Influence of arm positions on EMG-reaction time of the biceps brachii for elbow flexion and forearm supination.

The influence of starting positions of the arm on EMG-RTs of the biceps brachii muscle for elbow flexion and forearm supination was examined using 16 normal subjects. Two angles of the elbow joint, 45 degrees and 110 degrees flexion, and two positions of the forearm, 45 degrees supination and 90 degrees pronation, were used as the factorial combinations of all four. The EMG-RT for elbow flexion decreased in the order of 110 degrees Pronation greater than 45 degrees Pronation = 110 degrees Supination greater than 45 degrees Supination, and that for forearm supination decreased in the order of 45 degrees Supination greater than 45 degrees Pronation = 110 degrees Supination greater than 110 degrees Pronation. These results were kinesiologically interpreted that variations of EMG-RTs were based on the change in the number of synergic muscles participating in an intended movement and the muscle length of the prime mover at the start of the movement.