The influence of chronic low back pain on joint kinematics in multi-joint reaching movements with various loads. James S. Thomas, Daohang Sha, Christopher R. France, Kevin A. Swank, and Candace E. Kochman. School of Physical Therapy, Ohio University, Athens, OH

IntroductionPerforming multi-joint reaching tasks that necessitate some forward bending of the trunk become more complex following a low back injury due to the stresses placed on the spine. A reasonable assumption is that individuals adopt alternative movement strategies in the acute phase of an episode of low back pain. This is a normal response referred to as "protective guarding" and allows for healing of injured tissues. With chronic back pain, however, it is hypothesized that individuals become "stuck" in a dysfunctional movement pattern, which no longer protects healing tissues, but actually prevents full recovery. Consistent with this notion, preliminary data from our lab reveal that individuals with chronic pain avoid certain positions of the spine. However, it is not known whether individuals with low back pain adopt movement strategies simply to avoid certain positions of the spine or to avoid specific loading of the spine. In this experiment, participants reached for targets that required progressively larger forward displacement of the trunk. Additionally, spinal loading was manipulated by attaching small weights to the reaching limb. The purpose of this study was to determine whether participants with chronic back pain seek to avoid certain positions or loads during full body reaching tasks performed with various loads attached to the lumbar spine.

MethodsForty subjects (20 chronic back pain and 20 matched healthy controls-Table 1) reached with the right hand for three targets located in a mid-sagittal plane starting from a standing posture. First the horizontal and vertical location of the low target was calculated such that the participant could reach the target, in theory, by flexing the hips 60° with the shoulder flexed 90° and the elbow extended (See Figure 1). The horizontal distance calculated for the low target was used for the middle and high targets as well. Next, the vertical location of the middle and high targets were determined in a similar fashion by calculating the vertical location of the fingertip when the hips were flexed 30°(middle target) or extended 30°(high target). The target locations were chosen to create a task that progressively challenges the subject with larger excursions of the trunk. While standing on two force plates, subjects performed reaching movements with their right hand at a comfortable pace and were given no instructions on limb segment geometry. For the load conditions they held either a 2lb or 4lb dumbbell. For the no-load condition they held a wooden dowel. The participants completed 2 trials at each load condition and the presentation of the target height and load conditions were randomized. Kinematic data were sampled for 5 seconds at 100Hz using a seven camera Vicon MX-13 system. This system has a resolution of .10mm. The plug-in-gait marker placement for the infrared reflective markers was used with slight modification. Four markers were used on the posterior pelvis and virtual ASIS markers were determined from the sagittal plane orientation of the posterior pelvis. EMG data were collected bilaterally from the anterior deltoid, rectus abdominius, internal and external obliques, multifidus, and erector spinae with a Delsys Banoli system.

Data AnalysisFor each group, the changes in 3-D joint angles (from initial posture to target contact) of the right ankle, knee, hip, spine, shoulder, and elbow were analyzed using mixed-model ANOVAs in which target height and load were the within subject factors and group (low back pain or healthy) was the between subject factor. The latencies of trunk EMG relative to the onset of the deltoid were also analyzed.

ResultsUnexpectedly, there was no effect of group on lumbar flexion for these tasks. However, there was a significant effect of group on lumbar rotation (F=4.51, p<.05). Figure 2 illustrates that for each target height, participants with chronic low back pain had larger excursions of lumbar rotation compared to healthy controls, especially at the high target. Because the target was located in the midline, increased lumbar rotation allows the subject to reach the target with decreased flexion. As the load in the reaching hand increased, there was a significant increase in hip flexion (F=7.25, p<.05), lumbar rotation (F=13.6, p<.05), shoulder flexion (F=11.0, p<.05), and elbow flexion (F=4.8, p<.05) (see Figure 3). Subjects chose to use increased excursion at the shoulder, elbow, and hip in order to reach the target without increasing lumbar flexion. On average, across all target heights and load conditions, participants with chronic low back pain reached for the targets in 1423 ms (SD=22) while healthy participants without a history of back pain reached for the targets in 1202 ms (SD=22), F = 9.8, p<.01. (Figure 4). Figure 6 illustrates EMG recordings from the trunk and UE bilaterally during a reaching trial to the middle target. Patients with low back pain demonstrated a later onset of multifidus activation, but there was no consistent activation of the abdominal muscles. Analysis of latencies of the trunk muscles revealed significant group differences for the right multifidus, F=8.4,P<.001. There was no significant effect for the right erector spinae F=3.1, p=.093, the left erector spinae, F=3.2, p=.079 or the left multifidus,F=2.2, p=.146. (Figure 5.)

ConclusionsThe increase in joint motions may have been a result of an overshoot error in planning the movement task. However, we expected that lumbar flexion and rotation would be reduced as the load in the reaching hand increased in order to reduce peak loads on the lumbar spine. Nonetheless, our data indicate that individuals with chronic back pain actually use greater lumbar rotation to perform these multi-joint reaching tasks. Rotation of the lumbar region increases stress on lumbar discs and is a known risk factor for low back pain. The increase in lumbar rotation may result from poor trunk control or greater overshoot error due to increased latencis of back extensor muscle activation and could be a contributing factor in persistence of low back pain.

This research was supported by The National Institutes of Health Grant R01-HD045512 to J.S. Thomas

Figure 3. Effect of load on joint excursions for right handed reaches.

Figure 2. Effect of group on lumbar rotation at eache target height. Subjects with chronic low back pain had larger excursions of lumbar rotation compared to healthy controls.

A. Healthy Subject B. Chronic Low Back Pain Subject

R Multifidus

R Rectus Abdominus

R internal Oblique

R Erector Spinae

R External Oblique

R Deltoid

L Deltoid

L External oblique

L Rectus Abdominus

L Internal Oblique

L Erector Spinae

L Multifidus

R Deltoid

R External Oblique

R Rectus Abdominus

R Internal Oblique

R Erector Spinae

R Multifidus

L Deltoid

L External Oblique

L Rectus Abdominus

L Internal Oblique

L Erector Spinae

L Multifidus

Figure 6. EMG recordings from right and left trunk and UE musculature during a reaching trial to the middle target at a fast pace for a healthy (A) and low back pain subject (B)

Figure 1. Targets were located such that the subject could reach each target, with the shoulder flexed to 90 and elbow extended, by flexing the hips 30, or 60 degrees, or extending the hips 30 degrees.

Figure 5. Effect of group on onset latencies of the right eretor spinae (ERS_R), right multifidus (MULT_R), left eretor spinae (ERS_L), and L multifidus (MULT_L)

Latencies (ms)

0 100 200 300 400

ERS_R

MULT_R

ERS_L

MULT_L

Healthy ControlChronic LBP

Table 1. Characteristics of participants with and without chronic LBP.

Figure 4. Effect of group on movement time to reach the targets.

Healthy Controls Chronic LBP

Mean SD Mean SD

Age 24.2 5.7 23.9 7.5

Height (cm) 168.2 6.5 168.4 8.4

Weight (kg) 64.9 9.6 64.1 8.5

MPQ-PRI .55 .99 11.2 8.2

RMDQ .30 .73 7.0 4.9