Coordinating rules for multiple degree of freedom movements

We describe experiments in which a subject made planar movements of the arm. Elbow and shoulder joints were free to move in a horizontal at shoulder height. The subject grasped the handle of a parallel link drive air-magnet floating manipulandum which created a compliant field that was either predominantly inertial or predominantly viscous. We measured the movement of the hand the the force applied by the manipulandum and computed the net torques about the elbow and shoulder. The subject made fast movements in 8 different directions against two different external loads. We investigated whether the principle of linear synergy, which postulates that for the movements under investigation there exists a linear relationship between the joint torques applied to movements in this plane and with both of these loads. We conclude that it does for at least 7 of the 8 directions of movement that were examined. We discuss the implications of these observations for theories of how voluntary movement is planned and controlled.