Quadratus lumborum muscle

The quadratus lumborum muscle is an irregular quadrilateral muscle that forms part of the posterior abdominal wall.

Summary

• location:
  • dorsal abdominal wall
  • medial half of inferior margins of 12th ribs and upper four lumbar transverse processes
• blood supply: 
  • branches of the lumbar arteries
  • other smaller arterial branches, as described below
• innervation:
  • ventral rami of the 12th thoracic nerve
  • L1-L4 spinal nerves
• relations: thick, irregular, quadrilateral-shaped muscle sheet that lies in the posterior abdominal wall

Gross anatomy

Attachments

• insertion: iliolumbar ligament and internal rim of iliac crest
• action: extension and lateral flexion of vertebral column; fixes 12th rib during inspiration to stabilise the diaphragm

Relations

The muscle is a thick, irregular, quadrilateral-shaped muscle sheet that lies in the posterior abdominal wall.

Anterior relations:

• colon
• kidney
• psoas muscle (major and minor)
• diaphragm
• subcostal, ilioinguinal and iliohypogastric nerves

Blood supply

The quadratus lumborum muscleis supplied by:

• branches of the lumbar arteries
• the arteria lumbalis ima
• the lumbar branch of the iliolumbar artery
• branches of the subcostal artery

Nerve supply

• ventral rami of the 12th thoracic nerve
• L1-L4 spinal nerves

Action

Multiple actions, including:

• muscle of inspiration
  • fixation of the last rib
  • stabilisation of lower attachments of the diaphragm
    • proposed to provide a base for controlled diaphragmatic relaxation to facilitate precise adjustments required for speech and singing ¹
• when one muscle contracts, lateral flexion of vertebral column 
• when both sides contract, extends the lumbar spine

Variant anatomy

The number and extent of attachments to the last rib vary.

Related pathology

Implications in unilateral lower back pain.

May be enlarged in fast-bowl cricket players who injure their L4 pars interarticularis ³.