Origin: Iliac crest andiliolumbar ligament
Insertion: Last rib and transverse processes of lumbar vertebrae
Artery: Lumbar arteries, Iliolumbar artery
Nerve: Anterior branches of T12, L1, L2, L3, L4
Action: Alone, lateral flexion of vertebral column; Together,depression of thoracic rib cage
The Quadratus lumborum is irregularly quadrilateral in shape, and broader below than above. It arises by aponeurotic fibers from the iliolumbar ligament and the adjacent portion of the iliac crest for about 5 cm., and is inserted into the lower border of the last rib for about half its length, and by four small tendons into the apices of the transverse processes of the upper four lumbar vertebrae. 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 vertebrae, and is inserted into the lower margin of the last rib. In front of the Quadratus lumborum are the colon, the kidney, the Psoas major and minor, and the diaphragm; between the fascia and the muscle are the twelfth thoracic, ilioinguinal, and iliohypogastric nerves.
Variations.—The number of attachments to the vertebrae and the extent of its attachment to the last rib vary.
Nerve Supply.—The twelfth thoracic and first and second lumbar nerves supply this muscle.
Actions.—The Quadratus lumborum draws down the last rib, and acts as a muscle of inspiration by helping to fix the origin of the diaphragm. If the thorax and vertebral column are fixed, it may act upon the pelvis, raising it toward its own side when only one muscle is put in action; and when both muscles act together, either from below or above, they flex the trunk.