Which muscle is a synergist with quadratus lumborum when laterally flexing the spine to the right?

• A. Iliocostalis on right side
• B. Longissimus on left
• C. Spinalis
• D. Rectus abdominis

Answer: (A)

When you bend the spine to the right, the muscles on the right side that shorten contribute most to the movement. The quadratus lumborum on the right drives the lateral flexion by pulling the pelvis and lower rib cage toward that side. The iliocostalis on the right, part of the erector spinae, runs along the side of the back and also shortens on the right to help tilt and stabilize the spine in the same direction. This makes it a clear synergist with the quadratus lumborum for a rightward bend.

In contrast, the longissimus on the left would tend to assist a left-side bend or provide different stabilization, not support the rightward flexion. The spinalis is a more midline, smaller muscle with a limited role in lateral flexion, and the rectus abdominis mainly flexes the trunk rather than contributing to lateral bending. So the muscle on the right side of the iliocostalis best complements the quadratus lumborum for bending to the right.