Fig. 5.

Stepping responses. A: representative PD and control participants for 29-N backward pull. Top, perturbation force (solid black lines); bottom, AP position of CoM (dashed black), and heels (R, right, solid dark gray lines; L, left, solid light gray lines). Although the pulls were identical and the body displacement (CoM trajectory) and foot-lift time were similar, the PD patient took multiple short steps, whereas the control participant took a single longer step. B: group mean (±SE) length of first step for PD patients (squares, black lines) and controls (circles, gray lines) for each session (S1, OFF; S2, ON) in the backward (left) and forward (right) directions. C: same conventions as B for group mean (±SE) number of steps taken per trial.