Fig. 1.

Experimental setup. A: subjects stood up from a standardized position with the chair at 106% of fibular tuberosity (Fib Tub) height, feet width at 40% of greater trochanter (GT) stance height and the GT positioned ∼5 cm from the front chair edge. Three anterior-posterior foot positions were used and given by 20°, 10° and 0° of shank orientation relative to vertical. Subjects aimed to perform the movement at a uniform speed that was directed to make the total movement last 1, 2, 4 or 8 s. Immediately following each trial, a biofeedback display indicated the actual (black arrow) vs. instructed (gray arrow) movement durations. To prevent interfering with the task, feedback was not given during the trial. B: quantification of movement time (T_mov) and weight shift. Sit-to-stand (STS) T_mov was calculated from the onset of trunk (Trk) lean to when the acromion (Acrom) reached a vertical threshold. Weight shift was quantified by the maximal derivative ${\mathrm{(F}}_z^{\prime }$ max) and the duration of the final 50% of weight shift (T_ws). F_z, vertical force; t_start, start time; t_SO, seat-off time; t_end, end time.