Muscular strength is assessed by measuring the force exerted on an external object by muscle group. Since there are four flexor muscles at elbow, elbow flexion depends on muscle`s coactivation. The objective of this study is to investigate the changes of the role assigned to each muscle for three elbow angles and four exertion level. It should be noted that an EMG multiplexor is developed and bioelectric signals of four elbow flexors have been simultaneously measured. The results showed that the recruitment of biceps brachii is relatively constant but those of other muscles are varied depending on elbow angles and exertion levels.