Soreness in the upper arm of the left shoulder may be caused by trauma, myofasciitis and cervical spondylosis. 1.Trauma:When the patient’s soft tissues, muscle tissues and ligaments are injured. It may then lead to symptoms of soreness in the upper arm of the left shoulder. 2. Myofasciitis: myofasciitis is a kind of disease through the muscle and fascia aseptic inflammation, the patient will appear the symptoms of the left shoulder and upper arm pain, some patients may also continue to appear muscle spasm and motor dysfunction and other symptoms. 3. Cervical spondylosis: When cervical spondylosis occurs, it will compress the shoulder nerves, which will lead to the phenomenon of soreness in the upper arm of the left shoulder. When patients have left shoulder and upper arm pain, they should go to the hospital in time to get medical treatment, clarify the cause of the disease, and actively cooperate with the doctor for treatment.