There are four flexor muscles at elbow-biceps brachii, brachialis, brachioradialis, pronator teres. Muscle exertion force depends on the following conditions : elbow angle, external workload, and endurance time. The objectives of this study are to investigate the variations of the recruitment patterns of four muscles, and to analyze the changes of the role assigned to each muscle during elbow flexion. For three elbow angles, the transition point of type F motor unit of each muscle is inferred and the changes of the role assigned due to exertion level are identified by electromyographic analysis. The results showed that the recruitment pattern of biceps brachii is constant but those of other muscles are varied depending on elbow angle and exertion level. An EMG multiplexor is developed and simultaneous measurement of electromyographic signals of four elbow flexors is possible.