Which of the following is a bilateral action of the sternocleidomastoid?

• A. Elevate rib cage during inhalation
• B. Lateral flex the head to same side
• C. Rotate the head to opposite side
• D. Extend the neck

Answer: (A)

When both sternocleidomastoid muscles contract, they work together to move the neck and assist breathing. Bilateral contraction primarily brings the head toward the chest (neck flexion), and, importantly, acts as an accessory muscle of inspiration by elevating the sternum and upper rib cage. Elevating the rib cage during inhalation increases thoracic volume, which is why this action is described as a bilateral function of the muscle.

Lateral flexion to the same side and rotation to the opposite side occur with the muscle contracting on one side only, not both, so they’re unilateral actions. Extending the neck isn’t a typical bilateral action of the sternocleidomastoid; when contracting bilaterally, it tends to flex the neck rather than extend it.