EMG-reaction time of the biceps brachii in bilateral simultaneous motions.

EMG-RTs of the biceps brachii muscles were measured using electromyogram (EMG) in elbow flexion and forearm supination on 18 right-and 24 left-handed subjects for four tasks, flexion or supination (symmetrical) and flexion of one and supination of the other side (asymmetrical). For both subject groups, the EMG-RTs of flexion for both hands were not prolonged under asymmetrical tasks, but the EMG-RTs of supination were significantly prolonged on both sides. Comparing the coefficients of determination of the EMG-RTs of flexion to those of supination under four different tasks, those of the preferred hand for symmetrical and asymmetrical motions did not differ, but those of the nonpreferred side for asymmetrical motions were smaller than those for symmetrical motions in both subjects. These observations indicated prolongation of EMG-RT on the asymmetrical task was larger on supination than on flexion. It was suggested that the influence of strong timing constraints was greater on the auxiliary function than on the innate function of the biceps (elbow flexor). The steadiness of motor function of the preferred hand was also discussed in regard to hand preference.