Left back pain and weakness is considered to be caused by physiological factors such as weight bearing or strenuous exercise, or it may be caused by pathological factors such as trauma, cervical spondylosis, fasciitis, angina and so on. 1. Physiological factors: if the left shoulder is overloaded or strenuous exercise can lead to the fatigue of the left back muscles, the metabolic products produce too much, lactic acid accumulation, the pain and weakness of the left back can occur. It is a normal physiological phenomenon and can be relieved after rest. 2. Trauma: mostly caused by trauma or overstrain of shoulder and back muscles, which can cause localized pain and swelling, accompanied by subcutaneous bruising. 3. Cervical spondylosis: degenerative changes of cervical intervertebral discs, chronic injuries and accumulations lead to herniated cervical intervertebral discs, which stimulate or compress the nerve roots in the neck, and may cause stiffness and pain in the neck, or pain in the whole back of the shoulder. 4. Fasciitis: Chronic strain of the left scapular muscle can cause local aseptic inflammation, resulting in fasciitis, which can cause pain and weakness of the left back, and may also be accompanied by localized swelling. 5. Angina pectoris: a disease caused by insufficient blood supply to the coronary arteries and myocardial ischemia, manifested by chest tightness, chest pain in the precordial area, pain sometimes radiating to the left shoulder and back, accompanied by dyspnea, sweating, and a sense of near death. Left back pain weakness may have other causes, it is recommended to go to the hospital in a timely manner, improve the examination to clarify the cause of the disease, give targeted treatment or treatment.