biomechanics 1
TRANSCRIPT
Introduction to Biomechanics
Review of Mathematics and Mechanics
Properties of Biological Materials
Methodology in Biomechanical Studies
Clinical Biomechanics
Sports Biomechanics
Occupational Biomechanics
© 2003 Huei-Ming Chai at School of Physical Therapy, National Taiwan University, Taipei All Right Reserved
Introduction to Biomechanics
Objectives: After studying this topic, the students will be able to
1. describe the definition of Biomechanics 2. understand the development of Biomechanics 3. identify the scope of biomechanical studies and their applicaton 4. explain the common used physical quantities and their symbols
About BiomechanicsDefinition of BiomechanicsDevelopment of BiomechanicsScopes of BiomechanicsPhysical Quantity
1. Chaffin & Andersson, 1999: Chap 1 2. Luttgens, K. & Hamilton, N., 2002 Chap 1
About Biomechanics
Who should take this class?
physical therapist/ occupational therapist orthopedic/ occupational medicine/ rehabilitation medicine physician or nurse industrial/ production/ manufacturing/ process engineer ergonomist/ biomechanist/ kinesiologist coach/ athlete/ sports manager industrial hygienist/ safety manager/ labor relations manager forensic medicine physician, staff, spy..... entertainment specialist/ actor or actress dancer/ painter
Applications of Biomechanics
Physical Therapy Occupational Therapy Medicine
o Orthopedics o Sports medicine o Rehabilitation medicine o Occupational medicine o Forensic medicine
Engineering o Ergonomics (Industrial medicine) o bioengineering
Kinesiology (Movement science) Arts
o performance arts o fine arts o entertainment arts
Definition of Biomechanics
Board Definition of Biomechanics
the application of the principle of the physics and mechanical engineering sciences to the problem in the context of the living systems, which is a multidisciplinary study including
Physical properties of biological materials Biological signals and their measurements Biomechanical modeling and simulation Applications of biomechanics
Limited Definition of Biomechanics
the science that examines forces acting upon and within a biological structure and effects produced by such forces (Hay, 1973)
forces： external and internal forces effects：
1. movements of segments of interest 2. deformation of biological materials 3. biological changes in the tissues
Knowledge Needed in Biomechanical Studies
Mathematics Physics Mechanics
o statics o dynamics o fluid mechanics
Biology and Medicine Neurophysiology Behavior science
Development of Biomechanics
*** Please read Chaffin's book chapter 1 ***
Galioleo Galilei William Harvey Stephen Hales YC Fung WT Dempster Don B Chaffin David Winter Frankel and Nordin
Scopes of Musculoskeletal Biomechanical Research
Research directions of musculoskeletal biomechanical research
structure and/or physical properties of muscle, tendon, ligament, capsule, cartilage, and bone
effect of load and underload of speciifc strutures factors influencing performance
Subjects for human biomechanical studies
elderly vs. young kids vs adults women vs. men disable vs. able people athelets vs. sedentary people workers vs. non-workers
Methodology in Biomechanical Studies
anthropometric method performance limit evaluation kinesiology method
o kinematic method o kinetic method
biomechanical modelling method task analysis method
Physical Quantities
When you can measure what you are speaking out and express it in numbers, you know something about it!! -- Lord Kelvin
Physical Quantity： the quantity that can be used in the mathematical equations of science and technology
Physical quantity is objective and measurable.
Dimension System
Seven Fundamental Quantities
Unit Name Unit Symbol
Length (L) meter m
Mass (m) kilogram kg
Time (T) second s
Electric Current ampere A
Temperature degree of Klevin
Luminous Intensity candela cd
Amount of Substance mole mol
Derived Quantities displacement (d)
velocity (v) = dx/dt
acceleration (a) = dv/dt
angular velocity () =d/dt
force (F) = ma
moment of force (M): torque = Fd
work (W) = Fd
power (P) = W/t
energy (E)=mc2
momentum=mv
area (A)
volume (V)
density (D)=m/V
pressure (P)=F/A
Dimensionless Quantities
percentage percentile
the 5th percentile the 25th percentile = 1st quartetile the 50th percentile = 2nd quartertile (median) the 75th percentile = 3rd quartetile the 95th percentile the 99th percentile the 100th percentile = 4th quartetile
Unit Conversion
System of Unit
metric system CGS system MKS system SI system (Systeme International d'Unites; the International System of
Units)for details: http://physics.nist.gov/cuu/Units/index.html
English System
Unit of Mass
1 foot (lb) = 0.454 kg
1 kg = 2.205 lb
1 ounce = 28.350 g = 1/16 lb
Unit of Mass
1 foot (ft) = 0.305 m 1 m = 3.281 ft 1 inch = 25.4 mm = 1/12 ft
Standard Prefix
Name yotta tera giga mega kilo hecto deka
Symbol Y T G M k h da
Value 1024 1012 109 106 103 102 101
Name deci centi milli micro naro pico yocto
Symbol d c m n p y
Value 10-1 10-2 10-3 10-6 10-9 10-12 10-24
Review of Mathematics and Mechanics
Plane Geometry Plane Trigonometry Vector Basic Statics Basic Dynamics
Plane Geometry
angles, sides, and area of a triangle
where
angles, sides, and area of a polygon radius, diameter, circumference, and area of a circle arc length and area of a sector of a circle
Plane Trigonometry
define an angle between 2 lines units used to measure angles
o degree (deg) o radius (rad) = 57.9º
orthogonal projections of a line segment onto two perpendicular axes defintion of sine (sin) definition of cosine (cos) definition of tangent (tan) inverse trigonometric relationship：
o if sin= a then = sin-1 a o if cos= a then = cos-1 a o if tan= a then = tan-1 a
law of sine:
law of cosine:
solution of an arbitrary triangle knowing 3 sides to determine the angles knowing 2 sides and 1 angle to find the rest of the angles and sides knowing 2 angles and 1 side to find the rest of the angles and sides area of an arbitrary triangle
o
o where
Vector
scalar vs. vector
scalar quantities： quantities with magnitude only, e.g. speed of 5 m/s vector quantities： quantities with magnitude and direction, e.g. velocity
of 5 m/s to right vector addition or subtraction vector decomposition expressed by unit vectors
Review of Basic Statics
External ForcesInternal ForcesMechanical AdvantageCentroidEquilibrium of the Force SystemFree Body DiagramForce Couple
External Forces
Types of external forces
gravitational force ground reaction force friction force air or water resistance
Gravitational force (Force of Gravity)
g= 9.81 m/s2 W = mg 1 kg = 9.81 N
Ground reaction forces
force exerted on a body by the ground Fx Fy Fz Mx My Mz
Friction force
resistance of two moving objects Fs = ms N where ms = coefficient of static friction Fk = mk N where mk = coefficient of kinetic friction
Air or Water resistance
Fa = Av2c
Internal Forces
1. muscle force
2. forces from tendon, ligament, and other connective tissues
Mechanical Advantage (MA) of the Lever
Definition
the ratio between the length of the force arm and the length of weight arm
Types of Lever
1. first-class lever 2. second-class lever: force advantage 3. third-class lever:
advantage for speed or distance; most in open-kinematic chain motion
Centroid
Definition
the point that defines the geometric center of an object If the material composing a body is homogeneous, the weight can be neglected.
Equilibrium of the Force System
Definition
a condition in which an object is at rest if originally at rest, or has a constant velocity if originally in motion
Newton’s Laws of Motion
Only used for a particle with a mass and negligible size moving in a non-accelerating reference frame
first law (law of inertia)
o A particle originally at rest, or moving in a straight line with a constant velocity, will remain in this state provided the particle is not subjected to an unbalanced force.
o If the resultant force acting on a particle is zero, then the particle is in equilibrium.ie. If FR = 0 then v= constant
second law (law of acceleration) o A particle acted upon by an unbalanced force experiences an acceleration
that has the same direction as the force and a magnitude that is directly proportional to the force
o F= k (dmv/dt) = ma third law (law of action and reaction)
o the mutual forces of action and reaction between two particles are equal, opposite, and colinear
o Faction= -Freaction
Equation of equilibrium
requires both a balance of forces, to prevent the body from translating with accelerated motion, AND a balance of moments, to prevent the body from rotating
FR = 0 and MR = 0
Free Body Diagram (FBD)
Definition
a sketch of the outlined shape of the body which represents it as being isolated from its surroundings and all forces and couple moments that the surroundings exert on the body
Procedure for drawing a free body diagram
1. imagine the body to be isolated from its surroundings and sketch its outlined shape
2. identify all the external forces and couple moments that act on the body, including applied loads, reaction occurring at the supports or at points of contact with other bodies, and the weight of the body
3. label all forces and couple moments with proper magnitudes and directions
Force Couple
two parallel forces that have the same magnitude, opposite directions, and are separated by a perpendicular distance
FR = 0 but
The only effect of a couple is to produce a rotation or a tendency of rotation in a
specific direction
A couple moment is a free factor which act at any point since the couple moment depends only on the position vector directed between the forces and not the position vectors directed from the point O to the force
Review of Basic Dynamics
Dynamics is the study of the motion of bodies and the unbalanced forces that produce motion
Law of accelerationMechanical analysis methods used in dynamics
Law of acceleration
Newton's 2nd Law (Law of Acceleration)：A particle acting upon by an unbalanced force experiences an acceleration that has the same direction as the force and a magnitude that is directly proportional to the force
F = m a for a single particle only valid on an inertial frame of reference
Mechanical analysis methods used in dynamics
direct dynamics (forward dynamics)：mechanical analysis of a system that determines movement from forces
F known acceleration displacement e.g. using force plate to record forces
inverse dynamics：mechanical analysis of a system that determines forces from movement
displacement acceleration F e.g. using video-based motion analysis
relationship between forces and movement o A defined set of forces results in a specific movement. o A specific movement can be the result of an infinite number of
combinations of individual forces acting on a system
Biomechanics of Bone
Basic Concepts About BoneMechanical Properties of the Bone
Factors Affecting Bone Strength and Stiffness
Failure of the Bone
1. Nordin & Frankel, 2001: Chapter 2 2. Chaffin & Andersson, 1999
Basic Concepts About Bone
Functions of the Skeletal System
mechanical functions o to protect internal organs o to provide rigid kinematic links o to provide attachments sites for muscles o to facilitate muscle action and bone movement
physiological functions o to produce blood cells (hematopioesis) o to maintain calcium metabolism (mineral hemeostasis)
Long Bone
structure based on position o diaphysis o epiphysis o metaphysis
structures based on porosity o cortical bone
compact bone, cortex 5-30% of porosity
o cancellous bone spongy bone 30-90% of porosity
Bone Modeling and Remodeling
bone modeling： the process by which bone mass increased to alter the size, shape, and structure of the bone
bone remodeling： the process by which bone mass adapts, by change its size, shape, and structure, to the mechanical demands placed upon it
Wolff's Law o static stress model o Bone is deposited where needed and resorbed where not needed. o current concept： Bone modeling and remodeling occurs in response to
the mechanical demands placed upon it.
