title: Thresholds for inducing protective stepping responses to external perturbations of human standing.
abstract: 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.
authors.affiliation: Department of Physical Therapy and Human Movement Sciences, The Feinberg School of Medicine, Northwestern University, Chicago, IL 60611, USA.
authors.forename: M-L
authors.initials: ML
authors.lastname: Mille
authors.forename: M W
authors.initials: MW
authors.lastname: Rogers
authors.forename: K
authors.initials: K
authors.lastname: Martinez
authors.forename: L D
authors.initials: LD
authors.lastname: Hedman
authors.forename: M E
authors.initials: ME
authors.lastname: Johnson
authors.forename: S R
authors.initials: SR
authors.lastname: Lord
authors.forename: R C
authors.initials: RC
authors.lastname: Fitzpatrick
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000375:Aging / Q000502:physiology; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009068:Movement / Q000502:physiology; D004856:Postural Balance / Q000502:physiology*; D011187:Posture / Q000502:physiology*; D012684:Sensory Thresholds / Q000502:physiology*