Soreness in the left arm may be caused by serious diseases such as breast cancer, but most cases may be caused by cervical spondylosis, frozen shoulder, shoulder arthritis and other factors. 1. Breast cancer: As breast cancer cells spread, they will invade the surrounding tissue cells and metastasize to the local lymphatic drainage area, thus affecting the blood circulation in the arm and leading to the symptoms of soreness in the left arm. 2. Cervical spondylosis: prolonged bowing of the head or incorrect sitting posture will lead to degeneration of the cervical intervertebral disc structure and compression of the surrounding nerves, spinal cord and other structures, which will cause obvious soreness and discomfort in the left arm. 3. Frozen shoulder: Excessive pulling of the shoulder or poor posture will lead to degeneration of the shoulder joint and the surrounding soft tissues, resulting in aseptic inflammation such as hyperplasia and exudation, which will cause soreness and pain in the left arm. 4. Shoulder arthritis: excessive use of shoulder joint movement will lead to muscle spasm around the shoulder joint, fibrous degeneration and thickening of the joint capsule and other changes, which in turn cause shoulder pain and restricted movement. If your body has obvious left arm pain, you should go to the hospital in time, and standardize the treatment under the guidance of the doctor.