ADAPTATION TO UNEXPECTED VARIATIONS OF AN INERTIAL LOAD IN GOAL DIRECTED MOVEMENTS

Abstract Human motor behavior has been studied experimentally in goal directed movements. At the movement onset the inertial load to be displaced was changed unexpectedly. The adaptation of movement and muscular activity have been described with a moving average model. Significant adaptation effects were demonstrated in the first two or three movements in the new condition. Adaptation only partly compensated the mass effects: A higher mass led to a persistent reduction of movement velocity. Amplitudes of muscular activity showed no adaptation of muscular effort, only activation durations were modified. Thus the hypothesis that adaptation pursues a certain movement trajectory as a function of time had to be rejected. However, after scaling towards peak velocity, a global shape invariance was demonstrated in the movement trajectory. In the first movements after a change of mass effective and substantial modifications of muscular activity appeared about 90 milliseconds after movement onset. Earlier modifications suggest a force feedback enlarging the disturbance. Such force feedback may however increase system bandwidth as it will allow increased position/velocity feedback gains.