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Location & Overview
The quadratus lumborum muscle, abbreviated as the ‘QL muscle’, is located deep within the lower back musculature toward the posterior aspect of the body. It is classified as a muscle of the posterior abdominal wall and is located dorsal to the iliopsoas muscle. The QL muscle is quadrangular in shape, as reflected by the ‘quadratus’ part of its Latin name, although its shape may sometimes be irregular. It is positioned laterally on either side of the spine.[1,2]
The quadratus lumborum works alongside muscles such as the multifidus and erector spinae muscle group to extend the back and acts as an antagonist to the muscles of the anterolateral abdominal wall. However, the quadratus lumborum’s contractile force during trunk extension is comparatively weak compared with the multifidus and erector spinae muscle group. Nevertheless, it can still contribute to stabilization of the spine and pelvis, and it also assists with inspiration by bracing the twelfth rib.[1,2]
Origin & Insertion
The quadratus lumborum muscle originates on the inner lip of the iliac crest and iliolumbar ligament. The iliolumbar ligament is a strong ligament connecting the end of the transverse process of the fifth lumbar vertebra (L5) to the posterior and inner lip of the iliac crest, as shown in the images below. The quadratus lumborum inserts onto the internal surface and inferior border of the twelfth rib and the transverse processes of vertebrae L1-L4.[1]
Action & Function
The contraction of both sides of the quadratus lumborum muscle (bilateral contraction) assists in extending the trunk. On the other hand, the contraction of just one side of the muscle (unilateral contraction) helps with lateral flexion of the trunk.[1] The quadratus lumborum can also assist with inspiration (breathing in). It is thought that the quadratus lumborum is primarily designed as a respiratory muscle by bracing or anchoring the twelfth rib, allowing for a stable base for the diaphragm.[3]
Compared with other muscles, such as the erector spinae, the quadratus lumborum produces comparatively weaker forces when extending and laterally flexing the trunk. For instance, the quadratus lumborum can generate only up to 10% of the force that the erector spinae produces during these movements.[3,1] Even the compression forces produced by the quadratus lumborum are comparatively weak compared with those of the erector spinae and multifidus muscles. Therefore, it does not play a significant role in stabilization of the lumbar spine either, though it can contribute somewhat. This is thought to be because of its comparatively small size and limited number of muscle fascicles compared with the erector spinae and multifidus.[3]
Innervation
The quadratus lumborum muscle is innervated by segmental nerves from T12 and the upper lumbar spinal nerves. The T12 contribution is usually referred to as the subcostal nerve, rather than the 12th thoracic intercostal nerve. This nerve runs along the anterior surface of the quadratus lumborum before continuing toward the abdominal wall.[4,5]
The lumbar contribution is commonly described as coming from the anterior rami of the upper lumbar spinal nerves, especially L1-L4. Branches of the lumbar plexus run along the anterior surface of the quadratus lumborum. The iliohypogastric and ilioinguinal nerves, which arise from L1, also pass across the anterior aspect of the muscle before continuing toward the abdominal wall.[4,5,6]
Blood Supply
Blood is supplied to the quadratus lumborum mainly by branches of the lumbar arteries and by a lumbar branch of the iliolumbar artery. The lumbar arteries arise from the abdominal aorta and pass toward the posterior abdominal wall, where they supply muscles including the quadratus lumborum. The lumbar branch of the iliolumbar artery also supplies the quadratus lumborum and nearby posterior abdominal wall structures.[4,5,7]
Additional arterial contributions can come from branches of the subcostal artery and from the lowest lumbar arteries, which may arise from the median sacral artery. These smaller contributions help explain why the quadratus lumborum is usually described as having a segmental blood supply rather than relying on a single artery.[4]
References
- [1] Bordoni B, Varacallo M. Anatomy, Abdomen and Pelvis, Quadratus Lumborum. (Updated 2022 Jul 18). In: StatPearls (Internet). Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK535407/
- [2] Moore KL, Agur AMR, Dalley AF. Clinically Oriented Anatomy. 8th ed. Philadelphia: Lippincott Williams & Wilkins; 2017
- [3] Phillips S, Mercer S, Bogduk N. Anatomy and biomechanics of quadratus lumborum. Proc Inst Mech Eng H. 2008 Feb;222(2):151-9. doi: 10.1243/09544119JEIM266. PMID: 18441751.
- [4] Standring S. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. Amsterdam: Elsevier; 2021.
- [5] Dalley AF, Agur AMR. Moore’s Clinically Oriented Anatomy. 9th ed. Philadelphia: Wolters Kluwer; 2023.
- [6] Grzonkowska M, Baumgart M, Badura M, Dombek M, Wiśniewski M, Paruszewska-Achtel M, Szpinda M. Quantitative anatomy of the growing quadratus lumborum in the human foetus. Surg Radiol Anat. 2018 Jan;40(1):91-98. doi: 10.1007/s00276-017-1901-4. Epub 2017 Jul 29. PMID: 28756538; PMCID: PMC5820394.
- [7] Beveridge TS, Power A, Johnson M, Power NE, Allman BL. The lumbar arteries and veins: Quantification of variable anatomical positioning with application to retroperitoneal surgery. Clin Anat. 2015 Jul;28(5):649-60. doi: 10.1002/ca.22504. Epub 2015 Feb 2. PMID: 25644404.
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