What is the primary action of the sternocleidomastoid muscle when it contracts unilaterally (one side only)?

• A. Rotates the head to the opposite side and laterally flexes the neck to the same side
• B. Rotates the head to the same side and laterally flexes the neck to the opposite side
• C. Extends the cervical vertebrae and retracts the mandible
• D. Depresses the scapula and abducts the humerus

Answer: (A)

Verified step by step guidance

1. Identify the anatomical attachments of the sternocleidomastoid muscle: it originates from the sternum and clavicle and inserts on the mastoid process of the temporal bone.
2. Understand that when the sternocleidomastoid contracts on one side (unilaterally), it pulls the mastoid process toward the sternum and clavicle on that side.
3. Recognize that this unilateral contraction causes two movements: rotation of the head and lateral flexion of the neck.
4. Recall the direction of these movements: the head rotates to the opposite side of the contracting muscle, while the neck laterally flexes (bends) toward the same side as the contracting muscle.
5. Conclude that the primary action of unilateral contraction of the sternocleidomastoid is to rotate the head to the opposite side and laterally flex the neck to the same side.