Differences and similarities in electrical muscle activity for the abductor pollicis longus muscle divisions.

The abductor pollicis longus (APL) has to be viewed as an important muscle for moving and stabilizing the human thumb(10). This muscle has two divisions, a distal superficial division and a more proximal deep one. The electromyographic (EMG) signals from these divisions were measured for several motor tasks in order to investigate differences in activation and function. EMG signals were recorded with intramuscular wire electrodes in isometric as well as in dynamic contractions in different directions, both for the thumb and for the hand. The EMG signals of the right hand of eight subjects were scaled relative to the mean EMG value at the maximum voluntary isometric contraction (MVC) in order to compare relative muscle activity in various directions for different subjects. In 18 out of the 22 directions the EMG activity in the two divisions of the APL was modulated differently, indicating a different activation. The differences were most prominent in dorsal and palmar flexion of the hand. The results suggested that the APLdeep is activated to stabilize the carpal joint. The APLdeep is a direct mover of the carpal joint in dorsal flexion of the hand.