Fig. 1.

The experimental setup consisted of a custom-made device that limited index finger movement to the abduction-adduction plane. A force transducer was aligned with the radial border of the proximal interphalangeal joint to record index finger abduction force. Surface EMG from the first dorsal interosseus muscle was obtained with 4-mm-diameter electrodes. Two fine-wire electrodes (three 50-μm-diameter insulated stainless steel wires) were inserted into the first dorsal interosseus muscle to record the discharge of action potentials by single motor units.