The causes of shoulder muscle pain mainly include diseases of the shoulder joint itself, cervical spondylosis and shoulder tumor. 1. Diseases of the shoulder joint itself: they mainly include muscle fasciitis, synovitis, rotator cuff injury and periarthritis, etc. Diseases of the shoulder joint itself will cause accumulation of lactic acid, synovial membrane hyperplasia, muscle or tendon tear, degenerative lesions in the joint, etc., which will lead to abnormal impingement and extrusion of the joint and stimulate the nerve root to cause muscle pain symptoms. 2. Cervical spondylosis: It is mainly referred to as nerve root cervical spondylosis that causes shoulder muscle pain. Neurogenic cervical spondylosis is caused by degenerative changes of the cervical spine itself, which will result in the proliferation of bony cumbers, and the compression of the nerve root will lead to the radiating pain in the neck and shoulder, which is also commonly known as cervical-shoulder syndrome. 3. Shoulder tumor: when there is benign or malignant tumor in the shoulder, because the growth of tumor can stimulate and compress the shoulder muscle tissues, it will stimulate the shoulder myofascia and cause shoulder muscle pain. The symptoms of shoulder muscle pain alone cannot accurately determine the cause, but also need to be combined with the characteristics of medical history and other examination results to make accurate judgment, it is recommended to clarify the cause of the disease under the guidance of the doctor.