Control strategies for finger movement during touch-typing. The role of the extrinsic muscles during a keystroke.

A single keystroke during touch-typing is a rapid, goal-directed motion of the fingertip which consists of two single-direction movements. The neural control and the role of the finger extrinsic musculature during typing have not yet been explained. The fingertip motion and force, and the intramuscular electromyographic (EMG) activity (fine-wire) of the index finger extrinsic musculature were measured during touch-typing by ten experienced typists. The motions and forces were repeatable qualitatively across keystrokes. A three-burst EMG pattern was observed during a single keystroke. The three bursts were: (1) a burst of extensor activity lifted the finger before the keystroke; (2) a burst of flexor activity followed while the fingertip was moving downward; and (3) a second burst of extensor activity occurred as the fingertip reached the end of key travel. The timing of the third burst suggests the role of the extensors is to remove the fingertip from the keyswitch rather than stop the downward motion of the finger. The collision with the end of key travel stops the downward finger motion. The timing of the finger flexor EMG activity, burst 2, suggests that the flexor contraction principally overcomes the activation force of the keyswitch rather than accelerates the finger downward as expected.