Effect of warning signal on reaction time and EMG activity of the biceps brachii muscle in elbow flexion and forearm supination.

Using a reaction time (RT) task, the biceps brachii muscle was investigated in five healthy subjects to determine whether EMG in the initial phase of activity is dependent upon the direction of movement, i.e., elbow flexion and forearm supination, and/or influenced by the presence or absence of warning signal. Results showed that in the presence of warning EMG-RT of forearm supination was faster than that of elbow flexion, and that the reverse occurred in the absence of warning. The warning signal significantly reduced EMG-RT of both movements and the reduction of EMG-RT was larger in supination than flexion. Compared to forearm supination, large amplitude potentials were observed during elbow flexion. The warning suppressed EMG activities of both movements within 30 msec after their initiation. The present study indicates that the biceps brachii muscle exhibits a motor response program specific to the direction of movements and that preparatory set conditioned by warning shortens RT and also suppresses EMG activities.