Figure 2.

Experimental setup and protocol. A, Subjects sat in front of a desktop and applied forces on a transducer attached to a forearm, wrist, and hand splint. A flat monitor occluded the subject's hand and displayed a virtual scene colocated with the real desktop. B, Transparent spheres positioned on a horizontal plane with centers at the same height as the center of the palm indicated force targets that the subjects were instructed to reach with a smaller spherical cursor moving on the same plane according to the force applied (force control) or estimated from EMGs recorded from 13 arm and shoulder muscles (EMG control; see Materials and Methods). C, Subjects were instructed to perform a center-out reaching task in which they had to maintain the cursor in a central start location for 1 s, reach a target as soon as it appeared at one of eight peripheral locations, and maintain the cursor at the target for 0.2 s. D, Each subject performed a single experimental session consisting of 16 trials of maximum voluntary force generation in eight directions, 72 trials of reaching to targets in eight directions at three force magnitudes (10, 20, and 30% MVF) in force control, and the following blocks of 24 trials each in EMG control: one block of familiarization (FAM), two series of 24 blocks for each surgery type (6 baseline blocks, 12 surgery blocks, 6 washout blocks), and 6 additional baseline blocks.