What is the bilateral action of the iliocostalis, longissimus, and spinalis muscles?

• A. Extension of vertebral column and head
• B. Flexion of the vertebral column
• C. Lateral flexion of the spine
• D. Rotation of the pelvis

Answer: (A)

When both sides contract, the iliocostalis, longissimus, and spinalis act to extend the spine and the head. This group runs along the back, and their bilateral action pulls the vertebral column backward and raises the head, helping you stand upright from a flexed position. The part that reaches to the head—especially the superior fibers of longissimus—contributes to head extension as well. If the muscles contract on one side only, they mainly produce lateral flexion of the spine (and some rotation), which is different from the straightening motion seen with bilateral contraction. Flexion of the vertebral column is carried out more by abdominal muscles, and rotation of the pelvis isn’t a primary role of these back muscles.