Pain in the sternocleidomastoid muscle may be caused by the following reasons: First, it may be caused by trauma, such as accidental sprains or local contusions, which usually manifest as local swelling and, in severe cases, subcutaneous petechiae, and all have a clear history of trauma. Second, the pain may be caused by inflammation of the tissues near the sternocleidomastoid muscle, such as inflammation and enlargement of the lymph nodes near the sternocleidomastoid muscle, or inflammation of the thyroid gland, which induces pain in the sternocleidomastoid muscle. Usually, in this case, there is a granular hardness near the sternocleidomastoid muscle and pressure pain, which is caused by swollen lymph nodes. In the case of thyroiditis, the local enlargement is usually in the middle of the upper part of the neck and the enlargement moves up and down along with swallowing activities.