Psoas major

Dr Henry Knipe and A.Prof Frank Gaillard et al.

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

The psoas muscle lies in the gutter between the bodies and transverse process of the lumbar vertebra. Its vertebral attachment is to T12-L4 discs, the adjacent vertebral bodies and to fibrous arches that span the concavities of the sides of the upper four vertebral bodies (superficial part). Thus there is one continuous attachment from the lower border of T12 to the upper border of L5. It also attaches to the medial ends of the transverse process of L1 to L5 (deep part). 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 psoas fascia (part of the iliac fascia) invests the surface of the muscle, attached to the vertebral bodies, fibrous arches, and the transverse process, and extends along the pelvic brim attached to the iliopubic eminence at the margins of the muscle. The lateral edge blends with the anterior layer of the lumbar fascia (over quadratus lumborum). 

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 iliac 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 stabilize and flex the thigh 1-2. The antagonist is the gluteus maximus. 

Lower abdomen:

  • lumbrosacral plexus embedded within the muscle
  • anteriorly: genitofemoral nerve
  • laterally: iliohypogastric, ilioinguinal, lateral femoral cutaneous and femoral;
  • medially: obturator nerve and lumbrosacral trunk
  • posteriorly: part of the external vertebral venous plexus (in front of the transverse process), 4 lumbar arteries and veins pass beneath the four arches and run laterally behind the psoas accompanied by the sympathetic rami 

Pelvis and lower limb:

Lumbosacral triangle of Marcille:

  • contents from medial to lateral: obturator nerve, iliolumbar artery, lumbrosacral trunk, sympathetic trunk 
  • asymmetry of the psoas major muscle, which is usually not clinically significant 4
  • associated with psoas minor - origin from T12 and L1 vertebrae and inserted on the arcuate line and iliopubic eminence

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

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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Cases and figures

  • Figure 1: Anterior hip muscles
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