CONTROL STRATEGIES FOR FINGER MOVEMENT DURING TOUCH TYPING - THE ROLEOF THE EXTRINSIC MUSCLES DURING A KEYSTROKE

A single keystroke during touch-typing is a rapid, goal-directed motio n of the fingertip which consists of two single-direction movements. T he neural control and the role of the finger extrinsic musculature dur ing typing have not yet been explained. The fingertip motion and force , and the intramuscular electromyographic (EMG) activity (fine-wire) o f the index finger extrinsic musculature were measured during touch-ty ping by ten experienced typists. The motions and forces were repeatabl e qualitatively across keystrokes. A three-burst EMG pattern was obser ved during a single keystroke. The three bursts were: (1) a burst of e xtensor activity lifted the finger before the keystroke; (2) a burst o f flexor activity followed while the fingertip was moving downward; an d (3) a second burst of extensor activity occurred as the fingertip re ached the end of key travel. The timing of the third burst suggests th e 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 con traction principally overcomes the activation force of the keyswitch r ather than accelerates the finger downward as expected.