Mechanical Properties of the Bone
Bone Strength
ultimate stress the bone can sustain before failure o failure point in the stress-strain curve
ultimate strain the bone can sustain before failure energy the bone can store before failure
o size of the area under the entire curve
Bone Stiffness
the slope of the stress-strain curve in the elastic region metal >> glass > bone
Anisotropic Behavior of the Bone
anisotropy： the property of a material which exhibits different mechanical properties when loaded in different direction
Stiffness with respect to tension is maximal for axial loads and minimal for perpendicular loads.
for ultimate stress of cortical bone: compression > tension > shear
Factors Affecting Bone Strength and Stiffness
Gravity
positive correlation between body weight and bone mass
decreased bone mass in the weight bearing joints of astronauts
Muscle Activity
contraction of muscle alters the stress
distribution in the bone
contraction of the gluteus medius muscle
produces great compressive stress on the
superior cortex of the neck of the femur,
neutralizing the tensile stress and thereby
allowing the femoral neck sustain more load
Strain Rate Dependency
The stiffness of a bone changes with the rate of loading
when loads are applied at higher rate within the physiological limit, the bone o becomes stiffer o sustains a higher load to failure o stores more energy before failure
when a bone fractures, the stored energy is released. o single bone crack for a low-energy fracture o comminuted fracture of bone for a higher-energy fracture o severe destruction of bone before failure
Fatigue of Bone Under Repetitive Loading
Stress fracture may occur when a load of lower magnitude is applied repetitively. o march fracture
o spondylolithesis
Bone Geometry
Immobilization
Degeneration
Artificial Defects
stress raiser： defect length < bone diameter
o the stresses concentrate around the defect
o the weakening effect is marked under torsion loading
o example： compression hip screw
open section： defect defect length > bone diameter
o only the shear stresses at the periphery of the bone resist the torsion
o the shear stresses at the interior of the bone run in the same direction of the
torsion.
o example： bone graft
Failure of the Bone
Failure of bone may occur when the applied stresses exceed the ultimate strength
limit, which may result from excessive stresses, or weak material, or both.
Possible causes of bone failure
o excessive acting forces
o unfavorable acting moments
o small bone dimension
o excessive repetition of load application
Biomechanics of Collagenous Tissues
Basic Concepts About Collagenous TissuesMechanical Properties of the Collagen FiberFactors Affecting Strength of Collagenous Tissues
1. Nordin & Frankel, 2001: Chapter 3 & 42. Chaffin & Andersson, 1999
Basic Concepts About Collagenous Tissues
Classification of Collagenous Tissues
dense connective tissueo ligament： tensile stress o tendon： tensile stress
loose connective tissues o capsule： tensile stress o skin： tensile stress
cartilage o articular cartilage： compressive/ shear stress o fibrocartilage： compressive/ shear stress
Components of Collagenous Tissues
cell： fibrobalst or chondrocyte
extracellular matrix o fiber
collagen fiber： for strength elastin fiber： for flexibility retin fiber： for mass
o ground substance
Mechanical Properties of the Collagen Fibers
Structure
the most abundant protein in the body to resist tensile stress tropocollagen： 3 procollagen polypeptide chains ( chains) coiled about each
other into a right-handed triple helixes
Types
Type I found in bone, tendon, ligament, and skin
Type II found in articular cartilage, nasal septum, and sternal cartilage
Tensile Strength
Compressive Strength
only able to resist low compression loads buckle under compression load slenderness ratio
ratio of length to thickness
Creep Phenomenon
progressive deformation of a viscoelastic structure with time as the amount of load remains constant
Load Relaxation Phenomenon
progressive decrease in load with time as the deformation of the structure remains constant
Hysteresis (遲滯現象)
Energy stored in a viscoelastic material when a load is given and then relaxed.
aged heel pad： poor ability to absorb the shock
Factors Affecting Strength of Collagenous Tissues
Gravity
positive correlation between body weight and bone mass
decreased bone mass in the weight bearing joints of astronauts
Muscle Activity
contraction of muscle alters the stress
distribution in the bone
contraction of the gluteus medius
muscle produces great compressive
stress on the superior cortex of the neck
of the femur, neutralizing the tensile
stress and thereby allowing the femoral
neck sustain more load
Strain Rate Dependency
The stiffness of a bone changes with the rate of loading
when loads are applied at higher rate within the physiological limit, the bone o becomes stiffer o sustains a higher load to failure o stores more energy before failure
when a bone fractures, the stored energy is released. o single bone crack for a low-energy fracture o comminuted fracture of bone for a higher-energy fracture o severe destruction of bone before failure
Fatigue of Bone Under Repetitive Loading
Stress fracture may occur when a load of lower magnitude is applied repeatitively. o march fracture
o spondylolithesis
Bone Geometry
Immobilization
Degeneration
Artificial Defects
stress raiser：defect length < bone diameter
o the stresses concentrate around the defect
o the weakening effect is marked under torsion loading
o example：compression hip screw
open section：defect defect length > bone diameter
o only the shear stresses at the peripheryof the bone resist the torsion
o the shear stresses at the interior of the bone run in the same direction of
the torsion.
o example：bone graft
Biomechanics of Skeletal Muscle
Basic Concepts About Skeletal MuscleMechanical Properties of the Skeletal MuscleFactors Affecting Muscle Strength
Objectives: After studying this topic, the student will be able to
1. explain the relationships of fiber types and fiber architecture to muscle function 2. describe the effects of the length-tension and force-velocity relationships 3. identify the factors affecting the mechanical properties of the skeletal muscles
1. Hall, 2003: Chapter 6, pp.145-182 2. Nordin & Frankel, 2001: Chapter 6 3. Chaffin & Andersson, 1999
Basic Concepts About Skeletal Muscle
Functions of the Skeletal Muscle
To create motion by producing force To provide strength
Basic Behaviors of the Skeletal Muscle
extensibility： the ability to be stretched or to increase in length elasticity： the ability to return to the original length after a stretch irritability： the ability to respond to a a stimulus ability to develop tension： the ability to decrease in length Increase in tension does not imply decrease in muscle length.
Mechanical Model of a Muscle
contractile component： muscle fiber series elastic component (SEC)： tendon parallel elastic component (PEC)： muscle membrane
Structural Organizaiton of Skeletal Muscle
muscle fiber motor unit fiber types fiber architecture
parallel fiber arrangement： parallel to the longitudinal axis of the muscle, e.g. sartorius, masseter, biceps brachii, etc. pennate fiber arrangement： at an angle to the longitudinal axis of the muscle, e.g. rectus femoris, deltoid, etc. the greater the angle of pennation, the smaller the amount of effective force transmitted to the tendon
the angle of the pennation increases as tension progressively increases in the muscle fibers
Mechanical Properties of the Skeletal Muscle
Length-Tension Relationship
The tension that a muscle generates varies with its length found when a muscle under isometric contraction and for maximum activation of the muscle In a single muscle fiber,
peak force is noted at normal resting length. a bell-shaped length-tension curve
In a muscle, force generation capacity increases when the muscle is slightly stretched because of the effect of both active and passive components.
Force-Velocity Relationship
Muscle force decreases as the velocity of contraction increases (Hill, 1938) only true for concentric contraction Muscle force decreases with increased velocity of contraction during concentric contraction whereas it increases with increased velocity of contraction during eccentric contraction.
Eccentric strength of a muscle can exceed isometric strength by a factor of 1.5 to 2.0, but this is true only under electric stimulation of the motor neuron. does NOT indicate that the muscle cannot generate strong force at a fast speed
maximum strength can be generated either by recruitment of more motor unit or by increase in muscle length
Stretch-Shortening Cycle (SSC)
When a muscle is stretched just prior to contraction, the resulting contraction is more forceful than in the absence of the pre-stretch. possible contributors to forceful tension development
elastic recoil effect of the series elastic component of the actively stretched muscle stretch reflex of the forced lengthening muscle
example: wind-up during baseball pitching
Factors Affecting Muscle Strength
Body Temperature
Muscle function is most efficient at 38.5°C (101°F). elevated muscle temperature shift in force-velocity curve
increased maximum isometric tension nerve conduction velocity frequency of stimulation muscle force enzyme activity efficiency of muscle contraction elasticity of collagen extensibility of muscle muscle force
increased maximum velocity of muscle shortening requiring less motor unit to sustain a given load
body temperature too high heat exhaustion or heat stroke
Muscle Hypertrophy
by physical training cross-sectional area of muscle fibers number of muscle fibers change in proportion of muscle fiber types
by electric stimulation
Muscle Atrophy
cross-sectional area of fibers number of muscle fibers aerobic capacity by changing the proportion of muscle fiber types
sedentary people：# of type I fibers athletes： fiber type affected by that sport
Methodology in Biomechanical Studies
Objectives: After studying this topic, the students will be able to
1. identify the commonly used biomechanical instruments 2. describe the parameters used in biomechanical studies 3. compare the differences among different instruments that have the same function
Kinematic Analysis
Rigid Body Kinematics Measurement of Kinematic Variables Processing of Raw Kinematic Data Derived Kinematic Variables
Anthropometric Measurement
Application of Anthropometry in Biomechanics Measurement of Body Segment Length Measurement of Body Segment Mass Measurement of Center of Mass Measurement of Moment of Inertia
Kinetic Analysis
Basic Kinetics Mechanical Loads on the Human Body Instruments for Measuring Kinetic Variables Derived Kinetic Variables
Force and Strength
Relationship between force and body External force acting on the body Internal force generated by the body Stress and strain
Kinematic Analysis
Rigid Body KinematicsMeasurement of Kinematic VariablesProcessing of Raw Kinematic DataDerived Kinematic Variables
1. Hall, 2003：Chapter 2, 10 (pp.318-329), and 11 2. Chaffin & Andersson, 1999： Chapter 5-2
Rigid Body Kinematics
Application of Rigid Body Kinematics
rigid body kinematics： the study of motion of a rigid body without concerning its causes (e.g. forces) using 2D or 3D marker positions to determine limb segment position and orientation
assumption： body segment acts like a rigid body examples： reach forward movement can be regarded as a 3-segment movement
contributors
Marrey Eadweard Muybridge： a British landscape photographer
Reviews of Kinematics Terminology
types of motion： linear vs. angular motion reference system： relative vs. absolute reference system plane of motion：3 cardinal plane axis of motion：3 axes
Kinematic Variables
Variable name linear angular
position r (x, y, z) displacement s = r =
velocity v = dr /dt =d /dt
acceleration a = dv /dt =d /dt
Source of Errors in Application of Rigid Body Kinematics
not always represent true skeletal locations relative errors： the relative movement of two markers with respect to each other
resources： skin movement and movement of underlying bony structure error reduction：
invasive marker placement mathematical algorithms: smoothing techniques marker attachment system
absolute errors: the movement of one specific marker with respect to specific bony landmarks of a segment errors from inadequate placement of markers
Measurement of Kinematic Variables
Direct Measurement Techniques
universal goniometer： a protractor with two long arms source of errors： the location of the goniometer, the palpation of landmarks, and the estimation during reading
electric goniometer (elgon) first developed by Karpovich in the late 1950's
a goniometer with an electrical potentiometer at its axis continuous graphic recording of relative joint angle
advantages inexpensive immediate output
disadvantages relative data time consuming to fit and align too many straps and cables if a large number are fitted most joints do not move as a hinge cost for recorder or analog-to-digital converter
inclinometer： a gravity-based goniometer source of errors
the location of the inclinometer the different shape of muscles
accelerometer： a continuous recording of segment acceleration advantages
inexpensive immediate output
disadvantages relative data cost for recorder or analog-to-digital converter too many straps and cables if a large number are fitted sensitive to shock and easily broken noises increase during rapid movement or movement involving impact
system combining photocells, light beams, and timer： two or more records of time when each photocell is intercepted by the light beam and then the motion velocity can be calculated as the distance between two photocells divided by the recorded time.
