Human voluntary isometric maximum muscle strength during simultaneous bilateral versus separate unilateral elbow extension and flexion was measured. The following results were obtained. (1) Strength was reduced under the condition of simultaneous bilateral exertion of contralateral corresponding muscles as compared to the unilateral condition. (2) The decrease ratio of strength was 24.6 and 18.8% for the right and left for extension and 6.3 and 7.6% for the right and left for flexion. The extension-flexion difference was statistically significant but right-left difference was not significant. (3) Integrated EMG showed the tendency comparable to the strength and high correlation was found between strength and integrated EMG. (4) Decrease ratio was reduced when extension of one arm and flexion of the other arm was simultaneously performed. (5) Possible mechanisms subserving these findings were postulated and discussed.