Experimental apparatus and the placement of electrodes. (A) Electrode grids were placed on the biceps and triceps muscles of the participants. Single differential EMG signals were obtained by subtracting data from two longitudinally neighboring electrodes (e.g. green box). (B) Subjects grip a handle that is pressed upward or downward against the enclosure of the apparatus to achieve flexion or extension, respectively. Force sensors encased in the upper and lower enclosure provide force feedback. To achieve pronation or supination, subjects twist a handle. The handle is attached to a torque wrench providing the subjects with torque feedback.