Which muscle is primarily responsible for laterally flexing or bending the trunk?

The Quadratus Lumborum muscle plays a key role in laterally flexing the trunk due to its anatomical position and function. It is located in the lower back and extends from the lumbar vertebrae to the pelvis. When one side of the Quadratus Lumborum contracts, it pulls the pelvis and the ribcage closer together, resulting in lateral flexion of the spine. This action is essential for movements such as leaning to one side, and it helps stabilize the pelvis during various motions.

In addition to lateral flexion, the Quadratus Lumborum also supports the trunk and aids in maintaining posture. Its unique position in the posterior abdominal wall allows it to act effectively during these movements. While other muscles, such as the External Oblique, contribute to trunk rotation and flexion, they are not primarily responsible for lateral flexion. The Rectus Abdominis and Transverse Abdominis focus more on flexing the trunk forward and stabilizing the core, respectively, rather than bending it laterally.