The 'deceleration response' to transient perturbation of upright stance.

Characterizing the postural perturbation due to a transient support-surface displacement in terms of acceleration and deceleration events leads to a new framework for understanding the control of the stabilizing response. This study documents the existence of a 'deceleration response', which has not been reported previously, and explores the control of this response by varying the predictability of the timing and pattern of support-surface deceleration. Control features related to the capacity of the subject to predict deceleration were exposed, in part, by including novel 'tri-phasic' perturbations, in which the normal deceleration phase was replaced by a re-acceleration. In all trials, subjects showed a marked response to the onset of deceleration. The capacity of predict the timing and pattern of deceleration had a significant influence on the 'deceleration response', as well as the 'tonic' muscle activation that preceded the onset of deceleration.