Maximal isometric force and electromyographic (EMG) activity of the distal and proximal muscles during simultaneous and separate contractions of elbow flexion and hand grip contraction were investigated in 14 subjects. The subjects were asked to perform the force generation as strongly and quickly as possible for 2 s under three conditions : (1) hand grip only, (2) elbow flexion only, and (3) both contractions simultaneously. EMG activities were recorded from the flexor digitorum superficialis (FDS), extensor digitorum (ED) and biceps brachii (BB). The results are summarized as follows:1) Elbow flexion in the simultaneous condition showed decrements in the force and EMG of the BB during the rising phase. The hand grip, however, showed no decrements in the force or EMGs of the FDS and ED. 2) During the plateau phase, elbow flexion in the simultaneous condition exhibited a decrease in the force and EMG of the ED, while no changes were observed in the EMG of the FDS. Despite a decrement in the EMG of the BB, the force of elbow flexion was not decreased. These results suggest that there are differences in neural activity between the rising and the plateau phase s during maximal simultaneous elbow flexion and hand grip contractions. In the rising phase, the central nervous system is considered to exert inhibitory effects on the neural activities of the proximal muscles so that contraction of the distal muscles is preferred. During the plateau phase, the force decrement of the hand grip might be due to a strategy whereby the neural activity of the ED is decreased.