Simulated response to perturbation during first double support (7.5% gait cycle) for subjects using the GM activity of the inward (stance) leg, outward (swing) leg and a combination of both. Simulated change in stride width and margin of stability for a subjects using mainly the stance leg GM (subject 1), the swing leg GM (subjects 2) or a combination of both (subject 3). The average measured muscle activity of the stance (a) and swing (b) leg GM during unperturbed walking is shown in black (all unperturbed gait cycles in gray) and after perturbation in blue for the stance leg and green for the swing leg. The vertical black dotted line represents perturbation onset. The simulated margin of stability at first heelstrike after perturbation is shown for unperturbed walking (black), passive response (red), active response stance leg GM (blue), active response swing leg GM (green) and combined response of stance and swing leg GM (cyan).