Modulation of bilateral GM activity in response to platform translation during the gait cycle. Group mean +/- standard deviation of the GM response of the inward (a) and outward leg (b) to the platform translation as a function of onset timing of the perturbation during the gait cycle. GM response significantly different from zero (i.e. unperturbed walking) is highlighted with a square (p < 0.05). The relative change in GM activity of the inward versus outward leg in response to the perturbation of all perturbation trials for the four different timings of perturbation (c).