 0.54† L5 to Sa * 0.48†

0.49 ±

0.03

0.60 ±

0.12

* 0.69†

0.56 ±

0.07

0.70 ±

0.12

Table 5.5. LMT Type I fiber proportions (mean ± SD). † no standard deviation as only one biopsy sample taken from the deep region of LMT. PSIS, posterior superior iliac spine; Sa, sacrum; FBA, mean fiber bundle angle; X, absent at this level; *, only one segment present at these levels, hence mean FBA for segment equals mean FBA for level.

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The area occupied by Type I fibers was significantly different between the deep,

intermediate and superficial regions of LMT (p = 0.038). Figure 5.18 shows the

relationship between the proportion of Type I fiber area and region for LMT. A Tukey’s

post hoc test showed that the area occupied by Type I fibers was significantly different

between the deep and superficial regions (p = 0.012) but not between the deep and

intermediate levels (p = 0.09) or superficial and intermediate levels (p = 0.47) (Figure

5.19).

Figure 5.18. Mean area of Type I fibers for each region expressed as a proportion. If the letter(s) over the error bars are duplicated, then there is no statistical difference. If the letter(s) over the error bars are different, then there is a statistical difference.

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Figure 5.19. Comparison of the mean area occupied by Type I fibers between the deep, intermediate and superficial regions of LMT. A. The area occupied by Type I fibers (brown) was significantly different between the deep and superficial regions (p< 0.01); B-C. The area occupied by Type I fibers was not significant between the deep and intermediate and intermediate and superficial regions (p> 0.05).

In contrast to the area occupied by Type I fibers, the number of Type I fibers

present within the LMT was not affected by level (L1-L5), region (deep, intermediate or

superficial), or biopsy location (superior, middle or deep).

The area occupied by Type I fibers for individual fascicle of LMT varied

significantly across different fascicles of LMT (p = 0.019). Figure 5.20 shows the

relationship between the proportion of Type I fibers and the various fascicles of LMT.

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Figure 5.20. Mean areas of Type I fibers expressed as a proportion. Notice that within each band of LMT the area occupied Type I fibers increases from deep to superficial. The dotted line joining points helps illustrate this interesting “sawtooth” pattern and is not intended to depict continuous data.

The area of Type I fibers is observed to increase from the deep to superficial

regions of LMT within a given spinal level (e.g. L1), with this upward trend repeating

itself from L1-L5. When the fascicles of LMT are arranged from superior to inferior

along the horizontal axis of the scatter plot above, an interesting sawtooth pattern

emerges. Hence, the LMT is comprised of five primary bands (L1–L5) based on the area

occupied by Type I fibers within each fascicle of LMT. Each of these five bands is

supplied by its own medial branch of the posterior ramus as described previously in

section 5.2.2.

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5.3.2 Fiber type diameter The average diameter of Type I muscle fibers for deep, intermediate and

superficial LMT were 59.6µm, 56.8µm, and 55.5µm respectively (differences between

regions shown to be insignificant, p = 0.94). The corresponding values of Type II fibers

were 50.6µm, 49.7µm and 41.5µm, respectively, and were shown to be significantly

different from each other (p< 0.005) (Table 5.6).

Type I fiber diameter (um) Type II fiber diameter (um) LMT (n=1) Segment(s) Level Region Segment(s) Level Region

Superficial to L5 48.95 ± 8.43 47.18 ± 7.51 to S1 56.15 ± 8.09 49.49 ± 9.86

L1

to PSIS 53.20 ± 8.06

52.77 ± 8.59

44.35 ± 6.92

47.01 ± 8.32

to S1 53.01 ± 15.98 40.64 ± 9.62 L2 to PSIS 53.13 ± 14.43

53.07 ± 13.72 36.94 ± 11.51

38.80 ± 10.63

L3 to Sa * 61.07 ± 10.62 * 41.26 ± 9.85 L4 to Sa * 60.81 ± 8.27 * 37.14 ± 8.78 L5 to Sa * 57.77 ± 9.86

55.51a ± 10.92

* 33.93 ± 9.92

41.47a ± 10.45

Intermediate L1 to L4 * 57.81 ± 10.94 * 46.59 ± 8.52 L2 to L5 * 57.53 ± 8.61 * 50.24 ± 9.38 L3 to S1 * 56.71 ± 8.45 * 44.03 ± 10.37 L4 to Sa * 55.24 ± 8.06 * 57.89 ± 9.95 L5 - *

56.82a ± 8.97

* -

49.69b ± 10.77

Deep L1 to L3 * 53.59 ± 7.76 * 53.80 ± 8.59 L2 to L4 * 46.71 ± 10.97 * 40.83 ± 8.41 L3 to L5 * 57.03 ± 11.05 * 54.40 ± 7.99 L4 to S1 * 64.37 ± 9.77 * 54.48 ± 12.27 L5 to Sa * 76.46 ± 14.98

59.63a ± 14.88

* 49.27 ± 7.66

50.56c ± 10.23

Table 5.6. LMT fiber type diameters (mean ± SD). PSIS, posterior superior iliac spine; Sa, sacrum; FBA, mean fiber bundle angle; X, absent at this level; *, only one segment present at these levels, hence mean FBA for segment equals mean FBA for level. Type II fiber diameters were significantly different across different regions of LMT, p < 0.005.

Differences in Type I and Type II mean fiber diameter were investigated across

different regions (deep, intermediate, or superficial) and levels (L1, …, L5) of LMT, as

well as biopsy location (superior, middle or inferior) for a given fascicle of LMT.

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Type I fiber diameters increased as spinal level increases from L1 to L5 (Figure

5.21). Tukey’s post hoc test verified that fiber diameters measured for LMT at one

vertebral level were statistically significant only if compared with fiber diameters taken

from LMT two or more levels inferior. For example, the difference between the mean

diameters at L1 and L2 was not statistically significant, nor was the difference between

the mean diameters at L3 and L4. However, the difference in mean diameters between

L1 and L4, L1 and L5, L2 and L4, and L2 and L5 spinal levels were statistically

significant.

Figure 5.21. The relationship between mean Type I cell diameters and spinal level. If the letter(s) over the error bars are duplicated, then there is no statistical difference. If the letter(s) over the error bars are different, then there is a statistical difference.

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The size of Type I fibers within a given region of LMT depended on spinal level

(p = 0.013). This relationship is represented graphically Figure 5.22 and indicates that

deep LMT at L5 had the largest diameters.

Figure 5.22. The relationship between mean Type I fiber diameters and spinal level and region.

Type II fiber diameter decreased from the deep to superficial regions of LMT

(p<.0005). This relationship is depicted graphically in Figure 5.23. Tukey’s post hoc test

showed that the mean diameter of Type II fibers differed significantly between the deep

and superficial regions, and the intermediate and superficial regions of LMT (p < 0.001).

Biopsy location (i.e. superior, middle, inferior) did not significantly affect the fiber

diameters of Type II fibers (p = 0.26).

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Figure 5.23. The relationship between mean Type II fiber diameters and region. If the letter over the error bars is the same, then there is no statistical difference. If the letter over the error bars is different, then there is a statistical difference.

