Which neck muscle flexes the neck when both sides contract?

• A. Trapezius
• B. Sternocleidomastoid
• C. Splenius capitis
• D. Scalenes

Answer: (B)

The neck flexion produced when both sides contract is mainly the result of the sternocleidomastoid. This muscle runs from the sternum and clavicle up to the mastoid process behind the ear; when both left and right sternocleidomastoids contract together, they pull the head forward and down, bending the cervical spine so the chin moves toward the chest. If only one side contracts, it causes rotation to the opposite side and some lateral flexion, not pure flexion. The other muscles listed aren’t primary neck flexors: the trapezius mainly extends and stabilizes the neck and supports the shoulders; the splenius capitis extends and rotates the head; the scalene muscles elevate the ribs and assist with neck stabilization, contributing less to bilateral neck flexion.