Goal-directed arm movements. III: Feedback and adaptation in response to inertia perturbations.

Goal-directed shoulder-elbow movements with a maximal and with a submaximal velocity have been studied. 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 after a change of mass. 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, but activation durations were strongly 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 shape invariance was demonstrated in the movement trajectory. In the first moyements after a change of mass, effective and substantial modifications of muscular activity appeared about 90 ms after movement onset. Earlier modifications suggest a force feedback leading to a yielding towards the disturbance instead of a compensation. Such force feedback may, however, increase system bandwidth as it will allow increased position/velocity feedback gains.