Answer:
Trapezius.
Explanation:
Sternocleidomastoid muscles may be defined as one of the largest superficial cervical muscle. The main function of this muscle is the neck flexion and the head rotation.
Trapezius muscle is the large muscle that extend from the occipital bone. The main function of the trapezius muscle is the scapula movement and support of the arm. Trapezium muscle resist the neck flexion and works as the antagonist to the trapezius muscle.
Thus, the answer is trapezius.
