Trajectory formation of vertical arm movements through a via-point: a limit of validity of the minimum-jerk model.

The minimum-jerk model predicts the smoothest trajectory for a class of human movements and so provides us with a kinematic measurement of skilled motor performance. To establish the limits of the model's validity, the predicted and experimentally defined movement trajectories and the joint coordination were compared in two-joint arm movements, bringing the hand from the initial position to the final position through a specified point (a via-point). Kinematic data of the movements were obtained through the SELSPOT system. The movement path, tangential velocity, and coordinated change in positions of the shoulder and elbow joints evidently deviated from those predicted by the model. These results suggest that the minimum-jerk model is not valid for movements under extreme conditions which are highly dependent on musculoskeletal dynamics.