Activity patterns of upper arm muscles in relation to direction of rapid wrist movement in man

Adjustment of arm posture associated with rapid wrist movements was studied by EMG analysis. Seven healthy adults, seated and holding their right arm with the shoulder in a neutral position with the elbow in 90 degrees flexion and the wrist position neutral, were instructed to flex or extend the wrist as fast as possible. To examine whether the activity patterns of the upper arm muscles were related to the prime mover or the direction of the movement in space, the forearm was in two postures, supinate and pronate. The surface EMGs of biceps brachii, brachialis, triceps brachii and the prime movers were recorded along with the angular displacement of the wrist. The sequences of the upper arm muscle activities changed in relation to the direction of the movement. The earliest activities of the upper arm muscles were considered to counteract the dynamic perturbation induced by the rapid wrist movement. The onsets of the earliest activity of the upper arm muscles preceded the movement onset by 50-60 ms. These results revealed that the activity patterns of the arm muscles associated with the rapid wrist movements were functionally compatible with the anticipatory postural adjustment and were controlled according to the direction of the movement in space.