Human arm motion planning against a joint constraint

This paper seeks to define the mechanisms by which the human motor system finds optimal reaching solutions, when one of the arm joints is locked in place. Specifically, the paper studies how people solve the problem of motion planning when they lose the ability to move their elbow joint. Our hypothesis is based on the idea that the governing rules of motion planning will be consistent even under the given joint constraint, i.e. the hand will follow the shortest path with a bell-shaped velocity profile, while reaching from a start to an end position. We present an experimental protocol with human subjects to compare their hand paths with the geodesic curve in Euclidean space. The speed profiles of these trajectories are also compared to the modified output of the so-called minimum jerk model of Flash and Hogan. Our results indicate that arm reaching paths with an elbow joint constraint at a certain angle closely follow the geodesic and has a bell shaped speed profile. The future work involves extending this research to the shoulder and wrist joints.