What is an action of longissimus?

• A. Laterally flex vertebral column to the same side
• B. Extend the vertebral column
• C. Rotate the trunk
• D. Assist breathing

Answer: (A)

Longissimus, part of the erector spinae group, has actions that depend on whether it contracts on one side or both. When it contracts unilaterally, it pulls the vertebral column toward the same side, causing lateral flexion. This side-bending action is a clear and distinct movement produced by the longissimus, reflecting how the muscle stabilizes and steers the spine during subtle tilts and posture adjustments. The muscle can also contract bilaterally to extend the vertebral column, but the question focuses on a single, specific action, and the unilateral contraction that produces lateral flexion is a hallmark function. It’s not primarily involved in rotating the trunk or in assisting breathing as a main role, which is why lateral flexion to the same side is the best match for the action described.