Fig. 1.

Subjects moved the handle of a manipulandum into a circular target in 3 different postures. Visual feedback was held constant at the central location, that is, as if the subject were moving the robot's handle with their limb in the central posture (darkest trace in A). In experiment 1, subjects made movements to a single target at the central location from 8 discrete starting positions centered on the target (B); in experiment 2, subjects made movements from random starting positions centered on the target (C) in each of the 3 limb configurations shown in A. D shows tangential velocity for a representative subject. The area bounded by the curve represents ±1SE. Limb stiffness and movement variability were measured at the end of movements (approximate location of the gray arrow).