What is the primary function of the sternocleidomastoid muscle?

The primary function of the sternocleidomastoid muscle is indeed flexing the neck. This muscle, located on either side of the neck, plays a crucial role in the movement of the head and neck. When both sides of the sternocleidomastoid contract simultaneously, they work to flex the neck forward, bringing the chin towards the chest. Additionally, this muscle assists in turning the head when only one side contracts, allowing for rotation of the head towards the opposite side.

Other options do not pertain to the function of the sternocleidomastoid muscle. For instance, extending the back refers to movements associated with the muscles of the back, while moving the shoulder involves muscles such as the deltoids and trapezius. Rotating the hip is associated with the muscles in the pelvic region, completely unrelated to the sternocleidomastoid. Understanding the specific function of this muscle is important for recognizing its role in movements of the head and neck.