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Chapter 6 Discussion

6.1 Introduction

There has been a paucity of studies in the literature that concurrently examine the

morphology, architecture, fiber type distribution and nerve innervation pattern of the

muscles of the lower back throughout their entire volume. In order to begin the work of

examining the macro and micro anatomy of the back muscles in this manner, the largest

and most medial back muscle in the lumbosacral region, lumbar multifidus (LMT) was

selected for this study.

The importance of studying how LMT assists in stabilizing the lumbar spine cannot

be understated. The pioneering works of Crisco and Panjabi (Panjabi et al., 1989; Crisco

& Panjabi, 1991) have demonstrated that deep segmental muscles, such as LMT, can

significantly reduce intersegmental movement and hence augment stability. Clinically, it

has been shown that following an acute episode of LBP, one is 12.4x more likely to

suffer a reoccurrence if LMT is inadequately rehabilitated (Hides et al., 2001). In

addition, surgery performed on individuals with LBP may damage LMT muscle

architecture as well as its nerve supply potentially leading to functional deficits (Ward et

al., 2009), including lumbar instability. Failed back surgery may ironically be due to the

iatrogenic effect of disrupting the morphology, architecture, and/or nerve supply to LMT

as a consequence of surgery itself.

The morphology, nerve supply and fiber type characteristics of LMT have been

previously reported (Bogduk et al., 1982; Ford et al., 1983; Macintosh et al., 1986;

Rantanen et al., 1994; Bajek et al., 2000; Zhao et al., 2000; Yoshihara et al., 2001) but not

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through its entire volume. The current study, through meticulous dissection and

digitization of individual muscle fiber bundles and the medial branches to each of the five

bands of LMT has contributed to our understanding of LMT function. In addition, the

immunohistochemistry portion of this study which examined the distribution and size of

Type I and II muscle fibers throughout the volume of LMT has yielded some interesting

results which support the theory that LMT is subdivided into neuromuscular

compartments.

6.2 Morphology and Architecture

The morphology and architecture of LMT were captured throughout its volume by

digitizing up to 1400 individual muscle fiber bundles per specimen. This method, unlike

previous studies which simplified the morphology and architecture of LMT using straight

lines to represent muscle fascicles and their line(s) of action (Macintosh & Bogduk, 1986;

Macintosh et al., 1986) allows individual muscle fiber bundles to be three dimensionally

visualized and their architectural characteristics (i.e. fiber bundle length and fiber bundle

angle) to be quantified. The three dimensional models of the LMT produced in this

manner enables viewing and quantification of muscle architecture at a level of

complexity which could not be achieved previously (Agur et al., 2003; Kim, Boynton et

al., 2007)

As highlighted earlier, muscle morphology and in particular, architectural parameters

such as FBA, FBL, and muscle volume are important determinants of function which can

have significant effects on muscles’ force generating capability (Roy & Ishihara, 1997;

Lieber & Friden, 2000). For example, the orientation (i.e. angle) of muscle fiber bundles

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within a muscle may be used to determine its actions (Macintosh & Bogduk, 1986) and is

integral to ensuring proper electrode placement when carrying out electromyography power

spectrum studies (De Foa et al., 1989; Biedermann et al., 1991).

6.2.1 Morphology

Few studies have documented the morphology of LMT with “anatomical texts and

atlases describing muscle structure mainly in terms of their origins and insertions”

(Biedermann et al., 1991). In one cadaveric study (n=12), Macintosh et al (1986) found

LMT to consist of five bands (L1-L5), with each band originating from a lumbar spinous

process and inserting inferiorly onto mammillary process(es), the sacrum and/or the

ilium.

Similar to Macintosh et al (1986), the current study found LMT to consist of five

bands; each band attaching to a separate lumbar vertebra. However, closer inspection

revealed hierarchical arrangement of muscle fiber bundles. Muscle fiber bundles were

organized into groups to form fascicles with specific superior and inferior points of

attachment onto the lumbosacral spine. Fascicles in turn originating from L1 to L4

vertebrae were further organized into distinct regions: deep, intermediate or superficial.

Whereas the fascicles attaching to the L5 vertebra were organized into only two regions:

superficial and deep. The rationale for the absence of an intermediate region at L5 is not

entirely clear, but may have some influence on the higher incidence of pathology (e.g.

herniated discs) and instability (e.g. degenerative spondylolisthesis) observed at least at

the lumbosacral junction (Boden et al., 1996).More studies are needed to determine if

intermediate LMT is absent at L5 across different age cohorts.

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Furthermore, only the intermediate and superficial regions of LMT contained tendons

within their structure. The placement and gross morphology of tendons within a muscle

influences its function (Lundon, 2003). Hence, the position of tendons relative to

contractile tissues may influence the role that these two regions play in helping to

stabilize the lumbar spine. The intermediate regions of LMT contained tendons that were

both broad and flat and which seemed to anchor muscle fibers to bone inferiorly. In

direct contrast, the tendons associated with the superficial regions of LMT tended to be

thick and cylindrical, attaching muscle fiber bundles of LMT superiorly to the tip of the

spinous processes L1-L5. This arrangement of tendons within the structure of LMT may

have the following functional interpretation. The intermediate regions of LMT - with its

superiorly placed muscle fiber bundles and inferiorly placed tendons - have a line of

action that follows a caudal to cranial direction. This arrangement seems appropriate in

the intersegmental control of anterior translation. The superficial regions of LMT - with

its inferiorly placed muscle fiber bundles and superiorly positioned tendons – have a line

of action that is in a cranial to caudal direction. This arrangement is better suited in

helping maintain the lumbar lordosis, particularly with forward bending (Figure 6.1).

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Figure 6.1. Lateral view of lumbosacral spine showing superficial and intermediate regions of LMT attaching to the L1 spinous process. Yellow arrows depict the direction of the line of action for each region. 6.2.2 Measurement of architectural parameters of LMT The current study quantified key architectural parameters for LMT according to its

fascicular and regional arrangement of muscle fiber bundles. Differences in FBL, FBA

and muscle volume have important functional implications which are discussed in the

sections which follow. Before proceeding with this discussion, it is prudent to point out

some potential factors that could affect measurements taken from cadaveric specimens.

Cutts (1988) examined the effects of fixation on skeletal muscle tissue and found that a

significant loss in muscle length occurs when muscles are removed and measures before

fixation compared to those measured in situ while still attached to the bony skeleton. The

current study measured the FBL and FBA of individual muscle fiber bundles using the

digitizer directly (i.e. in situ) for each specimen. Although this reduced the possibility of

whole muscle length shrinkage and hence FBL/FBA changes due to fixation, it is still

possible that the embalming procedure could have altered these intrinsic architectural

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properties. In addition, “long-term degradation of muscle tissue may slowly, throughout

the course of storage, alter the architecture within skeletal muscle”(Martin et al., 2001).

6.2.2.1 Fiber Bundle Length (FBL)

Delp et al (2001) provided quantitative descriptions of muscle architecture for the

rectus abdominis, quadratus lumborum, and erector spinae muscles. Musculotendon

lengths, muscle lengths, fascicle lengths, sarcomere lengths, pennation angles, and

muscle masses were measured in five cadavers. While Delp et al (2001) provided new

architectural data for some of the abdominal and back muscles, these authors did not

provide descriptive data on other muscles important in spinal stability such as LMT.

