|
James L. Patton
* Sensory Motor
Performance Program Rehabilitation
Institute of Chicago &
Northwestern University Medical School 345 East
Superior St, Suite 1406 j-patton@nwu.edu |
Wynne A. Lee Physical
Therapy and Human Movement Sciences Institute for
Neuroscience Northwestern
University Medical School w-lee@nwu.edu |
Yi-Chung Pai Physical
Therapy and Human Movement Sciences Northwestern
University Medical School c-pai@nwu.edu |
Experimental
Brain Research, Volume 135 Issue 1 (2000) pp 117-126
SPRINGER DOI 10.1007/s002210000500
Accepted, May, 2000; Original submission January 7, 1999
This study tested the hypothesis that subjects improve
their relative stability as they learn a dynamic pulling task. Healthy adult
subjects practiced making brief horizontal pulls (<300 ms) on a handle to a
range of target forces ranging from 20 to 80 percent of their estimated maximum
for 5 days. They were instructed to always keep their feet flat, and begin and
end their motion in an upright posture. In order to do this, subjects had to
develop the appropriate body momentum prior to the pull, and then recover their
balance following the pull. We analyzed relative stability during balance
recovery, using two measures: spatial safety margin (minimum distance of the
center of pressure, COP, to the edges of the feet) and temporal safety margin
(minimum extrapolated time for the COP to reach the edges of the feet). We
hypothesized that: 1) Spatial and temporal safety margins would be
uncorrelated; 2) Safety margin means would increase with practice; and 3)
Safety margin standard deviations would decrease with practice. Two experiments
were conducted: one where subjects practiced three force targets and positioned
their initial COP in a small window, and one where subjects practiced two force
targets with no initial COP constraint. Results showed that spatial and
temporal safety margins were correlated but shared less than 6% variance,
indicating that they reflected different aspects of control. Safety margin
averages increased with practice and standard deviations decreased with
practice, indicating that the stability of balance control in the execution of
this task became more robust. We suggest that the nervous system could use
safety margins in both feedback and feedforward control of balance.
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