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 ev aluated in terms of the muscle activity and response dynamics of five subje cts exposed to horizontal forces at the pelvis producing sideways or forwar d sway. Rapid (EMG onset latencies of 70-80 ms recorded from the left glute us medius and gastrocnemius) and qualitatively different patterns of respon se were produced by forward pushes and pushes to either side. However, the EMG response to left push was constant in pat tern and timing, whether the push direction was constant and, therefore, predictable over a block of tri als or whether the left push trials were interleaved randomly with right pu sh or forward push trials. Moreover, there were no systematic effects of pe rturbation direction uncertainty on the latency and rate of increase of gro und reaction forces. We conclude that prior information does not speed post ural responses that differ quantitatively according to the direction of per turbation to balance.