Left and right arm muscle asymmetry may be caused by chronic strain injury, may also be caused by the two sides of the exercise intensity is different, may also be caused by cervical spondylosis and other factors. 1. Chronic strain injury: prolonged labor will lead to arm muscles and ligaments and other soft tissues strain, aseptic inflammation, which will lead to local congestion, edema, and thus left and right arm muscle asymmetry. 2. Both sides of the exercise intensity is different: long-term activities with one side of the arm, or exercise on one side of the arm muscle strength is too large, will lead to one side of the muscle thick, developed, thus making the arm muscles on both sides of the asymmetry. 3. Cervical spondylosis: cervical spine degenerative changes will squeeze the nerves around the cervical spine, so that the nerve nutrition is not sufficient, which will lead to one side of the arm than the other side of the arm slender, asymmetric. In order to clarify the specific causes of the asymmetry of the left and right arm muscles as soon as possible, it is recommended to go to the orthopedic department of the hospital in time for consultation and examination.