Time-Invariant Strategies in Coordination of Human Reaching

This paper addresses validation of a curvature-theory-based time-invariant inverse kinematic model and a related tracking algorithm for human motor control of reaching motions. Human subjects made unconstrained reaching motions in the horizontal plane to fixed targets at three self-selected speeds. Consistent shoulder/elbow joint speed ratios for motions to the same target across speeds were observed, indicating a time-invariant planning strategy. The inverse kinematic model’s technique of relating joint motions with a Taylor series expansion is in concert with the leading joint hypothesis. With this approach the tracking algorithm successfully replicated the experimental wrist trajectories, and also predicted the previously observed elbow-led motions for reaching in the ipsilateral hemifield. The elbow leads the arm motion in this hemifield because the shoulder approaches a dwell. A computationally frugal strategy of intermittent path correction based on two error parameters is proposed.