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In this study, we investigated the changes in the muscle activities of the brachialis (BR) and biceps brachii (BB) during dynamic elbow flexion under different movement velocity and load conditions. Twenty healthy adult males performed isotonic elbow flexions in the full range of motion (0–140^∘) under angular velocities of 30^∘/s and 60^∘/s, and with (30% maximum torque) and without load conditions. Muscle activity was measured using surface electromyography (EMG). The muscle activity of the BR and BB was compared to their response to different angle phases, angular velocity and load conditions. Both muscle activities of the BR and BB significantly increased in the initial angle phases of the elbow flexion. Muscle activity of the BR progressively increased with increasing elbow flexion, whereas that of the BB plateaued regardless of the velocity and load conditions. Specifically, BB muscle activity plateaued after an initial increase in the earliest phase at 60^∘/s with load conditions. It was suggested that BR and BB contributed to the control of the movement in a different way during dynamic elbow flexion.