Which structure is directly involved in the innervation of the quadratus lumborum?

The quadratus lumborum muscle is primarily innervated by the upper three to four lumbar spinal nerves. This innervation is crucial for the muscle's function, which includes lateral flexion of the lumbar spine and stabilization of the pelvis and lower back during movements. The specific spinal nerves that contribute to this innervation originate from the lumbar plexus, and their role in activating the quadratus lumborum highlights the importance of these nerves in lower back stability and mobility.

In contrast, the other nerve groups listed do not play a direct role in innervating the quadratus lumborum. Thoracic spinal nerves are involved in the innervation of muscles and skin in the thoracic region but do not target the muscles of the lower back directly. Lower sacral nerves primarily innervate structures in the pelvic region and the lower limbs, while cervical spinal nerves are responsible for the head, neck, and upper extremities. Thus, the involvement of upper lumbar spinal nerves in this context makes them the correct choice for the innervation of the quadratus lumborum.