Simplified arm control in goal-directed movement

Target reaching geometry and kinematics in free space, performed by able-bodied humans, were studied. The authors discuss the planar movements of the shoulder and elbow joints. They found that for goal-directed movements, different strategies generate a variety of arm trajectories. However, they established a common behavior in the phase space formed by joint angular velocities. They determined a synergistic parameter that describes this common behavior. The synergistic parameter is a zero-dimensional number derived from the initial and target positions, and it shows scaling of angular velocities in shoulder and elbow joints. The results of this study are meant for external control of arm movements in tetraplegic subjects.