Psoas major

The psoas major muscle (often referred to as the psoas muscle) is one of the muscles of the posterior abdominal wall and lies not in the retroperitoneum but posteriorly in the iliopsoas compartment.

The psoas muscle arises from the transverse processes, lateral aspects of the vertebral bodies and intervertebral discs of the T12-L5 vertebral column. It fuses with the iliacus muscle to form the iliopsoas muscle at the level of L5-S2 and passes inferiorly, deep to the inguinal ligament, to insert into the lesser trochanter of the femur 1-3.

The lumbar plexus is embedded within the muscle and its branches emerge from it 3:

The psoas muscle is enclosed by the psoas fascia and it is this that retains the pus in a psoas abscess 3

The muscle comes to lie medial to and fuses with the iliacus muscle, such that inferiorly the two are often referred to together as the iliopsoas muscle 1-2

The psoas muscle is innervated by the lumbar plexus via branches from L1-L3 (mainly L2) 3.

  • upper part: lumbar arteries
  • middle part: iliolumbar artery (main artery to muscle), deep circumflex and external iliac arteries
  • lower part: branches of the femoral artery 

The action of this muscle is complex, acting to both laterally flex the lumbar spine as well as stabilise and flex the thigh 1-2

  • asymmetry of the psoas major muscle, which is usually not clinically significant 4

From the Greek "psoa" meaning "loin" 3. The psoas muscle is referred to as the tenderloin by butchers.

Correct terminology is psoas major muscle (as opposed to just psoas muscle) to differentiate it from the psoas minor muscle.

Anatomy: Abdominopelvic
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Article information

rID: 10102
Section: Anatomy
Synonyms or Alternate Spellings:
  • Psoas muscle
  • Psoas major muscle

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