Which statement best describes the unilateral action of the sternocleidomastoid?

• A. Laterally flex the head and neck to the same side and rotate the head and neck to the opposite side
• B. Flexes the neck
• C. Elevates the rib cage during inhalation
• D. Protracts the jaw

Answer: (C)

When one side of the sternocleidomastoid contracts, it does not simply pull the head forward. Instead, the muscle rotates the head to the opposite side and brings the head toward the same side as the contracting muscle (lateral flexion). That combination—opposite rotation with same-side lateral flexion—is the hallmark of its unilateral action.

Elevating the rib cage during inhalation is a different action that occurs when both sternocleidomastoids contract together (bilateral action). This helps expand the chest for deeper breathing, rather than producing movement of the head to one side. The other movements listed aren’t functions of the sternocleidomastoid either, since jaw protraction involves jaw muscles rather than this neck muscle.

So the unilateral action described is the head rotating to the opposite side and laterally flexing to the same side.