Optoelectric Image Measurement Techniques
types of techniques classified by markers used
LED (light-emitting diode) reflective markers
classified by sampling frequency 60 Hz 120 Hz 240 Hz
advantages both absolute and relative reference system data unlimited markers minimal movement encumbrance able to be re-played frame by frame saving storage
disadvantages expensive need well-trained persons time consuming laboratory used only
considerations the clarity of the captured image the number of cameras used： more than 2 cameras is needed for a 3-D image the placement of cameras
Other Image Measurement Techniques
cinematography： 8/ 16 mm movie camera television + videography： 50/ 60Hz video camera
advantages： widespread availability, durability, and easy in use multiple exposure
ultrasound-based image system Zebris
magnetic-based image system
Processing of Raw Kinematic Data
Time-Domain Analysis
the signals are expressed as a time-dependent waveform an alternating signal is one that is continuously changing with time
Frequency-Domain Analysis
the signals are expressed as a frequency-dependent waveform, which can be the sum of a number of sine and cosine wave V(t) = VDC + V1sin(0t + 1) + V2sin(20t + 2) + + Vnsin(n0t + n) where 0 = 2 f0 n = the phase angle of the nth harmonic
Fourier series： the sum of the proper amplitudes of the harmonics Harmonic analysis (Fourier Transformation)： the mathematic process to transform given time-varying data to their frequency components
Digitization
Why needs digitalization? Continuous signal measurement is the most desirable because no data are lost. However, computer-based systems require periodic measurements since by their nature, computers can only accept discrete numbers at discrete intervals of time
analog to digital converter
Analog signals are continuous in time and amplitude. Digital signals are discrete in time and amplitude.
Sampling Theorem： the process signal must be sampled at a frequency at least twice as high as the highest frequency present in the signal itself If the signal is sampled at a too-low frequency, the aliasing error are obtained.
Smoothing and Filtering
Most of the signals from daily human movements are contained in the lower 12-14 harmonics. Source of noises
electronic noise in optoelectric devices spatial precision of the TV scan or film digitization system error in film digitizing
residual analysis
Derived Kinematic Variables
Displacement
the change of position that an object moves from one place to another a vector quantity that represents the straight-line distance and direction from point A to point B displacement vs. distance： distance magnitude of displacement, why? distance may be equal or greater than the magnitude of displacement
Velocity
change in position divided by change in time the first derivative of linear displacement
assumptions the raw displacement data have been smoothed by digital filtering the line joining xi+1 to xi-1 has the same slope as the line drown tangent to the curve at xi
velocity vs. speed
Acceleration
the rate of change in velocity i.e. the change in velocity in a given time interval the second derivative of linear displacement
or assumptions
the raw displacement data have been smoothed by digital filtering the line joining xi+1 to xi-1 has the same slope as the line drown tangent to the curve at xi
Angle
a vector quantity that is composed of two sides which intersect at a vertex
segment angle (absolute angle)：
the angle of one body segment which is measured in a counter-clockwise
direction starting with the horizontal plane equal to 0°
the absolute angle
in space
joint angle (relative
angle)：
the angle between
longitudinal axes of
two adjacent
segments
joint angle at the
anatomical position is
defined as zero
How to calculate angular velocity or angular acceleration??
What is the relationship between linear and angular kinematic variables?
Measurement of Anthropometric Data
Application of Anthropometry in BiomechanicsMeasurement of Body Segment LengthMeasurement of Body Segment MassMeasurement of Center of MassMeasurement of Moment of Inertia
1. Hall, 2003：Chapter 3 2. Chaffin & Andersson, 1999： Chapter 3 & 4
Application of Anthropometry in Biomechanics
Definition of Anthropometry
the study investigating the physical dimensions or other properties of the human body to determine the differences in the individuals and groups the science that deals with the measure of size, mass, shape, and inertia properties of the human body (Chaffin & Andersson, 1999)
Examples in Movement Science
length of body segment joint center of rotation angle of pull of tendons length and cross-sectional area of muscles
Knowledge Needed in Anthropometry
mathematics physics biomechanics biostatistics
Materials Used in Anthropometric Research
living body cadaver： fresh or frozen
Measurement of One Body Segment
Length of Body Segment Link
In motion analysis, the human body is considered to be a system of mechanical links, with each link of known physical size and form determination of link： the line draw along the longitudinal axis of the segment determination of center of rotation： the intersection of two segment links during motion link length = the distance between two centers of rotation error： <5%
Estimation of Link Length Using Bony Landmark
Dempster, 1955 identification of bony landmark located near the joint center of rotation link length = the distance between two bony landmarks R2 >0.9
link vs. segment link-to-length ratio (%)
humerus 89.0%
radius 107.0%
hand 20.6%
femur 91.4%
tibia 110.0%
foot 30.6%
Expressed Segment Length as a Percentage of Body Height
Drillis and Contini, 1966:
grouped link % of BH single link % of BH
total arm 44%
upper arm 18.6%
forearm 14.6%
hand 10.8%
total leg at stance 53.0%
thigh 28.5%
low leg 24.6%
foot 3.9%
Note: real foot length=15.2%
Measurement of Body Segment Mass
definition of mass
a physical quantity of matter composing a body symbol： m unit： kg(kilogram) in SI unit Can you distinguish mass from weight?
measurement of segment weight
If the location of the center of mass of the segment is known, then the weight of each segment can easily be calculated. Please see the next section Averaged density of the whole body d = 0.69 + 0.9 (h / w1/3) Segment density
immersion techniques the density of distal segment is greater than that of proximal density
Segment mass： expressed by the percentage of the total mass
grouped segment % of total body individual segment % of grouped
weight segment
head and neck 8.4%head 73.8%
neck 26.2%
torso 50.0%
thorax 43.8%
lumbar 29.4%
pelvis 26.8%
total arm 5.1%
upper 54.9%
forearm 33.3%
hand 11.8%
total leg 15.7%
thigh 63.7%
shank 27.4%
foot 8.9%
Measurement of Center of Mass
Definition of Center of Mass (COM)
the point where the entire weight of the body is concentrated the point in a body about which all the parts exactly balance each other Note：Can you distinguish the center of mass from the center of gravity (COG) or from the center of pressure (COP)?
Suspension Technique
A body segment is suspended in a frame from only one point and then the point where the gravity effect is equaled is the location of the center of mass
Moment Subtraction Method
developed by Williams & Lissner, 1977 example I： to measure the location of COM of a segment composed of the low leg and foot given： segment weight W
1. have the subject lie prone on a scale
2. measure the length from head to scale, L
3. measure the weight on the scale S
4. then have the subject bend one leg
5. measure the length from head to knee, X'
6. read the value on the scale, S'
7. the location of the COM of the low leg and foot is
equal to (X-X') from the knee joint
example II： to measure the mass of the segment composed of the low leg and footgiven： location of the COM of the segment composed of the low leg and footthe mass of the low leg and foot is
Segmental Zone Approach
developed by Miller & Nelson, 1976 Please check Chaffin's book for details
Ratios of Location of COM to Segment Length
Different values have been reported form different studies due to variations in the definition of segment length and different measurement techniques. Please check Chaffin's book for details
segment % from proximal end
upper arm 43.6%
forearm 43.0%%
hand 49.4%
thigh 43.3%%
shank 43.3%
foot 42.9%%
Measurement of Moment of Inertia
Definition of Moment of inertia
a physical quantity that an object resists to change or to action
or where mi = mass of the ith segment ri = perpendicular distance that the mass is located from a given axis of rotation of the ith segment moment of inertia acting around the axis of a joint
moment of inertia acting around the axis of a joint
Kinetic Analysis of Human Motion
Basic KineticsLoads Acting on the Human BodyInstruments for Measuring Kinetic VariablesDerived Kinetic Variables
1. Hall, 2003：Chapter 3, 12, and 14 2. Chaffin & Andersson, 1999： pp. 101-124, 146-158, 167-170
Basic Kinetics
Force
an action that changes the state of rest or motion to which it is applied
The action of a force results in acceleration of a body
F = ma
SI unit： Newton (N)
1 N = (1 kg)(1 m/s2)
external force vs. internal force strength： maximum force that a body can generate or be loaded
Body
an object that may be real or imaginary but represents a definite quantity of matter (mass), with certain dimensions, occupying a definite position in space rigid body vs. deformable body
Effect of forces on a body
in dynamic sense linear motion (translation) in the direction of net force rotary motion (rotation) in the direction of net moment
in static sense static equilibrium if the body is rigid or if the stress is low or if the duration is short deformation (shape changes) if the body is deformable
long-term effect on human body： biological changes growth injuries degeneration
Stress and Strain
stress： the intensity of force per unit area normal stress： the intensity of internal force acting perpendicular to a plane = F / A shear stress： the intensity of internal force acting tangent to a plane = F / A SI unit = N / m2 = Pa (Pascal)
strain： the degree of deformation per unit area
normal strain： the ratio of the change in length to the original length = L / L tensile strain is positive while compressive strain is negative shear strain： the intensity of internal force acting tangent to a plane = d / h SI unit noraml strain = % shear strain = rad
stress-strain curve elasticity： the ability of a body to resume its original size and shape on removal of the applied loads elastic (Young's) modulus： E = modulus of rigidity (shear modulus)： G = plasticity yield point failure point
strength： maximum force that a body can generate or be loaded e.g. muscle strength or strength of a material
Loads Acting on Human Body
Types of External Loads
tensile stress the force applied perpendicular to the body and take it apart the body tends to be elongated in the direction of the applied forces one kind of normal force
compressive stress the force applied perpendicular to the body and put it together the body tends to be shrink in the direction of the applied forces one kind of normal force
shear stress the force acting in directions tangent to the area resisting the force also named as tangential force
bending stress failure under bending stress
three point bending: failure at the point of the middle force four point bending: failure at the weakest point
torsion stress
combined stress
Factors Affecting the Extent of Deformation
mechanical properties size of the body shape of the body temperature humidity magnitude, direction, and duration of applied forces
Instruments for Measuring Kinetic Variable
Instruments for Measuring Muscle Forces
electromyography (EMG)： the technique of recording electric activity produced by the muscle
muscle activity： the change in electric current or voltage as tension is developed by a muscle
EMG signals： changes in electrical potential across the muscle finer membrane resting potential of a muscle fiber = -90mV action potential of a muscle fiber = 30-40 mV< motor unit action potential (MUAP): EMG signal from the depolarization of a motor unit<
to use electrodes recording the level of muscle activity at a given time interval types of electrode
surface electrode
wire electrode (indwelling electrode) needle electrode
parameters activity pattern integrated EMG pecentage of maximum voluntary contraction (MVC)
relationship between EMG and force not a linear relationship EMG records the recruitment of motor unit
dynamometer localized static strength measurement systems
hand-held dynamometer： electronic strain gaugedisadvantages： only measuring peak force seated strength tester
localized dynamic strength measurement systems Cybex isokinetic system： dynamometer Kin-Com isokinetic system： load cells