Bogduk et al (1992) constructed a model of the back muscles which included 49

fascicles of the lumbar erector spinae and LMT. To determine the physiological cross

sectional area of individual fascicles required quantification of both their volumes and

lengths. These authors explained these measurements were taken during the course of

previous morphological studies of these muscles (Macintosh et al., 1986; Macintosh &

Bogduk, 1991), but acknowledge that the data had not been previously reported. In the

model presented by Bogduk et al (1992) LMT consisted of 11 separate fascicles with

fiber bundle lengths ranging from 41 mm to 190 mm. The average FBL for each fascicle

of LMT in the current study was smaller than reported by Bogduk et al (1992). This is

likely due to the fact the values reported for FBL in the current study are of only the

contractile portions of each fascicle and does not include the length of any of its tendons.

Ward et al (2006; 2009) and Kim, Gottschalk et al (2007) studied the architectural

features of LMT in human cadavedic spines from T12 to the sacrum. LMT muscles were

isolated and measurements taken of muscle mass, ML, and FBL. “To compensate for

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variations in raw fiber length that occur because of the position of the spine during

fixation, muscle fiber lengths were normalized by scaling to the optimal sarcomere length

of human muscle (2.7µm)”(Ward et al., 2009). These authors reported the average FBL

of LMT as a whole to be approximately 5.66 ±0.65cm. In addition, these authors found

significant difference in FBL across segmental levels of origin with a trend towards

increasing FBL from T12 to L2 and then decreasing to L5.

In the present study, LMT shows a hierarchical arrangement of muscle fiber

bundles. As described previously, individual fiber bundles were arranged to form

fascicles, which in turn were organized into one of three distinct regions: superficial,

intermediate or deep. Average FBL measured ranged from 2.6 ±0.6 cm to 7.3 ±1.7 cm

(see Table 5.1) with superficial LMT demonstrating the largest average FBL (5.8 ±

1.6cm). The standard deviation calculated for each region and fascicle of LMT was

relatively small suggesting little variation over the ten specimens studied for this

architectural parameter.

The average FBL of superficial LMT determined in this study appear to

corroborate the earlier work of Ward et al (2009) for the LMT muscle as a whole. In

contrast however, the current study showed that FBL decreased from L1 to L5 for the

superficial region opposed to the increasing then decreasing trend reported by Ward et al

(2009) (Figure 6.3). Fascicles containing fibers with large FBL are designed to produce

more excursion (Ward et al., 2009). Hence, the decrease in FBL demonstrated from L1

to L5 in this study may reflect the greater importance of fiber bundles/fascicles

originating from more superior vertebrae (e.g. L1 and L2) in helping to maintain and

control the lumbar lordosis. The corollary of this is that fiber bundles/fascicles

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originating from inferior lumbar vertebrae (e.g. L4, L5) produce a smaller excursion and

hence are more important in controlling intersegmental movement.

L1 L2 L3 L4 L5 L1 L2 L3 L4 L5

0

2

4

6

8

10

Fibe

r Bun

dle

Leng

th

(FB

L)

Ward et al, 2009 Rosatelli et al, 2008

Spinal Level

Figure 6.2. Bar graph showing comparison of FBL values as a function of segmental level. 6.2.2.2 Fiber Bundle Angle (FBA)

Using photographs taken in the plane of the back of both male and female cadaveric

specimens DeFoa et al (1989) and Biedermann et al (1991) determined the fiber direction

of longissimus, iliocostalis and lumbar multifidus by measuring the angle subtended

between anatomical reference lines and the spine. For LMT, the FBAs’ reported for male

and female cadaveric specimens was 15.1° (range: 13.5°-18.0°) and 23.5° (17.5°-28.5°)

respectively (see Table 2.2). Although these authors did not specify from what region of

LMT (e.g. deep vs superficial) measurements were taken, one may speculate that the

values reported are representative of the most superficial portion of LMT as these are

more readily attainable. Assuming we are justified in comparing the average FBA for

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the superficial region of LMT in the current study (13.7° ± 6.9°) with the average FBA

calculated from previous studies (average male specimens: 15.1°; average female

specimens: 23.5°) we notice that for male cadaveric specimens the values are

comparable. A similar comparison cannot be made for the FBA of female specimens due

to insufficient numbers (i.e. one female specimen in this study). Hence, it is also not

possible to comment on gender differences in FBA.

By plotting the attachments of each fascicle of LMT from both cadavers (n=5) and

living subjects (n=21) onto clinical radiographs, Macintosh and Bogduk

(1986)determined that the “principal action of multifidus is posterior sagittal rotation”.

The current study corroborates this function; interestingly however the current study

found that the FBA for superficial LMT calculated at L5 was 10º greater than that reported

by these authors (Tables 6.1). Therelatively large FBA at L5 suggests that LMT at this

level is also important in controlling rotational movement in the transverse plane at the

lumbosacral junction.

As described above, Macintosh & Bogduk (1986) determined the action of LMT from

the orientation of its component fascicles (Macintosh & Bogduk, 1986). Each fascicle of

LMT however consists of many hundreds of muscle fiber bundles each with their own

unique FBA. Furthermore, the point of insertion of these fiber bundles may not be

uniformly distributed. This suggests that the net action of a given fascicle of LMT is

proportional to the spatial distribution of all its constituent muscle fibers and their

associated fiber angles (Figure 6.2). The strength of this study lies in the fact that the

results for mean FBA take into consideration both the number and relative distribution of

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muscle fiber bundles throughout the entire volume of LMT. This also helps to explain

the relatively large standard deviations observed for this architectural parameter.

Level FBA (°)

Rosatelli et al, 2008

Ant-Post Angle (°)

Macintosh et al, 1986

Muscle Fiber Angulation

(°) De Foa et al, 1989

Muscle Fiber Angulation (°)

Biedermann et al, 1991

L1-L4 15.9 ±6.7 14.8 ± 0.8 L1-L5 14.3 ± 8.0 15.0 ± 0.7 L1-S1 12.4 ± 8.9 12.6 ± 0.6

L1-Sacrum 16.4 ± 11.8 16.6 ± 0.9 L2-L5 14.0 ±3.9 18.8 ± 1.1 L2-S1 12.5 ± 3.9 18.0 ± 1.0

L2-Sacrum 16.0 ± 6.2 20.0 ± 1.6 L3-S1 16.4 ±8.9 23.2 ± 1.1

L3-Sacrum 11.7 ±5.4 19.6 ± 0.9 L4-Sacrum 12.6 ± 8.0/14.8 ±8.6 15.6 ± 0.9 L5-Sacrum 15.3 ± 8.4 5.4 ± 1.5

Male 15.1± 1.43 (13.5-18.0)

Female 23.5 ± 4.5 (17.5-28.5)

Table 6.1. Comparison of LMT muscle fiber angles of different studies including the current.

