The sternocleidomastoid is a prominent muscle located in the neck. It extends from the sternum and clavicle to the mastoid process of the skull. This muscle plays a crucial role in head movement, allowing for rotation and flexion of the neck. When both sides of the sternocleidomastoid contract, they help to flex the neck forward. When only one side contracts, it causes the head to rotate to the opposite side. This muscle is essential for various activities, including turning the head and maintaining posture.