Lateral balance organisation in human stance in response to a random or predictable perturbation

The effect of the predictability of perturbation to standing balance was evaluated in terms of the muscle activity and response dynamics of five subjects exposed to horizontal forces at the pelvis producing sideways or forward sway. Rapid (EMG onset latencies of 70-80 ms recorded from the left gluteus medius and gastrocnemius) and qualitatively different patterns of response were produced by forward pushes and pushes to either side. However, the EMG response to left push was constant in pattern and timing, whether the push direction was constant and, therefore, predictable over a block of trials or whether the left push trials were interleaved randomly with right push or forward push trials. Moreover, there were no systematic effects of perturbation direction uncertainty on the latency and rate of increase of ground reaction forces. We conclude that prior information does not speed postural responses that differ quantitatively according to the direction of perturbation to balance.