whole body static strength measurement system position of load cell can be adjusted to different heights position of load cell can be adjusted to different directions load cell can be attached with different handles
whole body dynamic strength measurement system isokinetic lift strength tester
using simple electromechanical measuring system for performing a lifting task components of the systemi. electronic load cell and velocity transducer connected to a
readout device ii. constant-velocity motor with adjustable speed control
isoinertial strength test (Liftest test) lifting loads with different weights until one’s psychophysiological limit is reached used for personnel selection in US military department
Factors Affecting Muscle strength gender
static strength: female = 65-85% of male knee isokinetic strength: 70-75% of male
age greatest around late 20’s at age of 40, 5% loss of young at ahe of 60, 20% loss of young
anthropometric variables body height lean body weight cross-sectional area of muscle
pain physical training
Instrument for Measuring External loads
force transducer： a force measuring device that gives an electric signal proportional to the applied force
types of transducer capacitive sensor conductor sensor strain gauze piezoelectic sensor
capacitive sensor consisting of e electrically conducting plates that lie parallel to each other, separated by a distance that is small compared to the linear dimensions of the plates
the space between the plates is filled with dielectric (non-conducting electrical material) A change in force produces a change in the thickness of the dielectric material which is inversely proportional to a current which can be measured F 1/Q where F= force, Q= total charge of on each plate
conductor sensor consisting of 2 layers of conductive material and a conductive material in between the space between the plates is filled with conducting material An increase in force produces a decrease in electric resistance between 2 plates
strain gauze made in electric types
electrical resistant transducer: wire piezoresistive transducer: silicon
piezoelectic sensor non-conducting crystal that exhibits the property of generating an electrical charge when subjected to mechanical strain, e.g. quartz
compressive forces produce a change in the electric charges on the surfaces where the force has been applied. shear forces produce a change in the electric charges on the surfaces perpendicular to the applied forces advantage: wide range in measurement of force
selection of force transducer capacitive or conductor sensors
for measuring forces on soft or uneven surfaces or pressure distribution less accurate (20% of error)
strain gauze or piezoelectic sensor for measuring forces on rigid body more accurate (5% of error)
Instrument for Measuring Ground Reaction Forces
force platform system： a ground reaction force measuring system that records forces in vertical, lateral, and anteroposterior directions with respect to the plate itself
types of force plate four-corner type： a rectangular flat plate with 4-triaxial force transducers mounted at each corner central support type： one centrally instrumented pillar which supports an upper flat plate
pressure plate system： a pressure map system that provides graphical or digital map of pressure across the plantar surface of the feet
types of pressure plate system mattress type shoe-insert type
Derived Kinetic Variables
Resultant Force
the net force resulting from the summation of several acting forces on a body
FR = Fi
SI unit： Newton (N)
1 N = (1 kg)(1 m/s2)
Pressure
the force over a given area
P = F / A
SI unit： Pascal (Pa)
1 Pa = (1 N) / (1 m2)
Moment of Force (Torque)
the effect of a rotary force acting on a body the product of force and the perpendicular distance from the point of force action to the axis of rotation
M = Fd or T = Fd
SI unit： Newton-Meter (N-m)
1 N-m = (1 N) (1 m)
Momentum
quantity of motion the product of the mass and its velocity of a rigid body in motion
L = mv
SI unit： kilogram-second (kg-s)
1 kg-s = (1 kg) (1 s)
principle of conservation of momentum： in the absence of external forces, the total momentum of a given system remains constant
m1v1 = m2v2
When a collision occurs between two objects, there is a tendency for both objects to continue moving in the direction of motion originally possessed by the object with the greater momentum. The magnitude of the final velocity is
v = (m1v1 + m2v2) / (m1 + m2)
Impulse
a large force applied to a rigid body through a small period of time the product of impulse force and the time over which the forces acts
impulse = F t
SI unit： Newton-second (N-s)
1 N-s = (1 N) (1 s)
relationship between impulse and momentum
impulse = F t = m a t = m (vi+1 - vi) = m vi+1 - m vi = L
Work
product of the force along the direction of displacement and the displacement of a rigid body in motion
W = F d
SI unit： joule (J)
1 J = (1 N) (1 m)
Power
the work done per unit of time the product of the mass and its velocity of a rigid body in motion
P = W / t = F d / t = F v
SI unit： watts (W) - 1 W = (1 N)(1 m) / (1 s) = (1 J)/ (1 s)
Clinical Biomechanics
Stance and Stability
COM, COG, and COP Stability and Equilibrium Stability at Quiet Stance Externally-Perturbed Stance Self-Perturbed Stance
Walking Locomotion Gait Parameters During Level Walking Kinematics of Level Walking Kinetics of Level Walking
Sit to StandWheelchair Propelling
Stance and Stability
COM, COG, and COPStability and EquilibriumStability at Quiet StanceStability at Externally-Perturbed StanceStability at Self-Perturbed Stance
1. Hamilton, N., & Luttgens, K. 2002. Kinesiology, Scientific Basis of Human Motion, 10thed. Boston: McGraw-Hill. Chapter 14, pp. 371-394 and Chapter 15, pp. 399-411
2. Chaffin & Andersson, 1999： Chapter 17 3. Hall, 2003：Chapter 13
Objectives: After studying this topic, the students will be able to
identify the center of mass, center of gravity, and center of pressure of human body and distinguish their differences describe the methods to measure limit of stability and the factors that affect stability and equilibrium explain the changes in center of mass and center of pressure at quiet stance and during different perturbed tasks
COM, COG, and COP
Posture and Balance
posture： a term to describe the orientation of any body segment relative to the gravitational vector balance： a term to describe the dynamics of body posture to prevent falling
Definition of Center of Mass (COM)
the point where all the mass of a body is concentrated the point about which a body would balance without a tendency to rotate
All the linear forces acting on the body is balanced, i.e. F = 0 All the rotary forces acting on the body is balanced, i.e. M = 0
Location of Center of Mass
its precise location depending on individual's anatomical structure habitual standing posture current position external support
NOTE： Location of COM remains fixed as long as the body does NOT change the shape
location in human body generally accepted that it is located at
~57% of standing height in males ~ 55% of standing height in females
varies with body build, posture, age, and gender infant > child > adult (in % of body height from the floor)
methods to estimate the COM of an object suspension method： to suspend an irregular-shaped object by a string and let it hang until it ceases to move
segment modeling method： weighed average of every segment of the entire body kinetic method： double integration of shear forces from the force platform
clinical method： measurement of the PSIS (posterior superior iliac spine) level in the sagittal plane
methods to locate the COM of one segment
COM parameters absolute position of the COM in the AP and ML positions excursion of the COM linear acceleration of the COM equals to the difference between the COP and COM
COP - dCOM = kawhere k = constant a = linear acceleration of the COM
since (GRF) (COP) - (BW) (dCOM) = I and , ,
so
Center of Pressure (COP)
the point where the resultant of all ground reaction forces act
COP parameters
absolute position of the COP in the AP and ML directions
excursion of the COP
safety margin
measurement of the position of the COP
single-force-platform method
two-force-platform method： measurement the COP with one foot standing on one force
plate and the other foot on the second force plate
Definition of Centroid and COG
centroid
the point that defines the geometric center of a body
If the material composing a body is homogeneous, the weight can be
neglected, i.e. centroid = COM
Note: human body is not homogeneous
center of gravity (COG)
the vertical projection of the center of mass to the ground
Stability and Equilibrium
Classification of Equilibrium
stable equilibrium occurs when an object is placed in such a position that any disturbance effort would raise its COM tend to fall back its original position e.g. BOS or COM
unstable equilibrium occurs when an object is placed in such a position that any disturbance effort would lower its COM tend to fall into a more stable position
neutral equilibrium occurs when an object is placed in such a position that any disturbance effort would not change the level of its COM tend to fall into a more stable position
Factors Affecting Stability
size and shape of base of support (BOS) wide-base stance tandem stance: standing with one foot ahead the other
stance with crutches
Pai et al., 1997： effects of velocity and position of COM on bas of support
height of COM relationship of COG to BOS mass of body friction segmental alignment sensory input
visual vestibular system proprioception other somatosensory system
psychological or mental status muscle activities
postural muscle： the muscle that acts to prevent collapse of the skeleton
slow twitch fatigue resistant
phasic muscle： fast muscle physiological and pathological factors
Tasks Used to Study the Stability of Erect Posture
quiet stance： to maintain static stability externally-perturbed stance： to regain dynamic stability
self-perturbed stance： to maintain dynamic stability
Stability at Quiet Stance
Postural Sway
the body sways back and forth like an inverted pendulum, pivoting about the ankle, at quiet stance
AP sway (anteroposterior sway)
sway in the sagittal plane
~ 5-7 mm at quiet stance in young adults
ML sway (mediolateral sway)
sway in the frontal plane
~ 3-4 mm during quiet stance in young adults
inverted pendulum model
the trunk sways around the ankle joint like an inverted pendulum
(GRF) (dCOP) = (BW) (dCOG) + I
assumptions
1. BW = GRF
2. body sway around ankle only
3. ankle acts as a hinge joint
relationship of COG and COP during quiet stance In the case if the COP ahead the COG (see the sketch below), a counter-clockwise moment (I) is present at the ankle joint, resulting in backward rotation of the trunk and the balance is regained. In the case if the COP behind the COG, a clockwise moment is present at the ankle joint, resulting in forward rotation of the trunk and the balance may be lost and possibly fall forward.
postureal sway strategy the timing and amplitude of the coordinated motor patterns at many joints in order to adjust (reactive or proactive) posture and balance ankle strategy vs. hip strategy
factors affecting postureal sway strategy age： highly correlated to falls in the elderly fatigue injury
bracing obesity stability of the external environment
Stability at Externally-Perturbed Stance
dynamic balance： the ability that the body regains balance at the moment of giving any externally-perturbed situation methods of external perturbation
changes in direction of perturbation by standing on a moving platform
horizontal translation
sagittal plane translation
changes in surrounding environment
horizontal translation on a moving platform
Nashner (1977)： first researcher to study the effect of a moving platform
COM sways backwards when the platform moves backwards
NOTE： Actually, what he did is to measure the COP rather than the COM.
bottom-up sequence of activities of the participating muscles
Stability at self-Perturbed Stance
dynamic balance- the ability that the body maintains balance during a functional task
methods of self perturbation stance with external support, e.g. using crutches or using canes change in base of support, e.g. wide-base stance, tandem stance, or one-leg stance moving one of body parts, e.g. fast arm raise, reach, or leaning closing eyes
relationship of COG and COP during forward reach movement
CNS regulates COG by controlling the net ankle moment that is expressed by COP (Fung and Winter, 1996)
Biomechanics of Walking
LocomotionGait Parameters During Level WalkingKinematics of Level Walking Kinetics of Level Walking
1. Simoneau G.G., 2002. Kinesiology of Walkign. In: Neumann, D.A. (ed). Kinesiology of the Musculoskeletal System: Foundations for Physical Rehabilitation. St. Louis, Missouri: Mosby. pp. 523-569.
