Medio-lateral foot placement controlled by stance leg GM (GM) activity in perturbations with onset during first double support. Stance leg GM activity was increased after perturbation (blue) compared to unperturbed walking (gray) (a). The isolated effect of stance leg GM response on walking kinematics was evaluated by comparing three forward simulations; (1) unperturbed walking (black), (2) passive response (red) and (3) active response (blue). The increase stance leg GM activity (b) and force tilts the pelvis upwards in the frontal plane (c) which stretches the swing leg hip abductors and therefore increases the swing leg GM force for the same activation (d). The resulting increase in swing leg hip abduction moment rotates the swing leg outwards and therefore increases the stride width (e). The resulting increase in stride width at heelstrike is shown in pane f.