Figure 6.3. The net action of a given fascicle of LMT is dependant on the number and distribution of muscle fibers bundles. A. Uniform number and distribution of muscle fibers (red and blue arrows). Resultant force vector (yellow arrow) assumes a median position with respect to all the fiber bundles. B. Non-uniform distribution of muscle fiber bundles (red and blue arrows). Resultant force vector is “pulled” towards the more densely packed, red arrows.

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6.2.2.3 Volume

As described in section 6.2.2.2, Bogduk et al (1992) measured the volume of

individual fascicle of LMT in order to derive their PCSA. As the authors explain, “these

measurements were taken during the course of previous morphological studies of the

muscle, but the data have not been reported previously”.

No other studies reported the volume of LMT as a whole or of its individual

fascicles or bands. The present study fills this void. In this study, the average muscle

volume for the three regions were: 5.6 ± 2.3ml for superficial LMT, 1.7 ± 0.4ml for

intermediate LMT and 0.7 ± 0.3ml for deep LMT. Small amounts of fluid and

connective tissue elements associated with cadaveric muscle tissue which could not be

eliminated are likely to have influenced volume measurements and is considered a source

of experimental error in this study.

Based strictly on volumetric data, superficial LMT because of its larger volume

compared with deep and intermediate regions is better suited to generate torque and

hence produce movement in the lumbar spine. Using the same train of thought, deep

LMT is least likely to produce any meaningful contraction due to its much smaller

volume. In addition to having the smallest volume, deep LMT also had the shortest

FBL (see section 6.2.2.2). Based on its architecture, small FBL and volume, deep LMT

is strategically positioned to provide proprioceptive feedback from the lumbar spine. The

proprioceptive information that deep LMT provides may assist the central nervous system

(CNS) in regulating the correct amplitude, direction and force output of the various

lumbar muscles, including of course LMT itself. Deep LMT is less likely to restrain

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inter-segmental movement. The exception is at L5 were the volume and length of its

fiber bundles is relatively greater.

6.2.2.4 Physiological Cross Sectional Area (PCSA) At the present time, the only method of determining the force exerted by individual

fascicles of the back muscles is to derive it from architectural features. In principle, the

maximum force exerted by a muscle is proportional to its size: either its cross sectional

area or in the case of an irregularly shaped muscle, its’ PCSA (Powell et al., 1984).

Hence, muscles with larger PCSA have greater force generating capabilities than those

with smaller PCSA. In addition, it is postulated that muscles with large PCSA but small

FBL are ideally suited for providing stability (Ward et al., 2009).

By knowing a muscles volume and mean fiber length, the PCSA of a muscle can

be estimated using the equation:

Muscle VolumeFiber Length

PCSA = (Lieber & Boakes, 1988)

As described previously, LMT consists of muscle fiber bundles which are

organized into discrete bands or fascicles. Each fascicle has specific points of attachment

onto the lumbosacral spine. Hence, it is possible to represent the PCSA for a given

fascicle of LMT using the equation 1.2 below:

Volume =

FBL fascicle

fasciclefascicle

PCSA

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In the current study the volume of each fascicle of LMT was determined using

water displacement. The FBL for each fascicle was determined by averaging the lengths

of individual fiber bundles. The PCSA for each fascicle of LMT was calculated and the

results are compared with previously reported values in Table 6.2.

Based on PCSA, the current study revealed some interesting features about the

fascicles and regions comprising LMT. It emerges that the superficial region of LMT,

whose fascicles, when summed together, exhibits the greatest PCSA and hence largest

total force in the sagittal plane compared with the intermediate and deep regions.

Furthermore, within the superficial region, the bands attaching to the L2 and L3 spinous

processes demonstrate the largest PCSA and hence produce the greatest amount of force.

This finding may reflect the potentiality of these bands in maintaining the lumbar lordosis

which is usually centered about the L3-L4 disc.

The findings of the current study are in line with those reported by Ward et al

(2009). Compared with the other back muscles the relationship between PCSA and FBL

differ significantly from the other back muscles. The large PCSA and short FBL

associated with LMT indicate that LMT can produce a large amount of force over a

relatively small operating range. This architectural design is best suited for muscles that

provide inherent stability rather than motion. Thus, the role of LMT may be to limit

excessive motion across individual motion segments (disk and facets) and thereby

balance the loads across the spine. This may be important to prevent adjacent level

degeneration. This coincides well with previous studies that support the importance of

this muscle for clinical function. Efforts to preserve multifidus muscle function, such as

with minimally invasive surgical techniques, are warranted.

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Previous investigations have shown that LMT can stiffen the spine and decrease

neutral zone motion in flexion and extension (Steffen et al., 1994) and lateral flexion

(Wilke et al, 1995) but not significantly in rotation (Wilke et al, 1995). The explanation

given for this finding was that LMT is at a mechanical disadvantage to control neutral

zone motion in this plane. The current study found that the FBA for superficial LMT

originating from L5 to be significantly larger then previously reported (Macintosh &

Bogduk, 1986). In addition the FBL measured at this level was paradoxically the

shortest. Based on these findings, the current study postulates that LMT may play a

larger role in controlling neutral zone motion in rotation at L5 than previously predicted

by Wilke et al (1995) which only investigated the influence of muscle forces at the L4-L5

segment.

LMT Mean

Volume (mm3)

Mean FBL (mm)

Mean PCSA (mm2) Current Study

Mean PCSA (mm2) Bogduk et al, 1992

Superficial to L5 1140 ±0.2 60.0 ±18 19.0 ±2.8 42 to S1 1560 ±0.6 68.8 ±17 22.6 ±9.2 36

L1 to PSIS 3900 ±0.7 84.0 ±20 46.4 ±8.9 60

to S1 1640 ±0.4 54.6 ±15 29.8 ±7.0 39 L2

to PSIS 6300±1.6 67.6 ±12 92.6 ±24.1 99 L3 to Sa 7040 ±1.7 56.7 ±11 125.7 ±29.6 157 L4 to Sa 3660±0.4 48.1 ± 12 76.2 ±7.5 186 L5 to Sa 2820 ±0.3 42.8 ± 17 58.8 ±6.0 90

Intermediate L1 to L4 1740 ±0.4 38.6 ±17 44.6 ±3.8 40 L2 to L5 1880±0.3 43.7 ±18 42.7 ±3.8 39 L3 to S1 1460 ±0.3 39.2 ±16 37.4 ±3.8 54 L4 to Sa 1760 ±0.5 40.8 ± 9 42.9 ±3.8 186 L5 - x x x 90

Deep L1 to L3 500 ± 0.1 25.5 ±6 19.2 ±3.8 NA L2 to L4 620 ±0.2 27.4 ±8 23.0 ±5.5 NA L3 to L5 560 ±0.2 25.7 ±8 21.5 ±7.0 NA L4 to S1 500 ±0.1 30.0± 13 16.7 ±3.3 NA L5 to Sa 1260 ±0.1 36.2 ± 14 35.0 ±2.5 NA

Table 6.2. Comparison of PCSA of the current and previous studies.