2. Hamilton, N., & Luttgens, K., 2002. Kinesiology, Scientific Basis of Human Motion, 10thed. Madison, WI, Brown & Benchmark. Chapter 19, pp. 467-494.
Objectives: After studying this topic, the students will be able to
identify different types of locomotion describe a typical gait cycle describe methods to measure the gait and the related parameters to understand ground reaction forces and how it works on the body during level walking explain the changes in kinematics and kinetics during level walking
Locomotion
Definition of Locomotion
the act or power of moving from place to place by means of one’s own mechanisms or power the result of the action of the body levers propelling the body
Types of Locomotion
on foot: walking, running, ascending or descending ramp or stairs, or jumping on wheels: bicycling, roller skating, ice skating, or wheelchair propelling on hands and/or knees or hands and feet: walking on hands, creeping or crawling, crutch walking, stunts rotary locomotion: cartwheels, handsprings, or rolls
A Typical Gait Cycle
the duration that occurs from the time when the heel of one leg strikes the ground to the time at which the same leg contacts the ground again 2 phases
stance phase (62%) swing phase (38%)
A typical gait cycle lasts 1-2 sec, depending on speed.
Stance Phase (Support Phase)
the duration when the foot in contact with the ground the duration from heel strike to toe off 3 subphases
initial contact period： from heel strike to foot flat
midstance period： from foot flat to heel off propulsive period： from heel off to toe off
Swing Phase (Recovery Phase)
the duration when the foot in the air the duration from toe off to heel strike 3 subphases
acceleration midswing deceleration
Gait Parameters During Level Walking
Recording the Gait Cycle
pneumatic switch (Marey, 1873)： 1st person to record the duration of sole contact electric switch (Scherb, 1927)： using 3 separate switches interrupted-light photography (Murray et al., 1964) pressure transducer (Andriachi et al., 1977) motion analysis system
Time Variables
stance time
single support time
double support time
duration： about 22% of the gait cycle totally
decrease when the speed of walking increases
increase in the elderly or patients with balanced disorders
swing time
stride or step time
Distance Variables
stride length
decrease in the elderly and increase as the speed of walking increases
step length
wide of base
degree of toe-out
Velocity Variables
cadence： steps per minute
comfortable speed： 80-110 steps/min
slow speed： <70 steps/min
fast speed： >120 steps/min
walking speed： distance/unit of time
increase with increased cadence and stride length simultaneously
decrease with decreased angle of toe out and increased limb length or
weight
increased speed results in decrease in duration of all the component
phases
walking velocity
Other Kinematic Variables
displacement of center of mass
angle change of each joint
linear acceleration
angular acceleration
Kinematics of Level Walking
Displacement of Body COM
Walking is a translatory motion of the body that is accomplished by the alternating rotary motions of both lower extremities
COM moves forward COM beyond anterior edge of BOS Þ the other foot moves forward to BOS
Vertical Displacement of Body COM
path： a sinosoid curve amplitude： ~2" highest point: immediately after COM passes over the WB leg lowest point: at the termination of the swing phase of the other leg
Lateral Displacement of Body COM
path： a sinosoid curve amplitude： ~2" to keep the COM over the weight-bearing foot
Kinetics of Level Walking
Forces That Control Walking
gravity (body weight) air resistance internal muscle forces ground reaction forces
normal component： vertical forces shear component ： anterior-posterior and medial-lateral friction forces
Ground Reaction Forces
definition： the forces applied to the body by the ground, as opposed to those applied to the ground, when an individual takes a step
in Cartesian ayatem： Fx, Fy, Fz, Mx, My, Mz vertical component
o double peaks
1st peak at heel strike：
the action of body
momentum
2nd peak at push-off：
contraction of calf
muscle
o peak value = 120% BW
o lower than BW during
midstance as a result of
balancing the upward
momentum of the COM
anterior-posterior component o the magnitude and direction of the
anterior-posterior shear force
depends on the position of the
COM relative to the location of the
foot
in the posterior direction at
heel strike for slowing the
forward progression of the
body
in the anterior direction at
toe off for propelling the
body forward
the larger the step length,
the greater the shear forces
because of the greater angle
of between the lower
extremity and the floor
o peak value = 20% BW
o sufficient friction force between
foot and ground is necessary for
preventing slipping down
o the propulsive force of one limb is
applied simultaneously to the
braking force of the other limb
when the weight is transferred from
one limb to the other
medial-lateral o the magnitude of the medial-lateral
shear force depends on the position
of the COM relative to the foot
in the lateral direction at
heel strike
in the medial direction at
the rest of stance phase
the larger the step width,
the greater the shear forces
because of the greater angle
of between the lower
extremity and the floor
o peak value = ~5% BW
o wide variety depending on
different foot types
Trajectory of Center of Pressure
At heel strike, the COP is located lateral to the midpoint of the heel At midestance, the COP moves more laterally
From heel off to toe off, the COP moves medially from the metatarsal heads to the bog toe
Joint Moment
At heel strike, the line of action of the
ground reaction forces passes posterior to the
ankle joint, posterior to the knee joint, and
anterior to the hip joint, leading to promote
ankle plantarflexion, knee flexion, and hip
flexion.
To prevent collapse of the lower extremity,
these external moments are counterbalanced by
internal joint reaction moments that are created
by ankle dorsiflexors, the knee extensors, and
the hip extensors.
net moment： the summation of the external
and internal moments
do NOT indicate the direction of
motion
e.g. cocontraction of agonisits and
antagonists
e.g. quadriceps avoidance
Joint Power
definition
the rate of work performed by controlling muscles
the product of the net joint moment and the joint angular velocity
significance： indicating the net rate of generating or absorbing energy by all
muscles and other connective tissues crossing the joint
positive value indicates power generation, reflecting a concentric
contraction
negative value indicates power absorption, reflecting an eccentric
contraction
Ankle Kinetics
definition
the rate of work performed by controlling muscles
the product of the net joint moment and the joint angular velocity
significance： indicating the net rate of generating or absorbing energy by all
muscles and other connective tissues crossing the joint
positive value indicates power generation, reflecting a concentric
contraction
negative value indicates power absorption, reflecting an eccentric
contraction
Biomechanics of RunningCharacteristics of Running CycleBiomechanical Analysis of RunningSpecial Considerations in SprintingSpecial Considerations in Jogging
1. Hamilton, N., & Luttgens, K. 2002. Kinesiology, Scientific Basis of Human Motion, 10thed. Boston, MA: McGraw-Hill. Chapter 19, pp. 480-484.
2. Adelaar, R.S. 1986. The practical biomechanics of running. American Journal of Sports Medicine 14:497-500.
3. Cavanagh P.R. 1987. The biomechanics of the lower extremity action in distance running. Foot and Ankle 7:197-217.
Characteristics of Running Cycle
Running Cycle
contact phase (support phase; drive phase)： one foot is in contact with the ground, i.e., from foot strike to toe-off
foot strike midsupport take off
swing phase： the lower extremity is swinging through the air, i.e., from toe-off to foot strike
follow through forward swing foot descent
Characteristics of Running
stride length and frequency tend to increase with increased running speed stride length depends on leg length, range of motion of hip, and strength of leg extensors stride frequency depends on speed of muscle contraction and the skill of running for speeds over 7 m/s, a increment in stride length is small but the stride frequency is significantly greater
Both feet tend to fall on the same line along the path of progression. With increasing running speed, duration of contact period decreases but that of swing phase increases. As the foot strikes on the ground, the foot is in front of the COM of the body but the distance from foot contact to the COG is shorter in running as compared to walking. This distance becomes shorter with the increase of the speed.
In barefoot running, the degree and duration of maximum foot pronation are increased as compared to that in running with shoes and/or foot orthoses.
Comparisons of Running with Walking
to distinguish walking from running a double swing phase during running while a double support phase during walking the body is totally airborne for a period of time during running whereas at least one part of the body (usually indicating foot) contact the ground for the whole gait cycle during walking
comparisons of kinematic and kinetic parameters of running with those of walking
running walking
entire cycle swing phase longer stance phase longer
duration of stance phase shorter longer
Double support period absent present
duration of swing phase longer shorter
floating period present absent
stride length longer shorter
stride freqency higher lower
position of body COM lower higher
vertical oscillation of body less more
COM
linear and angular velocity of lower extremity
faster slower
required ROM greater less
muscle activities greater less
leg drive during swing phase muscular momentum (pendulum)
foot progression line 1 line along midline of body 2 parallel lines
Ground reaction force 2.5~3 times body weight ~90% of body weight
Biomechanical Analysis of Running
Foot Strike
patterns of foot strike heel strike： better for long-distance running because the heel pad has a better ability to absorb high impact force midfoot strike or whole-foot strike forefoot strike
only can be used in sprinting metatarsalgia or stress fracture of the central metatarsal bones commonly occurs in the jogger with forefoot strike because of repetitive large loads onto the central metatarsal heads
At the moment of foot strike, the foot is slight supinated with the tibia in some external rotation. The most important event during foot strike is to absorb the initial impact of the foot striking the ground through
rapid extension of the hip flexion of the knee internal rotation of the tibia pronation of the subtalar joint shoes and/or orthoses
initial impact (impulse) impulse = F t initial ground reaction force = 2.5~3 times body weight, depending on the running speed heel pad has better ability to absorb initial impact than other adipose tissues in human body improvement in materials of shoes (e.g. air-cushioned shoes) or ground surface (e.g. PU or wooden surface) may decrease the initial impact
effect of lateral flare common used in jogging shoes because the heel flare increases base of support of the heel, resulting in decreased impact force per unit area at the moment of initial contact Heel flare shifts the initial contact point laterally, which increases length of the moment arm (lever arm) and then increase amount of ankle moment. This increase in ankle moment facilitates rapid pronation of the subtalar joint at the moment of landing, decrease the possibility of lateral ankle sprain
Takeoff
the greater the power of the leg drive, the greater the acceleration of the runner (F = ma) to make the foot act as a rigid lever to propel the body forward through
supination of the subtalar joint locking of the midtarsal joint
dorsiflexion (extension) of the MP joint of the big toe impulse = F t = m a t = m v = momentum
since running is a forward motion of the entire body, the horizontal component of the momentum is much more important than the vertical component
momentum： a product of mass and velocity momentum = mv impulse-momentum relationship： any changes in momentum equals to the impulse that produced it
concentric contraction of the gastrocnemius muscle the moment arm of the Achilles tendon increases during takeoff
moment of inertia is greatest at take-off during the entire running cycle the larger distance the body will move during swing phase depends on
less angle of takeoff higher speed of body projection at takeoff less difference in the height of COM at the moment of takeoff and landing
Swing Phase
reduce the moment of inertia by lifting the knee and the hip close to the body increase ROM of the lower extremity to bring the mass of the swing leg close to the hip and increase the angular velocity of the swinging leg
moment of inertia definition： the property of an object that causes it to remain in its state of either rest or motion (Hamilton & Luttgens, 2002) I = I0 + Ar2
where I0 = I about centroid axis A = area r = distance moment of intertia about centroid axis at different fixed-shape objects
circular area:： I0 = (1/4) r2 rectangular area：I0 = (1/12) b h3 Traingular area： I0 = (1/36) bh3
example： determine moment of inertia around centroid axis of a T-shaped beam
I = I0 + Ar2
= [(1/12)(2)(10)3(2)(10)(8.55-5)2] + [(1/12)(8)(3)3(8)(3)(4.45-1.5)2]=645.6
According to Newton's first law of motion, force is needed to change the velocity (amplitude and direction) of an object.
