Why would your sternocleidomastoid muscle be swollen?

The most likely cause of a swollen sternocleidomastoid muscle is a muscle strain or tear, which can happen during activities that involve sudden or forceful movements of the head or neck, such as whiplash injuries, contact sports, or heavy lifting. Swelling can also be a result of various medical conditions, such as:

1. Infection: Bacterial or viral infections can cause inflammation and swelling of the muscle.

2. Autoimmune disorders: Conditions such as rheumatoid arthritis or lupus can lead to inflammation and swelling of various muscles, including the sternocleidomastoid.

3. Fibromyalgia: This condition causes widespread musculoskeletal pain and tenderness, and can also affect the sternocleidomastoid muscle.

4. Tumors: Rarely, a swollen sternocleidomastoid muscle can be a sign of an underlying tumor, either benign (non-cancerous) or malignant (cancerous).

If you experience swelling in the sternocleidomastoid muscle, it's important to see a doctor for proper diagnosis and treatment. They can determine the underlying cause and recommend appropriate management, which may include rest, ice, pain relievers, physical therapy, or other interventions as necessary.