Scaling of joint torque during planar arm movements.

The present study scrutinized the "Motor Program" concept for aimed arm movements. Human subjects pointed at visual targets in a horizontal plane, with movements of varying starting positions, amplitudes and directions. We recorded movement kinematics and subsequently calculated the shoulder and elbow joint torque profiles. Our results indicate that the shape of torque profiles is rather uniform across movements and joints. We defined the size of those profiles by six "landmark variables", which could be subsequently reduced to three factors using factor analysis: one factor represented torque magnitude and two represented different aspects of torque timing. Additional analyses indicated that total torque duration is an important controlled signal. Our findings conform with the view that movements are executed by playing back scaled versions of prototypical joint torque profiles.