moment of inertia is greatest at take-off and least after acceleration has ceased
clearance of the foot from the ground is completed by ankle dorsiflexion knee flexion hip flexion
distance of a body moving in the air depends on the angle of take-off i.e. ths distance of the body COG ahead of take-off point the speed of the body projection at take-off the height of the COM at take-off and landing
muscle activities of the lower extremity during swing phase
joint motion force for movement muscle used
hip Flexion muscle iliopsoas + rectus femoris (concentric)
kneefirst 2/3： flexionlast 1/3： extension
first 2/3： momentumlast 1/3： muscle
first 2/3： --last 1/3： hamstrings (eccentric)
ankle dorsiflexion muscletibialis anterior + toe extensors (concentric)
Special Considerations in Sprinting
Definition
running distance < 400 m stance phase of sprinting is only 22% of the running cycle
Efficiency of Running -- to get maximum horizontal velocity without falling
increase in stride length speed = stirde length stride frequency stride length is dependent on leg length, angle of hip raising, and strength of the leg extensors stride frequency is dependent on speed of muscle contraction and the skill of runner
decrease in vertical displacement of the COM Given the same ground reaction force, the smaller the vertical component of the leg drive, the the greater the horizontal component of running velocity
foot strike close to center of gravity
better to use midfoot or forefoot strike in order to have line of gravity passing through the ankle joint If the foot strikes ahead the line of gravity, the ground reaction force creates a upward and backward moment that will retard forward motion. Therefore, as the running speed increases, the distance between the contact point of foot strike and the center of gravity decreases in order to reduce the stance and facilitate propulsion.
If the foot strikes behind the line of gravity, the ground reaction force create a upward and forward moment that will make the body fall forward
decease in lateral movements motions occurring in the entire lower extremity should be in the sagittal plane the arm movement is used to counterbalance rotation of the pelvis only
shortening of swing leg the shortening of swing leg shortens the moment arm to decreases moment of inertia and increase forward velocity the higher the knee lifts, the greater the velocity is created.
decrease internal resistance from the viscosity of the soft tissues warm-up and stretching exercises can reduce the viscosity of the soft tissues of the participating limbs
Sprint Start
crouching start (蹲踞起跑) the greater the power of the leg drive, the greater the acceleration of the runner (F = ma)
assistance of starting block (起跑架) make it possible that trunk inclines forward without overstretching the Achilles tendon provides a tilting surface against which the foot pushes horizontally while using total hip, knee, and ankle extension
the horizontal push-off force (impulse) results in an increased horizontal velocity (momentum)
Efficiency of Running
decrease in vertical displacement of the COM foot strike close to line of gravity decease in lateral movements shortening of swing leg increase in stride length
Shortening of Swing Leg
Increase in Stride Length
During the acceleration phase of the race, the trunk is more erect so that the length of the stride increase dependent on the angle that the hip joint raises
Biomechanics of Jogging
Definition
running > 1500 m classification of long-distance runners (Brody, 1980)
jogger： run 3-20 miles per week at a rate of 9-12 minutes per mile sports runner： run 20-40 miles per week and participate in "fun runs" or races of 3-6 miles long-distance runner： run 40-70 miles a week at a pace of 7-8 minutes per mile and may compete in 10,000 m races or marathons
elite marathoner： run 70-200 miles a week with a pace of 5-7 minutes per mile
Characteristics of Jogging
stance phase decreases to 31% should prevent repetitive impact stresses
heel strike or midfoot strike medial and lateral flares better material for heel pad
Throwing and Striking
Sequential Movements of the Body SegmentsBiomechanics of ThrowingBiomechanics of Striking
1. Hamilton, N., & Luttgens, K. 2002. Kinesiology, Scientific Basis of Human Motion, 10thed. Chapter 18, pp. 450-466.
Objectives: After studying this topic, the students will be able to
identify the sequential movement and give examples classify sports activities involving sequential movements according to the nature of force application identify the mechanical factors that affecting to throwing, striking, or kicking
Sequential Movements of the Body Segments
Definition of Sequential Movement
the movement that involves a sequential action of a chain of body segments, leading to a high-velocity motion of external objects (Hamilton & Luttgens, 2002, p.451)
results in the production of a summated velocity at the end of the chain of segment used the path of the external object motion is curvilinear in nature
examples a pitcher throws a baseball a young adult spikes a volleyball a batter hits a baseball
an elderly drives a golf ball a tennis player serves a tennis
Modification of Sequential Movement
objectives of sequential movements skill speed accuracy distance
components that are used to modify movement according to different objectives
numbers of body segment used range of motion (ROM) used lever length used
Classification by Nature of Force Application
momentary contact force imparted to an object through temporally contact with that object by a moving part of the body segment or by implement held or attached on the body segment the object may be either stationary or moving examples：
on moving object： baseball striking, soccer heading or kicking, volleyball set, or tennis driving on stationary object： golf
projection force imparted to an object through the end of a chain of body segments in order to develop kinetic energy, followed by a high-velocity motion of that object the object may be held in one hand or hands examples：
for distance： shot put, javelin, or volleyball serving for accuracy： baseball pitching or dart throw
continuous application force imparted to an object with the force continuously applying to that object examples：
against large resistance： pushing a desk or lifting weight maintain a position while waiting for a release： archery
Biomechanics of Baseball Throwing
Patterns of Throwing
overarm (overhead) sidearm underarm
Kinematics of Overarm Throwing
windup (cocking) phase
shoulder horizontal abduction and fully external rotation (closed-packed position)
trunk left rotation
prone to have shoulder impingement syndrome
acceleration phase
shoulder internal rotation
deceleration phase
checked by shoulder external rotators
follow-through phase
trunk rotation
Kinematics of Sidearm Throwing
preparation phase
shoulder horizontal abduction only
trunk right rotation
acceleration phase
shoulder horizontal adduction
deceleration phase
checked by deltoid posterior
follow-through phase
opposite hip internal rotation
Kinematics of Underarm Throwing
preparation phase
shoulder extension
elbow extension
acceleration phase
shoulder flexion (arm flexion)
deceleration phase
checked by shoulder extensors
follow-through phase
trunk rotation
Mechanical Factors of Throwing
ballistic movement of one segment
imparting force must overcome the inertial of an object
mass of object
internal resistance
friction between object and supporting surface
resistance to surrounding medium
force needed dependent on
speed of object
distance of throwing
accuracy of target： related to direction of the object after its release
direction of the object after release dependent on
direction of the object at the moment of release： path tangential to the arc of motion
gravity
air or water resistance
spin of the object
timing pattern of movement part
The slowest or heaviest part must start to move first, and the quickest and lightestone last
to facilitate use of stretch reflex
Biomechanics of Striking
Forehand Drive in Tennis
action： the player takes the racket to hit the ball and send it into the opponent's court
type of movement： ballistic movement participating lever： racket, racket-side arm, and trunk location fulcrum： the hip joint at non-racket side skill requirement： high speed and moderate accuracy
motion description back swing phase
the player pivots his body to have the non-racket
side face forward
the racket is taken back at the shoulder level
the body weight is over the foot of the racket side
the head of the racket is kept above the wrist
forward swing phase the player lowers down his body by flexing the knee to have the racket below the intended contact point the trunk rotates forward to shift the weight to the foot of the non-racket side the racket is perpendicular to the ground at the moment of impact
follow-through phase the body continues forward the racket arm swings across the body and up toward the chin
the effect of body spinning mechanical factors contributing the impact to the ball： the greater impart force will impart more momentum to the ball, leading to speed up the ball on its return flight
increase the lever-arm length by using a long-arm racket, keeping the arm straight
firmness of grip depends on muscle strength of wrist and finger flexors the angle of the racket face at ball hitting because the angle of rebound is highly correlated to the angle of incidence
actually, the ball is not a rigid body so that the angle of rebound is slightly less than the angle of incidence
Occupational Biomechanics the study of the physical interaction of workers with their tools, machines, and materials so as to enhance the worker’s performance while minimizing the risk of musculoskeletal disorders (Chaffin, 1994) applications
to improve working performance and efficiency to prevent occupational injuries to make industrial robots for high-risk or high-structured or repetitive works
Pushing and Pulling Push-and-Pull Motions Force Impart Biomechanics of Pushing a Cart
Load Lifting NIOSH Manual Materials Handling Limits Multi-Segment Biomechanical Model Biomechanics of Symmetrical Load Lifting
Seated Work Sitting Posture Anthropometric Dimensions of Seated Workers Seated Work Place and Layout Video Display Terminal Users
Application of Biostatistics
Design of Hand Tools
Vibration Environment
Pushing and Pulling
Push-and-Pull MotionsForce ImpartingBiomechanics of Pushing a Cart
1. Hamilton, N., & Luttgens, K. 2002. Kinesiology, Scientific Basis of Human Motion, 10thed. Boston: McGraw-Hill. Chapter 17, pp. 435-449.
