Practice-related changes in
lumbar loading during rapid voluntary pulls made while standing
Alison
H. Chang, PT, MS1 Wynne A.
Lee, Ph.D.1,2 James L.
Patton, Ph.D.3,4
Department of Physical
Therapy and Human Movement Sciences1
The Institute for Neuroscience2
Sensory Motor Performance
Program, Rehabilitation Institute of Chicago3
Department of Physical
Medicine and Rehabilitation4
Northwestern University
Medical School, Chicago, USA
in press, Clinical
Biomechanics
Please Address Correspondence
to:
Alison H. Chang, PT, MS,
Department of Physical Therapy and Human Movement Sciences, Northwestern
University Medical School
645 N. Michigan Avenue (Suite
1100), Chicago, IL 60611-2814
e-mail: hsini@nwu.edu
This paper is available at:
http://sulu.smpp.nwu.edu/SMPP_pub/ChangLeePattonClinBiomech2000_preprint.pdf
Abstract
Objective. To determine if five days of practice on a novel
dynamic, multi-joint pulling task resulted in lower magnitudes of lumbar
loading or a more consistent relationship between pulling force and lumbar
loading.
Design. A repeated measures design compared how practice
influenced the magnitude of lumbar torque and the correlations between lumbar
torque and pulling force.
Background. Previous studies suggest that practice can decrease
the magnitude of lumbar loading on simple manual material handling tasks, but
it is unknown whether practice reduces lumbar loading for more complex tasks.
Neither is it known whether the consistency of lumbar loading increases with
practice.
Methods. Ten healthy adults practiced impulse-like horizontal
pulls to targets equaling 20, 40, and 80% of their estimated maximal dynamic
pulling force over 5 days. Movements were unrestrained, other than keeping the
feet flat on the ground. We used a four-segment, sagittal plane inverse
dynamics model to compute lumbar, hip, knee, and ankle torques on Day 1 and Day
5 from ground reaction forces and moments, pulling forces, and kinematics.
Results. An analysis
of variance showed significant practice-related changes in lumbar torque at the
time of peak pulling force (lumbar torquepeakPF). The lumbar torquepeakPF
decreased for the 20% pulls, did not change for the 40% pulls, and increased
for the 80% pulls. Two subjects showed a significant decrease in lumbar torquepeakPF
for all three force levels. Coefficients of determination between pulling force
and lumbar torque (r2PF,LT: a measure of the consistency
of the relationship between these two variables) were significantly higher on
Day 5 than Day 1.
Conclusions. Practice on a novel pulling task changed the
magnitude of lumbar torques and increased their correlation with pulling force,
suggesting that subjects learned strategies that improve motor control of
lumbar torques.
Relevance
The study showed
that the magnitude and consistency of lumbar loading changed spontaneously as
subjects practiced a novel multijoint pulling task. Such changes may decrease
the risk of low back injury.