Pain on the lateral side of the arm is commonly caused by humeral epicondylitis, strain or injury of the upper arm muscles, and frozen shoulder, as follows: 1. humeral epicondylitis: commonly known as tennis elbow, the site is mostly on the lateral side of the elbow joint, mostly caused by excessive housework or physical work, which can manifest as aseptic inflammation of the lateral epicondyle of the humerus, and the pain can be significantly increased when lifting heavy objects or twisting towels. Rest, reduce the amount of housework, and at the same time, apply hot compresses to relieve pain. 2. Upper arm muscle strain or injury: Most of the pain is in the center of the outer arm. 3. Frozen shoulder: The pain of frozen shoulder can be around the shoulder, including the deltoid muscle, the outer arm, and sometimes the elbow joint.