What is the primary function of the sternocleidomastoideus muscle?

The primary function of the sternocleidomastoideus muscle, commonly known as the SCM, is to facilitate head rotation. This muscle originates from the sternum and clavicle and inserts into the mastoid process of the temporal bone behind the ear. When one side of the SCM contracts, it rotates the head to the opposite side, allowing for a range of head movements such as turning to look over one shoulder.

While this muscle does contribute to other movements, such as flexing the neck (when both sides contract), its most notable and primary action is head rotation. The other options address different functions or movements that are not the main role of this specific muscle. Understanding the anatomy and function of the sternocleidomastoideus is crucial for estheticians, as it plays a significant role in facial treatments and therapies involving neck and shoulder manipulation.