THE INFLUENCE OF CHRONIC BACK PAIN ON KINEMATIC RULES UNDERLYING MULTI-JOINT REACHING MOVEMENTS. J.S. Thomas, PhD, PT & Christopher R France, PhD* School of PhysicalTherapy, Department of Psychology*, Ohio University, Athens OH
Introduction
Reaching tasks such as ringing a doorbell, wiping a child’s face orretrieving the morning paper require the control and coordination of the trunk andlimb segments in order to perform these tasks smoothly and effortlessly. Given thenumber of segments involved in these whole-body reaching tasks there are aninfinite number of ways in which these tasks can be completed. Bernstein (1967)hypothesized that the CNS resolves the problem of kinematic redundancy byreducing the independent degrees of freedom required to complete the task. Byimposing some rules by which coordinated movements are performed, complexmulti-joint tasks are simplified.
Using principal components analyses of time series segment motionswe have shown that there are similar characteristics regarding the apportionmentof motion to segments in a whole body reaching task that can be generalizedacross healthy individuals. The purpose of this research was to determinewhether chronic low back pain leads to changes in the how motion is apportionedto various segments in a whole body reaching task.
Methods
The time-series changes in orientation of the forearm, humerus, trunk,pelvis, thigh, and shank were measured in 30 subjects (13 individuals with chroniclow back pain and 17 healthy individuals) performing whole-body reaching tasks.In this paradigm the targets were located around a clock face such that thesubject could, in theory, reach them by flexing the hips 20, 40, 60, and 80 withthe shoulder flexed 90 and the elbow extended (Figure 1). The target locationswere chosen to create a task that progressively challenges the subject with largerexcursions of the trunk. The segment orientation angles were measured in acounterclockwise direction as seen from the subjects right side, starting withhorizontal equal to zero degrees. Subjects reached for the targets at two speeds(self-selected and fast-paced) and were given no instructions on the limb segmentgeometry to use while performing these reaching tasks.
The time-series segment kinematics were analyzed by principalcomponent (PC) analyses to determine if there was a commonality amongst theshapes of the waveforms across segments, across trials, and across subjects.Principal component analyses were also performed on the scaling of the kinematicwaveforms across the experimental manipulations, to probe for any lawfulrelationships amongst their relative magnitudes.
Results
Comparison of waveform shapes: PC analyses of the time seriessegment motions revealed that the shape of the segment motion waveformswithin a given trial were nearly identical for both healthy individuals and those withchronic LBP. The EigenCurve illustrated in figure 2B accounts for 99.86% of thetotal angular variance of the time series waveforms in figure 2A. These findingsindicate that individuals with chronic LBP (at least confronters) use a similarshaped segment waveforms to perform these reaching tasks.
Comparisons of scaling of waveforms: PC analyses of the scalingcoefficients from each movement trial of every subject revealed that three principalcomponents could account for greater than 95% of the total angular variance forthese 6 DOF reaching tasks. This finding held for both healthy individuals andthose with chronic low back pain. However, examination of the factor loadings foreach group revealed important differences in these two groups. The factorloadings from the first movement component for healthy controls are as follows:shank=-.165, thigh=-.367, pelvis=.700, trunk=.893 humerus=.924, andforearm=.882. In contrast, the factor loadings for individuals with chronic backpain are: shank=.023, thigh= -.544, pelvis=.915, trunk=.829, humerus= .946, andforearm= .732.
Conclusions
Kinematic Rules: Based on the PC analyses of the segment kinematics of individuals withand without chronic low back pain performing reaching tasks that necessitate some forwardbending of the trunk we have identified rules that are consistent across groups and haveidentified how movement strategies may be adapted with chronic back pain.
Kinematic Rule 1a. Within any given movement trial of a multi-degree of freedom reachingtask, the shapes of the sagittal plane segment orientation waveforms are nearly identical.
Kinematic Rule 1b. There is one common waveform that describes the change in segmentorientations in the sagittal plane amongst all the segments used in a multi-degree of freedomreaching task that is valid for all subjects across target locations, movement speeds.
Kinematic Rule 2a. A relationship exists amongst the scaling coefficients of the kinematicwaveforms for these 6-DOF reaching tasks (in individuals with and without chronic back pain)such that the scaling of these waveforms can be well described by only 3 principalcomponents.
Kinematic Rule 2b. While commonalities exists in motor coordination strategies acrosssubject populations, chronic back pain leads to changes in how motion is apportioned tovarious limb and trunk segments in these multi-joint reaching tasks.
Figure 1. Target locations were determined from the subject's on armlength, hip-to-shoulder length and hip height Subjects could reach the lowtarget, in theory , by flexing their hips 60° (with the elbow extended and theshoulder flexed 90°) without any motion of the ankle, knee, or spine.
Figure 2. A) Time series segment motions for an individual with chronicLBP and B) The EigenCurve and six scaling coefficients derived from timeseries data with PC analyses. Greater than 98% of the total angularvariance from the time-series segment angle data can be accounted for bythe first principal Eigencurve.
Hip-to-shoulderlength
Hip Height
ArmLength
60° Target
Figure 3. The 24 time normalized first principal EigenCurves from this subjectsmovement trials are plotted. These waveforms and those from each movement trial ofevery subject were analyzed by PC analysis to determine the commonality of the shapeof the waveforms. Consistent with our previous findings from healthy subjects, we wereable to identify a common waveform in individuals with chronic back back pain.
Figure 4. The change in segment orientation from initial posture totarget contact ( i.e. ) for the thigh, pelvis, and trunk are plotted foreach trial of every subject. The data from the chronic low back paingroup and from healthy normal subjects have each been been fit by aplane. Individuals with chronic back pain used increased pelvic flexionand reduced trunk flexion to perform these reaching tasks. Theycompensated for these changes at the humerus and forearmsegments. These differences are consistent with the differences seenin the factor loadings of the 1st principal component.
Figure 5. Stick figures illustrating posture at contact with the 80 target in individualparticipants with and without low back pain. The individual with chronic pain tends toapportion motion to the limb and trunk segments such that the amount of lumbar flexionis reduced.
A
B
This work was supported, in part, by a grant from the National Institutes of HealthR01 HD045512-01, and an Ohio University Baker Award