The Quadratus Lumborum (QL) is a deep muscle in the lower back that acts as a primary stabilizer for the spine. It is frequently implicated in chronic lower back pain. To understand how the QL functions, it is necessary to examine the complex network of nerves that control it. This muscle’s unique anatomical location and multiple nerve supply are factors in various musculoskeletal issues.
Defining the Quadratus Lumborum
The Quadratus Lumborum is an irregularly shaped, quadrilateral muscle. It is located deep within the posterior abdominal wall, positioned on either side of the lumbar spine, and is considered the deepest of the back muscles. This muscle is situated lateral to the lumbar vertebrae and superficial to the psoas major muscle, forming a part of the thoracolumbar fascia system.
The QL has extensive attachments that anchor the lower rib cage to the pelvis and lumbar spine. Inferiorly, the muscle originates from the internal lip of the iliac crest and the iliolumbar ligament. The fibers extend upward to insert superiorly onto the inferior border of the 12th rib and the transverse processes of the first four lumbar vertebrae (L1 through L4). This multi-point connection links the movements of the trunk and the hips.
Primary Functions of the QL
The primary role of the QL is to stabilize the lumbar spine, especially during movements like walking or lifting objects. It works as an antagonist to the abdominal muscles, helping to maintain an upright posture and control load distribution across the lower back. When the muscles on both sides contract together, they contribute to the extension of the lumbar spine.
When only one side contracts, the muscle facilitates lateral flexion, or side-bending, of the trunk toward the active side. The QL also plays an accessory role in respiration by fixing the 12th rib during forced exhalation. This allows the diaphragm to contract more efficiently during inhalation. The muscle also assists in “hip hiking,” which involves elevating the pelvis on one side during the swing phase of walking.
The Specific Nerves Responsible for QL Control
The Quadratus Lumborum receives its motor control from a segmental nerve supply, meaning multiple levels of the spinal cord contribute to its function. The innervation primarily comes from the ventral rami, which are the anterior divisions of the spinal nerves. This nerve network originates from both the thoracic and lumbar spine levels.
Specifically, the muscle is supplied by the 12th thoracic nerve, known as the Subcostal nerve (T12), as well as branches from the Lumbar Plexus (L1-L4). The Subcostal nerve provides the most superior innervation, while the L1-L4 segments contribute sequentially.
Several named nerves branching from the Lumbar Plexus also contribute to the QL’s innervation, including the Iliohypogastric nerve (L1) and the Ilioinguinal nerve (L1). These nerves pass through and provide branches to the QL, linking its control to the network supplying the lower abdominal wall and groin area. The nerves enter the muscle predominantly from its ventral surface.
Clinical Significance of QL Nerve Dysfunction
Understanding the QL’s innervation is important because irritation or compression of its nerve supply can lead to complex pain syndromes. Since the T12, Iliohypogastric, and Ilioinguinal nerves pass through or lie near the QL, muscle tightness can physically entrap or inflame these nervous tissues. This nerve irritation can produce symptoms that mimic other conditions, making a precise diagnosis difficult.
Myofascial pain syndrome in the QL is a common cause of chronic low back pain, resulting from spasm and stiffness in the muscle fibers. This pain often refers to distant areas, sometimes radiating into the buttocks, groin, or hip. The referred pain patterns can mimic sciatica, leading to misdiagnosis.
Postural imbalances, such as those caused by prolonged sitting, can lead to the QL becoming overworked as it compensates for weaker core muscles. When the muscle becomes dysfunctional, it alters the distribution of loads in the lumbar area, perpetuating chronic pain. Diagnostic tools like ultrasound-guided nerve blocks targeting the QL’s fascial plane have become useful for both diagnosis and pain management.