Thresholds for Inducing Protective Stepping Responses to External Perturbations of Human Standing

• Published in: Journal of neurophysiology 2003-08, Vol.90 (2), p.666-674
• Main Authors: Mille, M.-L, Rogers, M. W, Martinez, K, Hedman, L. D, Johnson, M. E, Lord, S. R, Fitzpatrick, R. C
• Format: Article
• Language: English
• Subjects: Adult; Aged; Aged, 80 and over; Aging - physiology; Female; Humans; Male; Middle Aged; Movement - physiology; Postural Balance - physiology; Posture - physiology; Sensory Thresholds - physiology
• ISSN: 0022-3077; 1522-1598
• DOI: 10.1152/jn.00974.2002

1 Department of Physical Therapy and Human Movement Sciences, The Feinberg School of Medicine, Northwestern University, Chicago, IL; 2 Prince of Wales Medical Research Institute and University of New South Wales, Sydney, NSW 2031, Australia Submitted 29 October 2002; accepted in final form 2 April 2003 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 (T L ) that, 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 (T H ) 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, T L (91% vs. 107% BoS) and T H (59% vs. 72% BoS) were closer to the ankles, and the transition between T L and T H occurred at lower velocities (96% vs. 121% BoS.s – 1 ). Across the entire study population many sensorimotor factors were associated with T L and T H . 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. Address for reprint requests: M. W. Rogers, Department of Physical Therapy and Human Movement Sciences, The Feinberg School of Medicine, Northwestern University, 645 North Michigan Ave., Suite 1100, Chicago, IL 60611 (E-mail: m-rogers{at}northwestern.edu ).