Soreness and lack of strength in the left arm and shoulder are very likely due to cervical spondylosis, frozen shoulder, myofasciitis, severe injury to the shoulder, fracture dislocation, and severe soft tissue strain. It may also be caused by injury to the brachial plexus nerve, which is often seen with long-term strain, strenuous labor, and exertion. You can visit the orthopedic department and have the corresponding area examined. Acupuncture, physiotherapy and symptomatic pain relief should be applied, and the cause of the injury should be actively investigated and rest should be paid attention to prevent further aggravation of the injury. Generally, after a period of targeted physical therapy and treatment, the symptoms will gradually be reduced.