What is the action of iliocostalis lumborum?

• A. Extends and laterally flexes the spine.
• B. Flexes the spine anteriorly.
• C. Abducts the ribs.
• D. Elevates the shoulder girdle.

Answer: (A)

The action of iliocostalis lumborum is to extend the spine and, when contracting on one side, to bend the spine laterally toward that same side. This muscle is part of the erector spinae group, whose bilateral contraction straightens and stabilizes the trunk, helping you stand upright. When it contracts unilaterally, it pulls the spine to the side, producing lateral flexion. It doesn’t flex the spine forward—that role belongs to abdominal flexors—and it isn’t involved in abducting ribs or elevating the shoulder girdle, which are actions termed for respiration-related intercostal muscles or shoulder/neck muscles, respectively.