Modulation of stepping strategy in response to platform translation to the left during the gait cycle. Group mean +/- standard deviation of change in step width (a) and margin of stability (b) at the first three heelstrikes after perturbation onset compared to unperturbed walking as a function of the onset timing of the platform translation. The stars indicate results significantly different from zero (p < 0.05). An outward stepping strategy was used in response to the platform translation during first double support (c) (perturbation onset 7.5% gait cycle). An inward stepping strategy was used in response to the platform translation with onset during second double support (d) (perturbation onset 52.5% gait cycle).