Stumbling reactions in man: release of a ballistic movement pattern

Four different modes of perturbation were applied during gait, either alone or in combination, in order to determine the extent to which compensatory reactions in the leg are released as fixed patterns or, alternatively, are generated by feedback mechanisms. The first agonist burst of the usually triphasic pattern evoked by a perturbation was unchanged in amplitude and timing when a second disturbance of an opposite sense was induced a short time later. Following this burst, the remainder of the response evoked by the second disturbance appeared time locked to the second stimulus and was unchanged in form. It is concluded, that the first agonist burst induced by a perturbation is stimulus specific and immutable after release, while the appearance of the following bursts of the triphasic pattern is dependent on the actual conditions. This suggests that a similar mechanism as assumed for ballistic hand and finger movements is operative, with the difference that the compensatory reactions following gait perturbations are induced by peripheral signals.