Sudden soreness and weakness of the left arm may be caused by frozen shoulder, cervical spondylosis, myofasciitis and other diseases. 1. Frozen shoulder: If you often lift heavy objects or keep a posture for a long time, you may strain your shoulder, which may lead to frozen shoulder. In the early stage of frozen shoulder, when there is no adhesion of ligaments and joint capsule, there may be a sudden soreness and weakness of the arm. 2. Cervical spondylosis: The herniated cervical disc compresses the nerve root on the left side of the patient, which leads to arm pain and weakness, and the pain may extend to the shoulder, elbow and other parts of the body. 3. Myofasciitis: Shoulder injuries caused by incorrect exercise patterns lead to aseptic inflammation of the myofascia, which may cause left arm pain and weakness. If the symptoms persist, you should go to the hospital in time and standardize the treatment under the doctor’s guidance to avoid delaying the condition.