Responses of human ankle muscles to mediolateral balance perturbations during walking

Subjects walked on a treadmill and received pushes to the left and pulls to the right in various phases of the gait cycle. Muscle actions were divided into medium latency R1 (100-150 ms), long latency R2 (170-250 ms), and late action R3 (270-350 ms). Short latency responses, before 100 ms, were not observed but later responses were prominent. With inward perturbations (e.g. pushes to the left shortly before or during stance of the right foot) responses in RPL were seen. The forward roll-over of the CoP was briefly stalled in mid stance, so that the heel was not lifted. Stance was shortened. With outward perturbations, pushes to the left shortly before or during stance of the left foot, responses in all three muscles, LTA, LSO, and LPL were seen. Our interpretation is that these muscle activations induce a 'braking reaction' but could also contribute to the 'mediolateral ankle strategy'. The resultant balance correction is small but fast, and so diminishes the need for later corrections by the stepping strategy.