Fig. 3. Spatial and temporal effects of perturbation. The impact of perturbation on the perturbed leg itself was investigated over a walking sequence of three subsequent steps (-1, 0, +1), step 0 being the perturbed step (white). Bars indicate mean values of step parameters ± s.d. (Inner, N=51; Straight, N=64; Outer, N=58). Perturbation affected swing distance (A), swing duration (B), and duty cycle, i.e. the fraction of the step cycle spent in stance (C). Data of the perturbed step 0 and the subsequent unperturbed step +1 (grey) were tested with reference to step -1 (black), prior to perturbation (Wilcoxon test). Perturbation caused a reduction of swing distance and duty cycle and a prolongation of the swing duration, the only exception being swing distance of inner legs, which remained unchanged (A, left). Step parameters in unperturbed walking depend on the behavioural context and differ between straight and curve walking (Mann-Whitney U-test). Significance levels: ^***P<0.001; ^**P<0.01; ^*P<0.05.