Anatomy · Muscles

Psoas major

The hip flexor that is also the tenderloin

Posterior abdominal wall · flexes the hip

A long, thick muscle that lies alongside the lumbar spine, passes over the brim of the pelvis and under the inguinal ligament, and ends on the top of the thigh bone. Joined by iliacus it forms iliopsoas, the strongest flexor of the hip. In a butcher's carcass it is the tenderloin, or fillet.

How to spot them

▸A spindle-shaped muscle beside the lumbar vertebrae, running down and out towards the front of the hip
▸Its tendon is joined by the iliacus to reach the lesser trochanter of the femur
▸The lumbar plexus is embedded in its substance, and the ureter runs down on its front
▸Nerve supply from the front rami of L1 to L3

What to know

The muscle that lies beside the lumbar spine and flexes the thigh at the hip, its tendon reaching the lesser trochanter of the femur

Psoas major — with iliacus it forms iliopsoas.

The cut of beef sold as fillet, or tenderloin, is this muscle of the animal

Psoas major — in the ox it is the most tender cut because it does so little work.

Its tendon is joined by a fan-shaped muscle from the inner surface of the hip bone before both reach the top of the femur

Psoas major — the two together form iliopsoas, joined with iliacus.

A doctor lifts the thigh against resistance in a patient with abdominal pain; pain suggests an inflamed appendix or an abscess resting on it

Psoas major — the psoas sign.

A muscle threaded by the lumbar plexus, which forms within its substance

Psoas major — the plexus branches emerge from its borders and front.

Collections of pus from tuberculosis of the spine can track down its sheath to appear in the groin

Psoas major — a psoas abscess, classically from Pott's disease.

Runs from the twelfth thoracic vertebra to the fifth lumbar vertebra, then passes under the inguinal ligament to the thigh

Psoas major — attaches to the vertebral bodies and transverse processes of T12 to L5.

Key ideas

Because it crosses the hip joint at the front and attaches to the spine, tightness in the muscle tilts the pelvis forward and can pull on the lumbar spine; this is the basis of many claims about desk-work backache.
It is the deep posterior abdominal wall muscle that ties the spine to the leg, working with quadratus lumborum and iliacus in walking and in raising the trunk from lying flat.

Quick-fire facts

•Origin: sides of the vertebral bodies and discs from T12 to L4, and the front of the transverse processes of L1 to L5
•Insertion: lesser trochanter of the femur, by a tendon shared with iliacus (iliopsoas)
•Action: flexes the thigh at the hip, or the trunk on the thigh when the leg is fixed; helps to flex the spine sideways
•Nerve supply: anterior rami of L1 to L3, directly from the lumbar plexus
•Blood supply: lumbar branch of the iliolumbar artery, along with lumbar arteries
•Iliopsoas passes under the inguinal ligament and lies in front of the hip joint, with a bursa between them
•The lumbar plexus lies within the muscle; the genitofemoral nerve comes out on its front surface
•The ureter and the gonadal vessels lie on its front; a psoas abscess can spread down to the groin
•Psoas minor, a small tendon lying on top of it, is absent in some 40 to 50 per cent of people
•Beef tenderloin, the fillet in fillet steak, is the psoas major of the animal

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Sources

TeachMeAnatomy — Psoas major