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Quadratus lumborum muscle

The quadratus lumborum muscle (QLM) is a flat, quadrilateral muscle located in the posterior abdominal wall. It extends between the iliac crest and iliolumbar ligament inferiorly, and the 12th rib and transverse processes of L1–L4 vertebrae superiorly.

It lies deep to the erector spinae and psoas major muscles, forming part of the posterior abdominal wall, lateral to the lumbar spine. The muscle is enclosed within the thoracolumbar fascia, which connects it to the transversus abdominis and internal oblique muscles.

Functionally, the quadratus lumborum stabilizes the 12th rib during respiration, assists in lateral flexion of the trunk, and contributes to lumbar spine stabilization during movement. Clinically, it is an important site of chronic low back pain and plays a role in procedures such as QL blocks for regional anesthesia.

Synonyms

  • QL muscle

  • Quadratus muscle of the loins

  • Posterior abdominal wall stabilizer

Function

  • Unilateral contraction: lateral flexion of the trunk to the same side

  • Bilateral contraction: stabilizes lumbar spine and depresses thoracic cage

  • Fixes the 12th rib during inspiration and forced expiration

  • Contributes to core stability and load transfer in the posterior abdominal wall

Nerve Supply

  • Ventral rami of T12 (subcostal nerve) and L1–L4 spinal nerves

Arterial Supply

  • Lumbar arteries (branches of abdominal aorta)

  • Subcostal artery

  • Branches from iliolumbar artery

Venous Drainage

  • Follows arterial supply

  • Lumbar veins → ascending lumbar vein → azygos/hemiazygos system

  • Iliolumbar vein → common iliac vein

MRI Appearance

T1-weighted images:

  • Normal QL appears as intermediate signal muscle with visible fascicular architecture

  • Surrounded by bright retroperitoneal fat that provides contrast

T2-weighted images:

  • Normal muscle: intermediate-to-low signal intensity

  • Strain, edema, or hematoma: hyperintense signal areas

STIR:

  • Suppresses fat, highlighting muscle edema, inflammation, or hematoma as hyperintense foci

  • Sensitive for acute injuries and myofascial pain syndrome

T1 Fat-Saturated (Pre-contrast):

  • QL muscle shows intermediate signal lumen, standing out against fat-suppressed retroperitoneum

T1 Fat-Saturated Post-Contrast (Gadolinium):

  • Mild homogeneous enhancement in normal muscle

  • Pathology (myositis, abscess, tumor) enhances more markedly and heterogeneously

MRI Non-Contrast 3D Imaging:

  • Provides a 3D reconstruction of the muscle’s attachment to ribs, vertebrae, and iliac crest

  • Useful in preoperative mapping for regional anesthesia and spinal surgery

CT Appearance

CT Pre-Contrast:

  • Appears as a well-defined muscle density lateral to lumbar vertebrae

  • Detects calcifications or hematomas

CT Post-Contrast:

  • Enhances mildly and homogeneously in normal states

  • Highlights abscesses, tumors, or inflammatory changes within or adjacent to the muscle

  • 3D reconstructions show detailed relationships with ribs, spine, and iliac crest

MRI images

Quadratus lumborum muscle MRI axial image anatomy image -img-00000-00000

MRI images

Quadratus lumborum muscle MRI axial image anatomy image -img-00000-00000_00001

MRI images

Quadratus lumborum muscle MRI coronal image anatomy image -img-00000-00000

MRI image

Quadratus lumborum muscle mri axial anatomy image-img-00000-00000