Compensatory reactions to gait perturbations in man: short- and long-term effects of neuronal adaptation.

Short- and long-term changes in the cerebral potentials and the electromyographic (EMG) responses in the arms and legs evoked by gait perturbations were followed up over repeated trials in healthy subjects. Two different modes of muscle activation could be discerned. The first adapted to the specific motor task within a few trials and remained constant during the remaining experiments. This 'hard wired' and complex leg muscle reaction was shown to be task specific and necessary for an adequate compensation for the displacement. It was supposed that this reaction is mediated via a spinal pathway. The other mode, which included an early arm extension, disappeared successively as the motor task became familiar. These flexible EMG responses represent the 'protective' part of the reaction and are suggested to be mediated via a transcortical pathway, due to their close relationship to the cerebral potentials evoked by the perturbations.