Recovery from perturbations during paced walking.

The aim of the current study was to develop a safe, standardized, stability test and to explore a set of metrics to characterize the recovery of gait stability in healthy individuals following a single mechanical perturbation during steady locomotion. Balance perturbations were mechanically applied to the right foot of 12 healthy subjects during paced walking by translating a platform embedded in a 12 m walkway diagonally (+45/-135 degrees ) relative to the direction of travel approximately 200 ms after heel strike. We examined the medio-lateral (ML) displacement of the sternum before, during and after the platform translation. Measurements of ML position of the right and left shanks in relation to the position of the sternum were used as step-by-step estimates of the moment arm controlling ML motion of the body. We hypothesized that when gait is perturbed in the single stance phase of the step cycle via a translation of the support surface, a series of steps after the perturbation input will be altered reflecting an effort by the CNS to maintain the center of mass (COM) within the base of support and to stabilize the upper body for continued gait. Specifically, if the foot is perturbed laterally during mid-stance a widening of the upcoming step will occur and if the foot is perturbed medially a narrowing of the upcoming step will occur. This behavior was frequent for most subjects. Recovery of non-perturbation behavior was achieved on the third step after the platform translation. An additional strategy was seen for some subjects during lateral perturbation inputs. Instead of widening the upcoming step, these subjects acquired the support to stabilize the body by putting their left foot down very quickly with minimal change in stance width. The recovery profiles of the sternum, though directionally asymmetric, were similar in shape among subjects and roughly proportional to the magnitude of the platform translation. Five to six steps were required for complete recovery in the subjects tested in this study.

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