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6.3 Innervation

Muscle architecture is a primary determinate of function. Hence, the more complex

a muscles’ architecture, the more likely for it is to be subdivided functionally into specific

regions, with each region subserving a specific role (English et al., 1993). Not

surprisingly, LMT with its complex arrangement of muscle fiber bundles is ideally suited

to manage multiple roles simultaneously, orchestrated by the central nervous system via

an elaborate yet simplistic arrangement of nerve fibers.

As described previously, LMT is divided into architecturally distinct regions defined

by fiber bundle orientation i.e. fiber bundle length, fiber bundle angle and tendinous

attachments (Rosatelli et al., 2008). The muscle has three architecturally distinct regions:

superficial, intermediate and deep depending on spinal level. The L1-L4 lumbar

segments consisted of all three regions, while the L5 segment consisted of only two

regions: deep and superficial. This unique arrangement of fiber bundles supports the

hypothesis that LMT is composed of “neuromuscular compartments” which have

functional or task-oriented roles. Neuromuscular compartments are defined as

architecturally distinct regions within the muscle which are independently innervated by

an individual nerve branch. Each compartment contains motor unit territories with a

unique array of physiological attributes (English & Letbetter, 1982a). In other words, the

intensity and timing of motor unit activation of each compartment can be independently

controlled and can vary between regions.

The current study found that the medial branch of the posterior ramus innervated

each region of LMT separately. If a region contained more than one fascicle (e.g.

superficial LMT originating from the L1 and L2 spinous processes), then the medial

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branch supplying this region would further subdivide to innervate these fascicles. Since

intermediate LMT is absent at L5 it is not surprising that the medial branch supplied only

its deep and superficial regions. The absence of intermediate LMT and its nerve supply

at L5 may be due to changing functional demands placed on the spine as a factor of age

or possibly the results of denervation brought about because of pathology.

Since it is has been established “that individual alpha motoneurons innervate muscle

fibers of only single histochemical types” (English et al., 1993), this implies the muscle

fiber bundles may be organized into functional units or regions. The results of this study

support the hypothesis that LMT is divided into neuromuscular compartments. Each

fascicle or region is controlled independently by a separate branch from the medial

division of the posterior ramus, thus each region of LMT has the potential to carry out a

specific function. The exact nature of this depends in turn on the arrangement of its

muscle fibers (i.e. muscle architecture) and fiber type composition. Regions of LMT

having large PCSA, small FBL and a relatively high concentration of Type I fibers would

be expected to have a greater tonic stabilizing role.

6.4 FiberType Characteristics

Before the results of this section are interpreted it is important to address any

potential methodological aspects which could affect the data. First and foremost, since

only one male embalmed cadaveric specimen was used for this part of the study, the

results may not be reflective of the population. In addition, the number of biopsy samples

taken from each region (deep, intermediate and superficial) and vertebral level (L1-L5)

was not the same. The fewest number of biopsies was taken from deep LMT, while the

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greatest number of biopsies was taken from superficial LMT. Deep LMT being small in

size precluded the taking of multiple biopsy samples of this region. In contrast,

superficial and intermediate LMT constituted the bulk of LMT and therefore in order to

verify homogeneity of fiber types along the length of these fascicles more biopsies were

taken. Future studies making use of larger sample sizes would improve the

generalizability of the results. Other factors pertinent in this study, which may have

influenced the results, include: age, gender, medical history and the effect of chemical

fixation on measures of cell diameter and volume/area distribution.

Several studies have been performed to investigate the relationship between age

and the size, number and proportion of muscle fiber types. The majority of these studies

have looked at the quadriceps muscle, particularly vastus lateralis (Porter et al., 1995).

The general conclusion is that Type II fiber size decreases with increasing age, whereas

the size of Type I fibers does not appear to be affected (Porter et al., 1995). Other limb

muscles have been investigated to a much lesser degree with no comparable studies

performed for the back muscles. Alterations in the proportion of Type I and Type II

fibers have also been reported with increasing age. Pierobon-Bormioli et al (1981)

showed a higher percentage of Type I fibers taken from vastus lateralis in older subjects.

Larsson et al (1978) showed a similar result for quadriceps, but also found that the

distribution of Type II fibers to decrease linearly from the third to seventh decades. Later

studies seemingly contradict these earlier findings reporting no fiber type alteration with

increasing age (Grimby & Saltin, 1983; Sato et al., 1984; Lexell et al., 1986). Rat soleus

muscle shows an age-related decrease in the relative proportion of Type II fibers (Brown,

1987). This observed reduction may be indicative of a transformation from Type II to

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Type I fibers secondary to chronic increased use (Lieber, 2002). Alternatively, this may

be due to selective loss of Type II muscle fibers. In a series of elaborate and detailed

studies on the entire vastus lateralis muscle in humans, Lexell and colleagues confirmed

the large decrease in the number and total area occupied of Type II fibers (Lexell,

Henriksson-Larsen, Winblad et al., 1983; Lexell & Downham, 1992).

The specimen used in the current study was a fresh, 68 year old male cadaver.

As discussed above from previous studies, the size (i.e. diameter) of Type II fibers seems

to decrease with age while the proportion of Type I fibers seems to increase. In the

current study, the mean diameters of Type I fibers and Type II fibers was determined to

be 56.6µm and 45.4µm respectively which is in keeping with previously recorded values

(Mattila et al., 1986; Rantanen et al., 1993; Rantanen et al., 1994; Mannion et al., 2000).

The proportion of Type I fibers was 0.60 while the proportion of Type I fiber area was

0.70. These proportions are also similar to previously reported values for the erector

spinae and LMT (Mattila et al., 1986; Rantanen et al., 1993; Mannion, 1999).

Gender has also been shown to have an effect on the relative size and type

distribution of the erector spinae muscles in healthy subjects and those with chronic back

pain (Mannion, Dumas et al., 1997; Mannion et al., 2000). In healthy subjects, the

authors provided data showing that males had larger fibers than females for each fiber

type. In terms of fiber type distribution, no difference between males and females was

observed for the percentage of a given fiber type, however, the percentage of Type I fiber

area was significantly higher in females. Lastly, in females only, Type I fibers were

found to be considerably larger compared with Type II fiber (Mannion, Dumas et al.,

1997). In the current study, a significant difference was found between the diameters of

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Type I and II fibers (p < 0.005); however, no inference can be made as to whether gender

had any influence on this result.

Injury or pathology also seems to have an effect on fiber type distribution. In

patients undergoing back surgery due to herniated intervertebral discs, biopsy samples

taken from LMT, demonstrate selective atrophy of Type II muscle fibers (Mattila et al.,

1986; Rantanen et al., 1993) and a significantly lower proportion of Type I fibers in

individuals with low back pain (Mannion, Weber et al., 1997). However, examination

of muscle biopsies taken from cadaveric specimens with no history of lumbar pathology

or back pain has yielded similar results (Mattila et al., 1986; Rantanen et al., 1993).

Based on lack of relevant premortem data for the current specimen it is not possible to

determine if this or the type of tissue used (i.e. cadaveric) could have influenced the

studies results.

In addition to the alternations in the proportion of fiber type seen with age, gender

and pathology, considerable variation is also seen with the location or depth within the

muscle from which a particular muscle biopsy is sampled (Lexell, Henriksson-Larsen, &

Sjostrom, 1983; Lexell et al., 1988). For example, Dahmane et al (2005) found a

predominance of Type II fibers at the surface and Type I fibers in the deeper regions of

muscles in both upper and lower extremities.

