Figure 2.

Experimental setup. A, The subject's right arm was attached to the handle of the PFM with a custom-fitted thermoplastic cuff. Reaching movements were performed from a start position located at [0, 31] cm relative to the shoulder to a target located at [0, 56] cm. The PFM applied either a computer-controlled force field or controlled displacements to estimate stiffness during movement. B, Force produced by the three force fields: 0DF, −45DF, and 80DF (left to right). The force depended on the x-position of the subject's hand. As the hand position deviated either to the left or the right of the straight path joining the start and end targets, the force produced by the PFM increased. The direction in which the force pushed the hand was different for each field. C, Sample force patterns for two trajectories in each force field. Slightly diverging trajectories (shown in black dotted lines) result in widely different force patterns on the hand (compare black and gray arrows).