Which of the following statements about the sternocleidomastoid muscle is true?

The statement that the sternocleidomastoid muscle helps in rotating the head is correct because this muscle connects the sternum and clavicle to the mastoid process of the temporal bone in the skull. When one side of the sternocleidomastoid contracts, it can rotate the head to the opposite side while also tilting it toward the same side. This function is essential for various movements of the head and neck, making it an important muscle in activities like looking over one shoulder.

The other statements do not accurately describe the roles of the sternocleidomastoid muscle. For instance, while swallowing involves several muscles and structures in the throat and mouth, the sternocleidomastoid is not primarily responsible for this action. Additionally, while some muscles in the face contribute to expression, the sternocleidomastoid is not involved in this function; it is primarily a neck muscle. Lastly, regarding breathing, the sternocleidomastoid does assist to a degree by elevating the sternum during forced inhalation, but it is not the muscle that primarily aids in breathing, which is more reliant on the diaphragm and intercostal muscles.