6.4.1 Fiber Type Distribution

“Anatomical complexity is seldom fortuitous but develops in response to a

functional demand. Thus in very few instances can one simple function be assigned to a

particular muscle. Far more frequently, muscles fulfillboth a postural function involving

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tonic activity and also participate in movements involving phasic activity” (Johnson et

al., 1973). Tonic muscles have a higher percentage of Type I fibers and are capable of

sustained muscle activity over prolonged periods. Phasic muscles on the other hand have

a higher percentage of Type II fibers and tend to fatigue more quickly.

One of the primary functions of the lumbar muscles is to maintain an upright

posture. Of the back muscles, LMT is thought to play an important role in lumbar

stability, maintaining posture, and controlling shear forces. As such, it is not surprising

that studies on cadaveric specimens have established that the LMT and the thoracic

components of the erector spinae muscles have a higher percentage of Type I muscle

fibers (Johnson et al., 1973; Fidler et al., 1975; Jowett et al., 1975; Sirca & Kostevc,

1985; Thorstensson & Carlson, 1987; Jorgensen et al., 1993; Rantanen et al., 1993;

Mannion, Dumas et al., 1997; Mannion, Weber et al., 1997).

Although the concentration of Type I fibers is greater in the lumbar back muscles,

previous studies have found that the distribution of fiber types within a muscle to be

heterogeneous, that is, differing significantly between regions (Elder et al., 1982). As

mentioned previously, Dahmane et al., 2005 found that the distribution of muscle fiber

types varies as a function of depth, with a predominance of Type II fibers at the surface

and Type I fibers in the deeper regions of a muscle. If this is true, then regional

variations in the size and fiber type distribution through the volume of a muscle may

support the theory of “neuromuscular partitioning”. Neuromuscular compartments are

architecturally distinct regions within a muscle that are independently innervated by

separate nerve branches. These regions in turn have specific functional, task-oriented

roles.

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The current study found a predominance of Type I muscle fibers (60%) as well as

a higher proportion of Type I fiber area (70%) for LMT as a whole in keeping with

LMT’s theorized tonic stabilizing role. Interestingly, the area occupied by Type I fibers

was shown to increase from deep to superficial (Table 5.5), with the greatest difference in

mean proportions observed between its deep and superficial regions (p < 0.05). There

was no evidence of a difference between the deep and intermediate regions (p = .09) and

no evidence of a difference between the superficial and intermediate regions (p = .47).

These results parallel those of Johnson et al (1973) which found relatively more Type I

fibers in the superficial versus the deep regions of the lumbar erector spinae in a group of

6 post-mortem subjects. In direct contrast, Sirca and Kostevc (1985) found roughly the

same proportion of Type I and II fibers in the lumbar paraspinal muscles (longissimus

and multifidus), with slightly more Type I fibers in the deep regions of these muscles. In

a recent study by Dickx et al (2009) histological differences between the deep and

superficial regions of LMT were investigated using T2 relaxation times on MRI. Longer

relaxation times have are associated with muscles having more Type I fibers (Dickx, et

al., 2009). Fifteen health male subjects (mean age = 23.3 years) were investigated. The

authors found significantly higher relaxation times in the deep versus the superficial

region of LMT suggesting a higher percentage of Type I fibers in the deep region of

LMT. Since the use of MRI to investigate fiber type distribution in muscle is limited and

requires further exploration, the results of this study cannot be considered conclusive.

The proportion of Type I and II muscle fibers has also been shown to vary

systematically as a function of depth within other skeletal muscles (Lexell, Henriksson-

Larsen, & Sjostrom, 1983). Lexell and colleagues (Lexell, Henriksson-Larsen, &

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Sjostrom, 1983; Lexell & Taylor, 1989) showed that the proportion of Type I fibers and

the mean fiber area in the vastus lateralis muscle of young adult males was greater in the

deep regions of the muscle than the superficial regions. With increasing age, this

difference was much less marked and the systemic distribution of fiber types and fiber

areas diminished becoming more homogeneous (Lexell et al., 1988; Lexell & Taylor,

1991).

The conflicting results between this and previous studies may be due to several

factors. The small sample size used in this part of the study is clearly a limiting factor

which could have affected the results. Since only one specimen was used, this portion of

the study may be considered highly exploratory, and hence pilot in nature. It is without

question that further research in needed to analyse regional variations in fiber type

distribution through the volume of LMT using a larger sample size. In addition to small

sample size, the results of the current study may also be a consequence of intrinsic factors

related directly to the specimen such as age and functional adaptation.

In the current study, a greater proportion of Type I fiber area was found in the

superficial region of LMT. Older individuals, as was the case of the specimen used here

may require greater tonic muscle activity from LMT in order to facilitate the maintenance

and control over posture. Selective loss of Type II fibers and fiber type transformation

(Type II to Type I) in response to chronic increase use and changing functional demands

may have resulted in a higher proportion of tonic muscle fibers in all parts of LMT

particularly the superficial region. Interestingly, the proportion of Type I fiber area

within the deep region of LMT was greatest at L5 compared with all other levels, but was

shown not to be statistically significant (p=0.076). This observation may reflect the need

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to maintain optimal stability and the concurrent control of intersegmental movement

particularly at the lumbosacral junction.

6.4.2 Fiber Size Dubowitz et al (1985) report the average size of Type I fibers in human skeletal muscle to

be in the range of 50-55µm (males and females), while that of Type II fibers to be 55-

60µm in males and 45-50µm for females. In regards to the back muscles, the diameter of

Type I fibers is within the range given by Dubowitz et al (1985), while data on the

diameter of Type II fibers generally shows greater variability (Rantanen et al., 1994) .

In the current study the mean diameters of Type I and II fibers was 56.6µm and

45.4µm respectively which is similar in size to that reported previously. Our analyses

showed that Type I fibers had larger diameters compared with Type II fibers (p<0.005)

and that Type I fiber diameter increased as the spinal level increased from L1 to L5. This

increase may be related in part to the increasing functional demands for stability at lower

lumbar levels. Mannion et al (1997) proposed that “if an individual is not genetically

endowed with an excess of Type I fibers (by number) then the muscle seems to adapt by

modifying the relative size of the fibers types in an attempt to achieve the same end result

in relation to fatigue resistance. An alteration of fiber size is more readily achievable than

is a transformation from one fiber type to another (Goldspink, 1985)”.

As was stated previously, the current study found that the area occupied by Type I

fibers (i,e. proportion of Type I fiber area) was relatively large for the deep region of

LMT at the L5 level. Mannion et al (1998) showed that there is a significant correlation

between mean fiber size (i.e. distribution of fiber types and fiber diameter or CSA) and

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electromyographic measures of back muscle fatigability. Hence, the larger diameter and

proportion of Type I fiber area observed at more inferior levels of the lumbar spine

suggests that this region fatigues less readily. This allows LMT to function tonically

both to maintain posture and control intersegmental movement were it is needed most.

