We investigated muscle activation levels or patterns determined during two-joint arm movements. Twelve subjects were instructed to move their right (dominant) wrists in a horizontal plane. Surface electromyographic activities were recorded from the brachioradialis, biceps brachii, and triceps brachii. The muscle activation level was demonstrated as a function of both the elbow and shoulder torque in a 3-dimensional plot and was represented as an eigen plane and dynamic preferred direction (PD). The PD of the bi-articular muscles was suitable for its anatomical and functional roles; however, that of the mono-articular brachioradialis muscle was affected by the shoulder torque. These results suggested that the bi-articular muscle makes the main contribution toward the generation of the kinematic requirement and/or control of the endpoint movement.