2. Chaffin, D.B, & Andersson G.B.J., 1999. Occupational Biomechanics, 2nd ed.
Objectives: After studying this topic, the students will be able to
1. define push and pull patterns of motion 2. identify the the activities that involves push and pull patterns and give examples 3. analyze mechanical factors that affecting to push-and-pull activities
Push-and-Pull Motions
Definition
broad definition： a segment motion that involves moving an object, either directly by part of the body or by means of implement, in pushing and pulling pattern (Hamilton & Luttgens 2002, p.436)
a pitcher throws a baseball a tennis player serves a tennis a worker lifts a box from the floor onto an overhead rack an archer shoots an arrow from a bow
limited definition： a segmental motion that all forces are continuously applied onto an external object (continuous application pattern of sequential movement)
an individual pushes a desk across the room a traveler pulls his suitcase
Joint Action Patterns
simultaneous and opposite movement pattern in the upper extremity flexion in elbow with extension in shoulder
extension in elbow with flexion in shoulder
simultaneous movement pattern in the lower extremity simultaneous extension in the hip, knee, and ankle joints simultaneous flexion in the hip, knee, and ankle joints
at the distal end of the movement chain, a rectilinear path of motion is present. All forces produced by segmental motion are applied directly to the object and applied in the direction of motion. (Hamilton & Luttgens 2002, p.436) results： maximum forces and/or maximum accuracy but no tangential forces
trade-off in velocity and accuracy
Force Imparting
Mechanical Factors to be Considered
source of force by hand by foot by head by trunk by implement
force magnitude of force direction of force point of force application
stability of the body at the moment of giving motion the interaction between the body and the surface that supports it characteristics of the moving object
Magnitude of Force
The force to move an object must be greater enough to overcome the resultant of the following forces
internal resistance (moment of inertia)
friction between the object and the supporting surface resistance of the surrounding medium, such as air or water
For maximum force production, the maximum number of segments should be used through the largest safe range of motion. For maximum force accuracy, the minimum number of segments should be used through the smallest possible range of motion.
Direction of Force
The direction the object moves is determined by the direction of the resultant of all forces imparting on it For maximum force production, the segments involved should be aligned with the intended direction. If the object is subject to move along a preset path (e.g. a sliding door), any component of force not in this direction will be wasted and may act to increase resistance. If that force is greater enough, then some destructions will occur.
Point of Force Application
Force applied in line with the COM of an object will result in linear motion of that object, provided the object is freely movable; otherwise, it will result in rotary motion.
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Biomechanics of Pushing a Cart
Economy of Effort
use lower extremities ( friction)
force applied in line with the object’s COM and in desired direction
Load Lifting
NIOSH Manual Materials Handling LimitsMulti-Segment Biomechanical ModelBiomechanics of Symmetrical Load Lifting
1. Chaffin, D.B, & Andersson G.B.J., 1999. Occupational Biomechanics, 2nd ed.
Objectives: After studying this topic, the students will be able to
1. understand the NIOSH standards2. identify the the activities that involves lifting patterns3. analyze mechanical factors that affecting to lifting activities
NIOSH Manual Materials Handling Limits
About NIOSH
full name： National Institute for Occupational Safety and Health reported statistics of overexertion injuries
~ 1/4 of all reported occupational injuries is overexertion injuries < 1/3 of the patients with low back pain returned to their previous work ~ 2/3 of overexertion injury claims involves lifting loads and ~ 1/5 involves pushing or pulling loads
Manual Material Handling (MMH)
types of manual materials handling lifting： to move a load from a lower place to a higher place press down： to press a load in a downward direction pushing/ pulling： to move a material with continuous force application carrying： to move a material horizontally from one place to another holding： to hold a material without any motion
characteristics of major components affecting manual materials handling system (Herrin et al., 1974)
worker： physical measures, sensory processing capacities, motor capacities, psychomotor (interface for mental and motor processing), personality, training/ experience, health status, and leisure time activities material/ container characteristics
load： weight, pushing/pulling force requirements, and mass moment of inertia dimensions： size of unit workload, e.g. height, width, breadth, and form distribution of load： location of COM of the unit workload respect to the worker couplings： simple devices used to aid in grasping and manually manipulating the unit load, e.g. texture, handle size, shape, and location stability of load： consistency of COM location, especially for handling liquids or bulk material
task/ workplace： workplace geometry, time dimension of the task (frequency, duration, and pace), complexity of the load, and environmental factors work practices： operating practices under the control of the individual worker, work organization, and administration of operating practices
1981 NIOSH Lifting Guide for evaluation and control of symmetric, sagittal plane lifting includes both biomechanical spinal compression force limits and psychological limits in order to predict incidence and severity of overexertion injuries factors would lead to a hazardous lift
weight of object lift (L) location of object COM horizontally from the ankle (H) location of object's COM at the beginning of lift (V) vertical traveling distance of hands from origin to destination of object (D) frequency of lifting duration of the period which lifting takes place
Lifting Hazard Levels
Action Limit (AL) epidemiological data indicates that some workers would be at increased risk of injury on jobs exceeding the AL biomechanical studies indicates that L5/S1 disc compression forces can be tolerated by most people, but not all, at about 3400 N level, which would be created by conditions at AL physiological studies indicates that the average metabolic energy requirement would be 3.5 kcal/min for jobs performed at the AL Psychological studies indicates that > 75% of women and 99% of men could lift the load at the AL
Maximum Permissible Limit (MPL) = 3AL epidemiological data indicates that musculoskeletal injury rates and severity reates are significantly higher for most workers placed on jobs exceeding the MPL biomechanical studies indicates that L5/S1 disc compression forces cannot be tolerated over the 6400 N level in most people, which would be created by conditions at AL physiological studies indicates that the average metabolic energy requirement would exceed 5.0 kcal/min for most workers frequently lifting loads at the MPL Psychological studies indicates that only <1% of women and ~25% of men could lift the load above the MPL
categories of lifting hazard level above MPL： unacceptable between AL and MPL： unacceptable without administrative or engineering controls below AL： appropriate for most workers
Multi-Segment Biomechanical Model
Biomechanical Model
definition model is a representation of a system, based on some simplifications and assumptions, to make it easily understand (Chaffin & Andersson, 1999)
purposes of biomechanical modeling to understand easily about a complex system e.g. beam model of the plantar fascia to explore each component of a complex system and their interactions to simulate some conditions that are rare, dangerous (e.g. ultimate strength of biological tissues), hard to be measured (e.g. intradiscal pressure), or time- and/or cost-consuming tasks (e.g. zero-g conditions)
to predict some outcomes or potential hazards without real practice, e.g. prediction of maximum allowable load
Single Body Segment Static Model
The force to move an object must be greater enough to overcome the resultant of the following forces
internal resistance (moment of inertia) friction between the object and the supporting surface resistance of the surrounding medium, such as air or water
Example： An anthropometrically averaged-sized worker holds a even-distributed load in both hands, with forearm in the horizontal position, at waist height in front of his body.Question： What rotation moments and forces are acting on his elbow?Model used： static model since the task is only holdingAnswer：
Single Segment Dynamic Model
As a body segment is rotated about a joint center, inertial forces act at the COM of the segment
o tangential force： force tangent to the arc of motion
o contrifugal force： force along the radius of the arc of motion to pull away from the center
of rotation
o centripetal force： the reaction force of centrifugal force to hold the structures together o moment at the joint is equal to the sum of the moment from the weight of the segment
(the static gravity effect), the instantaneous acceleration effect due to the tangential force,
and the rotation acceleration effect due to the mass distribution
Biomechanics of Load Lifting
Joint Reaction Forces and Moments -- Static Model
load lifting can be simplified and regarded as a 5-link static model if the velocity is minimum.
For each joint, the resultant force and moment should be equal to zero. force component： weight of each limb, load, and reaction force of the adjacent joint
moment component： the moment produced by the weight of each segment, the moment produced by the load, and the moment produced by the reaction force of adjacent joint
what would happen about the reaction forces and moments if the posture is changed?
when the lifting is completed with both knees keeping straight when the lifting is completed with both elbows keeping straight
Reaction forces are only affected by the load. for each joint, reaction force Rloaded = Rload=0 + load
Reaction moments are largely affected by both the load and lifting postures, e.g.
arm reaching out trunk leaning forward knee bending for each joint, reaction moment Mloaded = Mload=0 + (load)(disanceload-to-
joint) exercise： please try to set up a 3D model for lifting
Dynamic Lifting Strength
highly correlated to the posture as the lifting task is performed major errors in earlier lifting research
using static strength to measure the capacity for a dynamic task basic assumption： to move a maximum load in a very slow speed can be regarded as a static task may be under-predicted by as much as 54% because the effect of acceleration is not considered
using vertical lift type of test instead of actual lift pathway in reality, when a load is lifted, the path of motion is a combination of vertical lift and toward body pulling
Multi-Segment Dynamic Model of Load Lifting
highly correlated to the acceleration of lifting first peak： at first 200-400 ms 2nd peak： for accuracy
larger moment are present at th hip joint as compared to the moments at upper extremity
Low Back Biomechanical Model
use the load moment at lumbosacral disc (L5/S1) as the basis for settig limits for lifting and carrying loads since 85-95% of disc herniation occurs at the L5/S1 and L4/L5 levels Morris, Lucas, and Bressler (1961) using static sagittal-plane model
extensor errector spinae： exerting force at 5 cm posterior to the center of L5/S1 IVD (intervertebral disc) abdominal pressure： in front of the L5/S1 IVD resulting on large disc compression force that was confirmed by Machemson and Elfstrom (1970)
Chaffin 1975 using add hip-sacral link and lumbar-thoracic link to refine the above model
length of the hip-sacral link is approximately 20% of that of the shoulder-hip link pelvic angle from the horizontal is approximately 45 deg. estimation of compression force estimation of force of erector spinae at the L5/S1 level
estimation of abdominal muscle forceFabd = PabdAdiagram
where average Adiagram = 465 cm2
estimation of moment at the L5/S1 level
Asymmetrical Lifting
isometric lifting strength decreases 20% for the task requiring left/ right trunk rotation and decreases 26% for the task requiring trunk backward rotation
Seated WorkSitting PostureAnthropometric Dimensions of Seated Workers
Seated Work Place and LayoutVideo Display Terminal Users
1. Chaffin, D.B, & Andersson G.B.J., 1999. Occupational Biomechanics, 3rd ed. New York: John Wiley & Sons. pp.355-392.
Objectives: After studying this topic, the students will be able to
1. understand the biomechanics of sitting posture 2. identify the anthropometric measurements for the seated workers 3. understand the guideline for seated work place design and layout 4. understand the common problems and solutions for VDT users
Sitting Posture
Definition
a body position in which the weight of the body is transferred to a supporting area, mainly by the
ischial tuberosities of the pelvis and their surrounding tissues (Schoberth, 1962)
body weight transferring through
the ischial tuberosity to the seat and then to the floor
the foot directly to the floor
the forearm to the armrest and then to the floor
the back and pelvis to backrest and then to the floor
comparisons of sitting posture with standing posture
Sitting posture provides stability required on tasks with high visual and motor control.
Sitting posture is less energy consuming than standing posture.
Sitting posture places less stresses on lower extremities than standing posture.
Sitting posture lowers hydrostatic pressure on lower extremity circulation.
The pelvis rotates backward and the lumbar spine flattens when standing to sitting.
Although seated work provides some advantages for the workers, it is obvious that the work
place should be assessed carefully so as not to introduce musculoskeletal problems.
