Final answer:
The trapezius muscle acts as an antagonist to the unilateral action of the left sternocleidomastoid during neck extension movements.
Step-by-step explanation:
The muscle that is antagonistic to the unilateral action of the left sternocleidomastoid is the trapezius muscle when it is involved in extension and hyperextension of the neck. The sternocleidomastoid muscle is a major muscle that laterally flexes and rotates the head, and it is innervated by the accessory nerve. Unilateral action of the left sternocleidomastoid causes the head to rotate to the right. In contrast, the trapezius muscle acts as an antagonist to the sternocleidomastoid during head extension movements. The trapezius and sternocleidomastoid muscles also work together in certain movements. They can act as synergists for lateral flexion of the head towards the shoulder. Additionally, both muscles play a role in elevating the scapula and clavicle, and their function is significant in changing the position of the head. Strength testing of these muscles, which involves flexing and extending the neck or shrugging the shoulders against resistance, is common in examining the motor functions that are controlled by the spinal nerves and the accessory nerve.