The quadratus lumborum muscle, informally called the QL, is a paired muscle of the left and right posterior abdominal wall. It is the deepest abdominal muscle, and commonly referred to as a back muscle. Each is irregular and quadrilateral in shape. The quadratus lumborum muscles originate from the wings of the ilium; their insertions are on the transverse processes of the upper four lumbar vertebrae plus the lower posterior border of the twelfth rib. Contraction of one of the pair of muscles causes lateral flexion of the lumbar spine, elevation of the pelvis, or both. Contraction of both causes extension of the lumbar spine. A disorder of the quadratus lumborum muscles is pain due to muscle fatigue from constant contraction due to prolonged sitting, such as at a computer or in a car. Kyphosis and weak gluteal muscles can also contribute to the likelihood of quadratus lumborum pain. Structure The quadratus lumborum muscle originates by aponeurotic fibers into the iliolumbar ligament and the internal lip of the iliac crest for about . It inserts from the lower border of the last rib for about half its length and by four small tendons from the apices of the transverse processes of the upper four lumbar vertebrae. The number of attachments to the vertebræ, and the extent of its attachment to the last rib, may vary. Also, occasionally, a second portion of this muscle is found in front of the preceding. It arises from the upper borders of the transverse processes of the lower three or four lumbar vertebræ, and is inserted into the lower margin of the last rib. Relationships Anterior to the quadratus lumborum are the colon, the kidney, the psoas major muscle, (if present) the psoas minor muscle, and the diaphragm; between the fascia and the muscle are the twelfth thoracic, ilioinguinal, and iliohypogastric nerves. The quadratus lumborum muscle is a continuation of transverse abdominal muscle. Nerve supply Anterior branches of the ventral rami of T12 to L4. Functions The quadratus lumborum can perform four actions: Lateral flexion of vertebral column, with ipsilateral contraction Extension of lumbar vertebral column, with bilateral contraction (based on line of force passing ~3.5 cm posterior L3 rotation axis) Fixes the 12th rib during forced expiration. The quadratus lumborum assists the diaphragm in inhalation Elevates the Ilium (bone), with ipsilateral contraction ("hip hiking") Additional functions: Vertical stabilization of pelvis, lumbar spine, and lumbosacral junction.