Types of Sitting Posture
middle sitting COM of the upper body directly above ischial tuberosity floor support ~25% subtypes：
relaxed middle sitting with the lumbar spine straight or slight kyphosis
supported middle sitting： with the lumbar spine straight or slight lordosis
forward sitting (forward leaning sitting) COM of the upper body in front of ischial tuberosity floor support >25% subtypes：
forward rotation of the pelvis with the lumbar spine straight or slight kyphosis little rotation of the pelvis but with large kyphosis of the lumbar spine sitting on a chair with a forward sloping seat： with the lumbar spine slight lordosis
backward sitting (backward leaning sitting) COM of the upper body behind ischial tuberosity floor support <25% subtypes：
backward sitting without lumbar support： backward rotation of the pelvis and kyphosis of the lumbar spine backward sitting with a lumbar roll support： backward rotation of the pelvis and lordosis of the lumbar spine
Standard Sitting Posture
chin in
neck flexion 5-10 º
keep lumbar lordosis
hip： 85-100 º
tibia： perpendicular to the floor
foot flat on the floor
Sitting on a High Chair
should have a foot support without foot support, the weight of leg will form a moment at the hip joint to create anterior tilt of the pelvis, and then increase lumbar lordosis that might result in low back pain
Semi-Sitting Posture
good for ‘active’ worker e.g. grocery check-out
person
to encourage mobility
to allow rapid changes between sitting and
standing
to preserve lumbar lordosis
inclination of the seat starts just in front of the
ischial tuberosity to have full support of the trunk and
the thigh
Anthropometric Dimensions of Seated Workers
Vertical Anthropometric Measurements
All of the anthropometric measurements are based on the position when an individual sits with the popliteal fold 3-5 cm above the seat, with knee flexion of 90º, and with the foot flat on the floor.
sitting height： the vertical distance from the floor to the posterior aspect of the mid-point of the thigh shoulder height： the vertical distance from the sitting height to the superior aspect of the acromion elbow height： the vertical distance from the sitting height to the tip of the olecranon with the elbow being flexed to 90º and the upper arm being vertical thigh height： the vertical distance from the floor to the highest point of the thigh patellar height： the vertical distance from the floor to the superior aspect of the patella orbital height： the vertical distance from the floor to the orbit
Sagittal Anthropometric Measurements
abdominal depth： the sagittal distance from the posterior aspect of the buttocks to the anterior aspect of the abdomen external sitting depth： the sagittal distance from the posterior aspect of the buttocks to anterior aspect of the patella internal sitting depth： the sagittal distance from the posterior aspect of the buttocks to the posterior aspect of the popliteal fold
Transverse Anthropometric Measurements
shoulder width： the transverse distance between the tips of both acromion processes buttocks width： the maximum transverse distance at the buttocks external elbow width： the transverse distance between the tips of both olecrani when the arms are placed at shoulder abduction of 90º
Seated Work Place and Layout
Dimensions of the Seat
seat height = sitting height
3-5 cm below the knee fold when the low leg is vertical; otherwise it will cause
compression of the posterior aspect of the thighs
3-5 cm above popliteal level if the chair is tiltable or the seat slope is forward
(Bendix, 1987)
seat width
seat depth (length)： 10 cm less than the internal sitting depth in order to facilitate rising from the
chair
seat slope
backward slope of 5º
adjustable seat slope： better used in the office
forward slope of 20º
shape of the seat： Front part of seat should be contoured so that the edges of the seat should not
be detectable during seated work.
friction properties
softness： pressure should be avoided on the posterior aspect of lower thigh
adjustability
climatic comfort
Dimension of the Backrest
Either with backrest or with lumbar support will decrease the pressure under the ischial tuberosity.
Backrest should not restrict trunk or arm movements
backrest top height = backrest bottom height + backrest height
backrest bottom height
backrest center height
backrest height
backrest width
backrest horizontal radius： concave from side to side to conform the body contour
backrest vertical radius： convex from the top to the bottom to conform to the lumbar lordosis
backrest-seat angle
pivoting and recline possibility
softness
adjustability： adjustable in the vertical and/ or horizontal planes
climatic comfort
Dimension of the Armrest
Armrest can reduce the loading on the spine and facilitate the rising from the chair
armrest length
armrest width
armrest height = elbow height
shoulders shrug if the armrests are too high
trunk slumps or leans to one side if the armrests are too low
armrest-to-armrest width
distance from armrest front to seat front
Dimension of the Chair Base
number of feet base diameter use of caster or wheel
Dimension of the Workbench
Not necessarily the same for all types of work factors affecting workbench dimensions
size of the workpiece motions required by the task performer overall work layout
workbench top height 3-4cm above the elbow level (Bendix, 1987) Key board height = workbench top height if the computer is used
workbench bottom height： greater than the thigh height in order to ensure sufficient space for the thigh workbench surface
size large enough to accommodate work objects but not too far to reach friction high enough to prevent sliding of work
inclination of workbench surface The influence on lumbar posture from inclined table surfaces was actually greater than the influence of the seat slope. (Bendix, 1987) for reading： a slope of 45° for writing: a flat desk
field of vision VDT must be placed to prevent forward head or trunk flexion of the user focal distance: 20-40 cm
Video Display Terminal Users
Definition
maintaining the same posture > 2 hours for one specific computer work repeated using the same key(s) or mouse NOTE： In most developed countries, approximately ¾ of labors is sedentary workers (Reinecke et al. 1992)
Cumulative Traumatic Syndromes in VDT Users
Hultgren & Knave1st, 1974 1streporter about soft tissue problems among VDT users Muscle fatigue, soreness, stiffness, cramps, numbness, and/or pain were frequently found in VDT users associated with the frequency of key strikes
More than half of computer users have reported local pain. (1991 US statistics)
location of pain neck and shoulder pain: 67% low back pain: 40% wrist pain: 29%
resulting in increase in medical expenditure Increase in work compensation decrease in productivity
possible causes physiological factors
Endurance time decreases significantly when the posture required more than 30% of the strength of back muscles (Jorgensen, 1970)
intradiscal pressure changed during various sitting postures
If the trunk leans forward, the moment loaded on the lumbar disc increased as the sine of . For example, if the trunk leans forward at an angle of 30º, then the moment is Wd(sine30º), i.e., 0.5 Wd.
flextion of the neck depends on the visual demand and the height of work surface.
environmental or task factors malposture or maintaining the same posture for a long period of time improper workplace repetitive motions
psychological factor work stress time stress
social factors prevention of cumulative traumatic syndromes
to decrease the sustained duration muscle cannot sustain contractions over ~15-20% of their maximum strength without fatigue
to decrease the frequency to increase muscle strength in the posture where the task requires
Biomechanical Considerations in VDT Workplace Design
chair chair with armrest
seat slope
chair base
better to have 5-
foot support
radius = 30-35cm
use of casters or
wheels
computer desk to provide sufficient space for the legs i.e. work bench bottom height thigh height If the desk is too low, an individual tends to lean forward and lower and protract the shoulder joints. If the desk is too high, an individual tends to elevate and shrug the shoulder joint which is susceptible to muscle fatigue.
keyboard keyboard height (from middle row to floor): 70-85 cm keyboard distance (from middle row to table edge): 10-26 cm in the position to have minimum wrist extension, flexion, and ulnar deviation
screen screen height (from center of screen to floor): 90-115 cm screen inclination: 88-105° screen distance (screen to table edge): 50-75 cm
body posture visual distance (from eyes to center of screen) viewing angle (from eyes to center of screen)： < 20º trunk-seat angle： most people uses the backward leaning posture that causes in a decrease in lumbar lordosis and is susceptable to herniation of the intervertebral disc. elbow angle： ~ 90º shoulder flexion angle： as small as possible
Application of Biostatistics Hazard LevelsNormal DistributionInferneces from Sampling Distribution
1. Chaffin, D.B, & Andersson G.B.J., 1999. Occupational Biomechanics, 3rd ed. New York: John Wiley & Sons. pp.355-392.
Objectives: After studying this topic, the students will be able to
1. understand the biomechanics of sitting posture 2. identify the anthropometric measurements for the seated workers 3. understand the guideline for seated work place design and layout 4. understand the common problems and solutions for VDT users
Hazard Levels
Action Limit (AL)
epidemiological data indicates that some workers would be at increased risk of injury on jobs exceeding the AL biomechanical studies indicates that L5/S1 disc compression forces can be tolerated by most people, but not all, at about 3400 N level, which would be created by conditions at AL physiological studies indicates that the average metabolic energy requirement would be 3.5 kcal/min for jobs performed at the AL Psychological studies indicates that > 75% of women and 99% of men could lift the load at the AL
Maximum Permissible Limit (MPL) = 3AL epidemiological data indicates that musculoskeletal injury rates and severity reates are significantly higher for most workers placed on jobs exceeding the MPL biomechanical studies indicates that L5/S1 disc compression forces cannot be tolerated over the 6400 N level in most people, which would be created by conditions at AL physiological studies indicates that the average metabolic energy requirement would exceed 5.0 kcal/min for most workers frequently lifting loads at the MPL Psychological studies indicates that only <1% of women and ~25% of men could lift the load above the MPL
categories of lifting hazard level above MPL： unacceptable between AL and MPL： unacceptable without administrative or engineering controls below AL： appropriate for most workers
Normal Distribution
Definition
normal distribution (Gaussian distribution)
a distribution followed the curve of
a symmetrical bell-shaped curve with the mean value of and the standard deviation of standardized normal distribution： given = 0 and =1
68.3% of population fall within 1 standard deviation from the mean 95.0% of population fall within 1.96 standard deviation from the mean 95.4% of population fall within 2 standard deviations from the mean 99.0% of population fall within 2.58 standard deviation from the mean 99.7% of population fall within 3 standard deviations from the mean
Central Tendency
mean (： the average value of all observations in a population
for example： a population of 18 observations as follows
observation 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
value 6.8 5.3 6.1 4.3 5.0 7.1 5.5 3.8 4.6 6.0 7.2 6.4 6.0 5.5 5.8 8.8 4.5 5.9
the mean = (6.8 + 5.3 + 6.1 + ... + 5.9)/ 18 = 104.6 / 18 = 5.81 median (Md)： the middle observationin the above example, the values in rank-order are
observation 8 4 17 9 5 2 7 14 15 18 10 13 3 12 1 6 11 16
value 3.8 4.3 4.5 4.6 5.0 5.3 5.5 5.5 5.8 5.9 6.0 6.0 6.1 6.4 6.8 7.1 7.2 8.8
the median = 0.5 (5.8 + 5.9) = 5.85 mode： the value that occurs most frequentlyin the above example, mode are 5.5 and 6.0.
Variability
range = maximum - minimum variance (²)：
standard deviation ()：
Percentiles
definition： a number that indicates the percentage of a distribution that is equal to or below that number method： to rank all observations in an ascending order, divide them into 100 subgroups, and then assign one subgroup as a percentile mean =50th percentile for a normal distribution In occupational Biomechanics, we usually report
1st percentile = - 2.326 5th percentile = - 1.645 25th percentile = - 0.67 50th percentile = 75th percentile = + 0.67 95th percentile = + 1.645 99th percentile = + 2.326
Influence from Sampling Distribution
Central Limit Theorem
sampling distribution： select many samples from the target population, compute the mean in each sample, and then the distribution of all these means is the sampling distribution the mean of the sampling distribution of means is equal to the population mean the standard deviation of the sampling distribution of means is called as standard error of the mean (SEM)
If the population distribution is normal, then the sampling distribution is normal, too.