6.5 Functional considerations

It has been reported that the control and stabilization of the lumbar spineis

mediated through the interaction and activity of several trunk muscles (Panjabi, 1992b,

1992a), with strong evidence demonstrating LMT in augmenting spinal stiffness (Panjabi

et al., 1989; Kaigle et al., 1995; Wilke et al., 1995). Despite this evidence, several of the

clinical beliefs regarding this muscle have not been substantiated (MacDonald et al.,

2006). The results from the current study challenges some previous theories concerning

the role of LMT and the principles used therein to retrain this muscle using therapeutic

exercise.

Detailed dissection and digitization of the entire LMT for ten specimens produced

accurate 3D computer models of this muscle in situ using Autodesk® Maya® 2009. In

particular, the deep region of LMT was observed to have morphological, architectural

and fiber type characteristics which do not appear to support its role (except perhaps at

the lumbosacral junction) in controlling lumbar segmental stability directly. Based on

our results, deep LMT is best suited to provide proprioceptive feedback to the CNS.

Moseley et al (2002) demonstrated that the deep and superficial fibers of LMT are

differentially active during single and repetitive movements of the arm. Specifically,

they were able to show that the deep fibers of LMT became activated in a “non-direction-

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specific manner to modulate spine compression for the control of intervertebral and

rotational forces” (Moseley et al., 2002). However, the current study demonstrated that

the size (i.e. volume and PCSA), architecture, and morphology of deep LMT to be

inadequate to facilitate the control of intersegmental movement. Instead, the intermediate

region of LMT, with its larger volume, PCSA, and complimentary muscle architectural

seems better suited for controlling shear and/or torsion forces. Since the deep LMT

becomes activated irregardless of the direction of limb movement this should not be

interpreted as direct evidence in support of its theorized role in augmenting spinal

stiffness. Instead, the non-direction specific activity observed in deep LMT may reflect

its function to act as a specialized mechanoreceptor providing sensory information back

to the CNS.

Since deep LMT is likely a spinal proprioceptor this region of the muscle should

demonstrate low levels of continuous EMG activity. This is necessary in order for the

CNS to continuously monitor the “state of lumbar stability”. In addition, since it takes

time to initiate a response based on sensory input, the CNS may generate an underlying

level of tonic activity in deep LMT to increase the “readiness” of the spine to small

changes in FBL. The ability to detect small changes in FBL is especially important

around the neutral zone, where the spine exhibits the least amount of stiffness and hence

where intersegmental movements between vertebrae are the greatest. The degree of

readiness of deep LMT may be modulated by feedback (closed loop control system) from

muscle spindles contained within its structure. It is the stretch reflex and its control of

gamma motorneuron excitability that controls the sensitivity of the sensory component of

muscle spindles.

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From the preceding discussion, one might infer that deep LMT is likely to contain

a high concentration of muscle spindles within its structure. Indeed, future studies would

need to be conducted to verify this postulate. Interestingly, the results of this study

demonstrated that the area occupied by Type I fibers was greater in the superficial versus

the deep region of LMT. However, the gamma spindle system is known to facilitate the

alpha motoneurons that control Type I muscle fibers (Johansson & Sojka, 1991). At first

glance, the results of this study seem paradoxical. One explanation for this difference

might be attributed to the age of the specimen used during the fiber typing portion of this

study. Older specimens may have greater degenerative changes affecting the articular

triad consisting of the intervertebral discs and zygapophyseal joints. As such, spinal

stiffness increases with age decreasing the intersegmental nature of the spine. As the

degrees of freedom available to the spine decreases so to will the need of the CNS to

monitor the intersegmental movement between vertebrae. The proportion of Type I

fibers within the deep region of LMT may consequently decrease thereby increasing

relative proportion of Type I fibers observed in the superficial region of LMT.

The results of this study have important clinical implications. Firstly, clinicians

may find themselves rethinking some aspects of how they retrain LMT, in particular the

deep region in clients with LBP. The common practice of palpating deep LMT in

patients with LBP for the purpose of isolating and hence isometrically contracting this

region of the muscle should be reevaluated.

The results of this study suggest that all regions of LMT have an important role in

promoting and maintaining spinal stability. Therefore to improve the effectiveness of

therapeutic exercise programs designed torestore normal LMT function it is important to

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optimize the activation and control over all of its regions concurrently whenever possible.

Therapeutic exercises that focus on the strength/endurance of LMT while concurrently

facilitating its intersegmental stability and proprioception functionsmay improve clinical

outcomes in patients with LBP. Further clinical research is necessary in this area.

Back surgery is performed routinely to relieve pain and improve quality of life in

many patients with LBP (Chou, et al., 2009; Wilkinson, 1983). Unfortunately, in up to

10% of cases, surgery leaves the client no better and sometimes worse (Wilkinson, 1983).

A multitude of reasons may contribute to the relatively high rate of failed back surgery,

including the patients’ age, sex, activity level, medical history, and socioeconomic status.

In addition, alterations to muscle architecture due to preexisting pathology do not

spontaneously “correct” themselves following back surgery and may in fact be induced or

even perpetuated with surgery. Hence, by restoring “normal” muscle architecture prior to

back surgery it may be possible reduce recovery times. The current thesis has been able

to document with cutting edge three-dimensional techniques the morphology and

architecture of LMT throughout its volume and as such provides the prerequisite data

with which to make comparisons with pathological muscle tissue. In addition, the use of

real time ultrasound to retrain LMT in patients with LBP may be expanded to include the

non-invasive visualization of its muscle architecture well as assists in fine wire electrode

placement of specific regions of LMT to study their activation patterns. This may in turn

aid clinicians in monitoring the effectiveness of specific therapeutic intervention

strategies. Surgeons may find the data helpful when selecting patients most appropriate

for surgery, and thus more likely to have a positive outcome.

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Another possible reason for failed back surgery is that the innervation to LMT via

the medial branch of the posterior ramus may be disrupted causing localized muscle

atrophy and segmental instability. Minimal invasive surgery that does not disturb the

neuromuscular arrangement and continuity of the back muscles is an integral part of

achieving good post-operative outcomes. Not surprisingly, a thorough understanding of

the spatial arrangement of both contractile (i.e. muscle fiber bundles) and non-contractile

(i.e. tendons, aponeuroses, and nerves) tissue elements is a necessary substrate in the

formulation of new surgical techniques with the least iatrogenic effects.

6.6 3D Reconstruction and Modelling: Pros and Cons

The visualization and quantification of skeletal muscle architectural parameters is

a worthwhile endeavor as it allows muscle function to be better assessed. As Macintosh

and Bogduk (1986) correctly point “this can be done most accurately in cadavers in

which all the fascicles of a muscle can be dissected, and their attachments visualized and

plotted directly”. This process although capable of yielding accurate architectural data,

“is a time-consuming process if large numbers of observations are desired to cover

biological variability” (Macintosh and Bogduk, 1986). Although more rapid approaches

of determining muscle architecture using magnetic resonance imaging (MRI) and

computerized tomography (CT) have been used, these methods cannot account for

changes in fiber length and fiber angle that occur along the muscle length (Lieber &

Friden, 2000). In addition, although plotting the attachments of muscle fascicles on 2D

clinical radiographs may be a more rapid approach to discerning the possible actions of a

muscle, it is limited in that this method “collapses” the 3D architecture of muscles into a

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few representative line vectors. Only rough approximations of muscle force can be

generated using this approach.

