Submitted on October 29, 2002
Accepted on April 2, 2003
Thresholds for inducing protective stepping responses to external perturbations of human standing
Marie-Laure Mille1, Mark W. Rogers1*, Kathy M. Martinez1, Lois D. Hedman1, Marjorie E. Johnson1, Stephen R. Lord2, and Richard C. Fitzpatrick2
1 Department of Physical Therapy and Human Movement Sciences, The Feinberg School of Medicine, Northwestern University, Chicago, IL, USA
2 Prince of Wales Medical Research Institute and University of New South Wales, Sydney, NSW, Australia
* To whom correspondence should be addressed. E-mail: m-rogers{at}northwestern.edu.
Standing subjects were unexpectedly pulled forward to identify a threshold boundary that evokes stepping in terms of the size of the pull relative to the base of support (BoS). Performances in a range of sensorimotor tests were correlated with the threshold boundary parameters. Younger and older subjects were studied to identify age-related changes in stepping and the threshold boundaries. The threshold boundary had a forward limit (TL) which, when crossed, always made subjects step no matter how slowly they were pulled. As velocity increased, the threshold position that produced a step shifted nearer to the ankles. Eventually a pull velocity was reached above which velocity had no further effect and a position threshold (TH) was identified behind which subjects never stepped. Thus, the position threshold boundary for stepping is a posterior-going sigmoidal function of perturbation velocity. Older subjects stepped more than the young (69% vs. 40% of trials). For the older subjects, TL (91% vs. 107%BoS), and TH (59% vs. 72%BoS) were closer to the ankles, and the transition between TL and TH occurred at lower velocities (96% vs. 121%BoS.s-1). Across the entire study population many sensorimotor factors were associated with TL and TH. However, these associations were not present when age was removed as a factor. Thus, although the older subjects use protective stepping more often, this cannot be attributed directly to the sensorimotor factors tested here. It can be explained by stepping as a triggered response to the perturbation event rather than later sensory input about body movement.
