Consider muscle strain, cervical spondylosis, and epicondylitis of the humerus. 1. Muscle strain: If the arm is in a fixed position for a long time, it can cause muscle strain and pain on the inner side of the arm, which will be aggravated after exertion and alleviated after rest. 2. Cervical spondylosis: Herniated discs or osteophytes compressing the nerves can cause radiating pain in the arm, some may have numbness, and the fingers may be weak in grasping. 3. Epicondylitis of the humerus: frequent flexion and extension of the elbow joint or often do rotate forward and backward movements can cause chronic aseptic inflammation of the flexed muscles at the epicondylar stop of the humerus, causing pain in the inner side of the arm. The pain on the inner side of the arm needs to be examined in the hospital if the pain persists without relief.