Psoas major muscle

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

Summary

• origin: vertebral bodies, intervertebral discs and transverse processes of T12 to L5
• insertion: lesser trochanter of the femur
• innervation: branches of the L1-L3 roots of the lumbar plexus
• action: lateral flexion of the trunk; stabiliser and flexor of the hip

Gross anatomy

Arising from the transverse processes, and lateral aspects of the vertebral bodies and intervertebral discs of T12 to L5. 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 branch emerge from it ³:

• anterior aspect: genitofemoral nerve
• lateral border: iliohypogastric, ilioinguinal, lateral femoral cutaneous and femoral nerves
• medial border: obturator nerve and lumbosacral trunk

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 iliopsoas muscle ^1-2. 

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

Action

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

Variant anatomy

• asymmetry of the psoas major muscle, which is usually no clinical significance ⁴

History and etymology

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

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

Related pathology

• psoas abscess
• psoas haematoma