Psoas major
Musculus psoas major
Also known as: psoas, tenderloin
A long, fusiform muscle running alongside the lumbar spine to the lesser trochanter. With iliacus it forms iliopsoas, and the lumbar plexus is embedded within it.
Overview
Psoas major arises from the sides of the T12–L5 vertebral bodies, the intervening discs and tendinous arches, and the lumbar transverse processes. It descends along the pelvic brim, passes behind the inguinal ligament, joins iliacus and inserts on the lesser trochanter. The lumbar plexus forms within its substance. Branches emerge from its lateral border (iliohypogastric, ilioinguinal, lateral femoral cutaneous and femoral nerves), its anterior surface (genitofemoral nerve) and its medial border (obturator nerve and lumbosacral trunk). It is enclosed in the psoas fascia, which is attached above to the medial arcuate ligament of the diaphragm. Infection can therefore track within the sheath from the spine to the groin.
Anatomy
- Origin
Bodies of T12–L5 and the intervening discs; transverse processes of L1–L5.
- Insertion
Lesser trochanter of the femur (with iliacus).
- Action
Flexion of the hip; lateral flexion of the lumbar spine; flexion of the trunk.
- Innervation
Direct branches from the anterior rami of the lumbar spinal nerves.
- Nerve roots
L1–L3
- Arterial supply
Lumbar arteries; iliolumbar artery; external iliac and femoral branches.
- Relations
The lumbar plexus lies within it. Its lateral border lies against quadratus lumborum. The sympathetic trunk lies on its medial edge and the vertebral bodies. The kidney and ureter lie anterior to it.
Function
- Flexes the hip
- Flexes and laterally bends the lumbar spine; stabilises it
Clinical significance
Retroperitoneal bleed
Haematoma within the psoas sheath, for example in patients on anticoagulants or after femoral catheterisation, compresses the femoral nerve. This causes quadriceps weakness and anterior thigh numbness.
Compiled from standard anatomy and physiology references — see the Muscular System sources. For education only; not medical advice.