Quadratus lumborum muscle

At a glance

English

Quadratus lumborum muscle

Latin (TA2)

Musculus quadratus lumborum

System

Muscle

Region

Back

Side

Paired, left and right

About the quadratus lumborum muscle

Quadratus lumborum, often shortened to QL, forms much of the back wall of the abdomen on each side of the lumbar spine. It is irregular and roughly rectangular and lies in its own compartment of the thoracolumbar fascia, deep to the erector spinae and beside psoas major. Although it is a muscle of the abdominal wall, it is usually discussed with the back because of where it lies and what it does.

Below, it arises by aponeurotic fibres from the iliolumbar ligament and from a few centimetres of the iliac crest's inner lip. Above, it attaches along the medial half of the lower border of the twelfth rib and, by small tendons, to the tips of the transverse processes of L1 to L4. Some people have an extra front layer running from lower lumbar transverse processes to the last rib. In front of the muscle lie the kidney, the colon, psoas and the back of the diaphragm, and the subcostal, iliohypogastric and ilioinguinal nerves cross its front surface.

Contracting on one side, the muscle bends the lumbar spine to that side, hitches the hip up on that side, or both, as when swinging a leg through while walking. Contracting on both sides, it helps to extend the lumbar spine and keeps the pelvis and lower spine steady. It also holds the twelfth rib down, so the diaphragm has a fixed base during deep breathing in and the rib is anchored during forced breathing out.

It is supplied by the T12 to L4 ventral rami and by the lumbar arteries, with the subcostal and iliolumbar arteries. Prolonged sitting, weak gluteal muscles, scoliosis and a rounded upper back can overload it, and it is a frequent source of one-sided low back pain with tenderness over the iliac crest. Anaesthetists inject around it to block pain after abdominal surgery.

Attachments, action and supply

Origin

Iliolumbar ligament and inner lip of the iliac crest.

Insertion

Medial half of the lower border of the twelfth rib and tips of the L1 to L4 transverse processes.

Action

Bends the lumbar spine to its own side and hitches the hip; helps extend the lumbar spine on both sides; fixes the twelfth rib in breathing.

Nerve supply

T12 to L4 ventral rami.

Blood supply

Lumbar arteries, with branches of the subcostal and iliolumbar arteries.