What is the only true lateral flexor of the spine?

The quadratus lumborum is considered the only true lateral flexor of the spine due to its anatomical position and function. This muscle, located in the lower back, plays a crucial role in lateral flexion, allowing the trunk to bend sideways. It originates from the iliac crest and inserts into the lower rib and lumbar vertebrae, enabling it to assist in stabilizing the pelvis and lower spine during movement.

While the other muscles listed contribute to trunk movement and stability, they do not primarily function as lateral flexors. The external and internal obliques assist in trunk rotation and flexion, and they have some involvement in lateral flexion, but their main roles are more dynamic and rotational rather than strictly lateral. The latissimus dorsi primarily functions in actions like arm adduction and extension, making it less relevant to lateral flexion of the spine.

In summary, the quadratus lumborum's specific orientation and attachment points enable it to effectively facilitate lateral flexion of the spine, distinguishing it as the primary muscle responsible for this action.