Psoas major muscle

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.

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

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:

• 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 the iliopsoas muscle ^1-2. 

Innervation

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 to both laterally flex the lumbar spine as well as stabilise and flex the thigh ^1-2. 

Variant anatomy

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

History and etymology

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.

Related pathology

• psoas abscess
• psoas haematoma