Which action is performed by the internal oblique?

• A. Rotate vertebral column to same side
• B. Rotate vertebral column to opposite side
• C. Flex trunk forward
• D. Extend the spine

Answer: (A)

Unilateral contraction of the internal oblique rotates the trunk toward the same side. This ipsilateral rotation happens because the muscle fibers run obliquely upward from the pelvis to the lower ribs, so pulling on one side twists the torso in that direction. The internal oblique also helps with lateral flexion on the same side and, when both sides contract together, contributes to trunk flexion and abdominal compression. Rotating to the opposite side is the action of the external oblique, not the internal oblique. Extending the spine is mainly the job of the back extensors, while pure flexion of the trunk is a broader effect of several abdominal muscles, not the defining action of the internal oblique.