The quadratus lumborum is a muscle of the posterior abdominal wall. It is a thick muscular sheet, quadrilateral in shape, and positioned superficial to the psoas major.
- Attachments: Originates from the iliac crest and iliolumbar ligament. The fibres travel superomedially, inserting onto the transverse processes of L1 – L4 and the inferior border of the 12th rib.
- Actions: Extension and lateral flexion of the vertebral column. It also fixes the 12th rib during inspiration, so that the contraction of diaphragm is more efficient.
- Innervation: Anterior rami of T12 – L4 nerves.
- Blood supply: Lumbar, median sacral, iliolumbar and subcostal arteries.