The middle joint of the arm belongs to the elbow joint, and the pain in straightening may be caused by exfoliative osteochondritis, synovitis, soft tissue injury and so on. 1. Exfoliative osteochondritis: a joint disease in which the articular cartilage and its subchondral bone are limited due to various pathogenic factors, and gradually separated from the normal bone around it. It may be caused by sports injuries, local ischemia, heredity and other reasons. Due to the stimulation of inflammation, the elbow joint can be painful when it is straightened. 2. Synovitis: synovitis of elbow joint may be caused by excessive joint movement, joint injury, infection, etc. Due to local stimulation, pain may appear when straightening the elbow joint. 3. Soft tissue injury: soft tissue injury of the elbow joint caused by collision, beating, etc. Due to the injury of the elbow joint, pain can also occur when straightening the elbow joint. Straightening the elbow joint pain may also be caused by other reasons, it is recommended to go to the hospital in a timely manner, follow the doctor’s instructions for treatment, to avoid delaying the condition.