Purpose The Aim of this study is to determine the muscle activity of the upper extremity depending on the forearm position during the shoulder external rotation exercise. Methods 23 normal adult males were selected to perform shoulder external rotation exercises using three different position (supination, neutral, and pronation), while subject’s shoulder and elbow was at 0° abduction and 90° flexion, respectively. Electromyography (EMG) was attached to the infraspinatus, posterior deltoid, biceps brachii, triceps brachii, brachioradialis and extensor carpi ulnaris to observe muscle activity. Results With the different positions of the forearm, there was significant difference in muscle activity for the posterior deltoid, biceps brachii, brachioradialis, and extensor carpi ulnaris(p<.05). However, there were no significant difference between the infraspinatus and triceps brachii. Conclusion Shoulder external rotation exercise with a forearm supination position and external rotation can safely be used as efficient method for a rehabilitation of one’s shoulder.