Pain in the lateral upper arm is commonly caused by the following two factors: first, deltoid fasciitis; second, subacromial deltoid bursitis. The first one is due to muscle fatigue after frequent activities and labor in the upper arm. When repeated fatigue occurs, muscle strain and aseptic inflammation will occur, and some painful excitation points will appear on the myofascial surface of the deltoid muscle, resulting in pain in the lateral upper arm, which usually does not exceed the elbow, but mostly in the middle and upper third of the upper arm. If there is repeated supination of the shoulder joint, the bursa under the acromion and deltoid may be irritated, resulting in bursitis, which can cause pain in the lateral aspect of the upper arm and the anterolateral aspect of the shoulder.