Which muscle is a primary neck muscle involved in head rotation?

The sternocleidomastoideus muscle plays a crucial role in the rotation of the head. This muscle, commonly referred to as the SCM, is located on each side of the neck and is notable for its long, strap-like structure that runs from the sternum (breastbone) and clavicle (collarbone) to the mastoid process behind the ear.

When the sternocleidomastoideus muscle on one side contracts, it causes the head to rotate towards the opposite side. For instance, if the right sternocleidomastoideus contracts, the head turns to the left. Additionally, this muscle assists in tilting the head to the side and also plays a role in flexing the neck when both sides work together.

Other options like the frontalis, masseter, and deltoid do not primarily serve the purpose of head rotation. The frontalis is mainly involved in facial expressions and raising the eyebrows, the masseter is crucial for chewing as it is a primary muscle of mastication, and the deltoid is responsible for shoulder movements. This clear distinction in their functions highlights why the sternocleidomastoideus is specifically identified as the primary muscle for head rotation in this context.