Which of the following is an action of the sternocleidomastoid?

• A. Rotate the head and neck to the opposite side
• B. Elevate the ribs
• C. Extend the spine
• D. Retract the scapula

Answer: (A)

The key action of the sternocleidomastoid is to rotate the head and neck to the opposite side when it contracts on one side. This happens because the muscle runs from the sternum and clavicle up to the mastoid process, so pulling on one side twists the head away from that side. When both sides contract, it helps flex the neck. The other options describe actions not primarily performed by this muscle: raising the ribs is more about respiration with muscles like the scalenes or intercostals; extending the spine is done by the back extensor muscles; and retracting the scapula is accomplished by the rhomboids and trapezius. So rotating the head and neck to the opposite side is the best match.