What muscle is primarily stretched when the trunk is laterally flexed?

When the trunk is laterally flexed, the muscle primarily being stretched is the quadratus lumborum. This muscle is located in the lower back and plays a crucial role in lateral flexion and stabilization of the spine. When performing lateral flexion, one side of the quadratus lumborum contracts to pull the trunk towards that side, while the opposite side is stretched.

The erector spinae, while involved in trunk movements, primarily assists with extension and not lateral flexion specifically. The pectoralis major, being a muscle of the upper chest, is not directly involved in trunk lateral flexion and is more relevant to movements of the shoulder and upper arm. Similarly, the obliques help with rotation and flexion of the trunk, but they do not undergo significant stretching during a lateral flexion movement in the same way the quadratus lumborum does. Thus, the role of the quadratus lumborum in lateral flexion makes it the primary muscle that is stretched in that movement.