Pain in the lateral muscles of the right upper arm can be caused by the following 2 common causes: 1. Myofasciitis of the deltoid muscle. Because of repeated exercise or excessive labor in the right upper arm, strain on the deltoid muscle occurs, and after repeated strain, local inflammatory exudation and adhesions occur, resulting in aseptic inflammation and painful excitation points on the myofascia of the deltoid muscle, causing pain, called myofasciitis. 2. Subacromial bursitis of the deltoid muscle. Because of the repeated lifting activities of the shoulder joint, there is constant friction between the subacromial bursa and the greater tuberosity of the humerus, and the friction will stimulate the bursa between the two, that is, the deltoid muscle and the subacromial bursa, and after repeated stimulation, bursitis will occur, causing pain in the lateral upper arm.