The Quadratus Lumborum Muscle is a deep, quadrangular muscle located in pairs on the posterior abdominal wall of the back. Located on either side, it is the deepest muscle between the abdominal and back muscles and is often referred to as the back muscle. It derives its name from its quadrangular shape.

Anatomy

Morphology and Connection Points

The Quadratus Lumborum Muscle is a thick, vertical muscle located in the deep layer of the lumbar region and plays an important role in trunk stability. It originates from the wings of the ilium at the upper part of the pelvis, specifically from an area approximately 5 centimeters along the medial lip of the iliac crest, and from an aponeurotic structure via the iliolumbar ligament. The fibers extend upwards to attach to the lower posterior border of the twelfth rib and the transverse processes of the first four lumbar vertebrae via four small tendons. The number of these attachment points, and particularly the degree of rib attachment, can vary anatomically among individuals. In some individuals, a second muscle segment with a different origin, located anterior to the main muscle, may be present. This variation should be considered, particularly in clinical and surgical evaluations.

Embryological Origin and Classification

The Quadratus Lumborum is a muscle of somitic mesenchyme, embryologically derived from paraxial mesoderm. Developing from the dorsal muscle mass, this muscle is classified as a muscle that specifically provides postural control and stabilizes the lumbar spine. It is located anterior to the deep back muscles and is closely related to the abdomino-lumbar muscle group.

Nerve Innervation and Vascular Supply

The Quadratus Lumborum muscle is innervated by the T12 intercostal nerve and the ventral branches of the L1–L4 spinal nerves. This neural supply allows the muscle to perform both segmental and global postural control functions. Its vascular supply is via the lumbar arteries, iliolumbar artery, and subcostal artery. These vessels should be carefully monitored due to the muscle's deep location.

Function and Clinical Relationships

When contracted unilaterally, the quadratus lumborum muscle provides lateral flexion of the lumbar spine, meaning it helps to bend the trunk to the side. It also lifts the pelvis, achieving unilateral pelvic elevation. The combined contraction of both muscles allows for extension of the lumbar spine, meaning it bends the trunk backward. This muscle plays a key role in maintaining upright posture and ensuring spinal stability. Clinically, quadratus lumborum muscle dysfunction is a common cause of low back pain. Conditions that lead to postural disorders, such as prolonged sitting, desk work, and driving, can lead to muscle fatigue and chronic spasm in this muscle. Kyphosis, weak gluteal muscles, and pelvic imbalances can also increase the load on this muscle, triggering pain. Overuse, trauma, or strain can increase muscle tenderness, leading to limited mobility in the lumbar region. Postural analysis, along with physical examination, and, when necessary, imaging methods can be used in the diagnosis of such clinical conditions. The rehabilitation process; should include postural awareness, pelvic balance exercises and targeted mobilization techniques.