Although 3D modeling and reconstruction of skeletal muscle in situ using

digitization of individual fiber bundles is a more tedious and time consuming process, it

has the advantage of providing a complete architectural data set. The models generated

using this approach represents the closest approximations of the in situ characteristics of

skeletal muscle and can be used to predict the gross (e.g. torque producing) and subtle

(e.g. control of intersegmental movement) actions of muscles. Finite element models

may be produced with this data and in conjunction with specific tissue properties (e.g.

tensile strength of tendons and ligaments and load tolerance limits of vertebrae and

intervertebral discs). In addition, the ability to add or subtract individual muscle fibers,

fascicles, or entire sections of muscle provides spinal biomechanists with a model to test

the affects of muscle atrophy or dysfunction on the liability of different elements of the

spine to injury. Finally, for educational purposes three dimensional reconstructions of

muscle also permit clinicians, researchers and medical/allied health students to visualize

muscle at a level of complexity and acuity that could not be achieved previously. These

models may be developed further providing virtual surgery constructs to help plan and

execute various surgical procedures.

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Chapter 7 Conclusions

1. LMT is a highly complex muscle with a hierarchal arrangement of muscle fiber

bundles/fascicles which are grouped into architecturally distinct regions: superficial,

intermediate, anddeep.

2. Fascicles of LMT attaching to the L1 to L4 vertebrae contained superficial,

intermediate and deep regions, while fascicles attaching to the L5 vertebra contained

only superficial and deep regions. The intermediate region was absent at L5.

3. Mean FBL was significantly different between regions of LMT (p < 0.05), increasing

from deep (2.9 ± 1.1 cm) to superficial (5.8 ± 1.6 cm).

4. Mean FBA was not significantly different between regions, but showed an increasing

trend from superficial (13.7 ± 1.1°) to deep (18.3 ± 1.1°).

5. Mean volume and PCSA increased significantly (p< 0.005) from the deep to

superficial region.

6. The superficial, intermediate and deep regions of LMT were independently

innervated by branches of the medial branch of the posterior ramus.

7. The area occupied by Type I and Type II fibers was significantly different (p< 0.01)

between the deep and superficial regions of LMT possibly reflecting functional

diversification between these regions. In this pilot study using one fresh cadaveric

specimen, the superficial region contained 75% Type I fibers while the deep region

had only 56% Type I fibers.

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7.1 Functional paradigm LMT has neuromuscular compartments or regions based on muscle architecture,

nerve distribution pattern and fiber type composition. The findings suggest that each

region has a specific function, working together synergistically to control movement

and/or provide stability to the lumbar spine.

Clinically, PCSA is proportional to torque production, while FBL is proportional to

excursion (Lieber et al., 1992). In addition,asmall FBA relative to the long axis of the

spine maximizes torque produced in the sagittal plane. The primary function of

superficial LMT is posterior sagittal rotation (i.e. extension) of the lumbar spine due its

long fiber bundles (5.8 ± 1.6 cm), small fiber bundle angles (13.7 ±6.9º), and large

physiological cross sectional area (70.3 ±3.0 mm3). Within the superficial region, FBL

decreased from L1 (7.3 ± 1.7 cm) to L5 (4.8 ± 1.7 cm). Muscle fiber bundles originating

from upper lumbar vertebrae (e.g. L1, L2) may produce larger excursions and are likely

more suitable in controlling and maintaining the lumbar lordosis. In contrast, fiber

bundles originating from lower lumbar vertebrae (e.g. L4, L5) may produce smaller

excursions and are better suited in restraining intersegmental movement. In principle,

muscles with a greater proportion of Type I fibers fatigue less readily compared with

those having a predominance of Type II fibers. In the current study the area occupied by

Type I fibers was greatest for the superficial region and is believed to be in response to

increase use and changing functional demands placed on this region of LMT, particularly

in older individuals. Superficial LMT may be tonically active in controlling anterior

sagittal rotation (forward flexion) thereby helping to maintain the lumbar lordosis during

forward bending.

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Conversely, small FBL, large FBA and small PCSA is indicative of limited excursion

range and torque production. Deep LMT is not well designed to produce torque or

control intersegmental movement. Rather, it is well positioned to possibly function as a

specialized mechanoreceptor providing proprioceptive feedback to the CNS. Indirectly,

the relatively smaller proportion of Type I vs Type II fibers in this region does not

support its proposed function in controlling intersegmental movement.

Intermediate LMT has architectural and histochemical properties which are

“intermediate” in value between the superficial and deep regions. Also, the placement of

the tendon relative to the muscle belly suggests this region produces torque in a cranial to

caudal direction and may facilitate the control of intersegmental movement. In addition,

the absence of intermediate LMT at L5 may increase the incidence of certain pathological

conditions, including spondylolisthesis or disc prolapse in certain older individuals.

Further study is needed to determine if the absence of intermediate LMT is a function of

age and/or pathology.

As outlined above, each region of LMT has characteristic morphological,

architectural, and histochemical properties which assist in ascribing specific roles to each

region. The precise activation, timing and control over each of these parts by the central

nervous system could be facilitated by the independent innervation observed to each

region. Injury to one or more of these branches can lead to movement impairment

syndromes and/or compromise stability. Great care is needed to preserve the nerve

supply to all regions of LMT in patients undergoing surgery for LBP.

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Chapter 8 Future Direction

The study undertaken for this thesis has many possible future directions, some of

which are outlined below.

1. Develop ultrasound techniques to directly visualize and isolate different regions of

LMT for the purpose of electromyography, muscle biopsy and muscle retraining.

2. In light of new morphological, architectural, and nerve distribution data:

a. Re-evaluate clinical beliefs of the role of deep and superficial regions of LMT

in the control of stability and movement.

b. Develop new therapeutic treatment techniques to restore and/or optimize LMT

function in patients with LBP.

3. Explore the proprioceptor potentiality of deep LMT in motor control firstly through

direct investigation of muscle spindle distribution through the volume of LMT and

indirectly through studies which examine impaired proprioception in patients with

LBP.

4. Provide the impetus for further studies which examine the muscle architecture, nerve

innervation and fiber type distribution of other paraspinal muscles, such as those in

the cervical region which are commonly injured in whiplash.

5. Produce a dynamic 3D model of the entire musculoskeletal lumbar spine that is able

to predict the magnitude and direction of forces placed on it as a function of daily

activities, work or trauma.

133

6. Model pathologic muscle using architectural and ultrasound data collected from

patients with LBP, disc pathology or trauma, thus creating a tool to assess function

following surgical intervention.

7. Use the innervation data for LMT to:

a. Selectively target and denervate specific regions of LMT to examine its

effects on lumbar stability.

b. Improve radiofrequency neurotomy procedures involving the medial branch to

optimize therapeutic effects.

c. Improve surgical skill to lessen the trauma and improve outcomes after back

surgery.

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