Psoas major

At a glance

English

Psoas major

Latin (TA2)

Psoas major

System

Muscle

Region

Lower limb

Side

Paired, left and right

About the psoas major

Psoas major is a long, spindle-shaped muscle on each side of the lumbar spine, at the back of the abdominal cavity. It runs from the last thoracic vertebra down over the brim of the pelvis and beneath the inguinal ligament into the thigh. Low down it merges with iliacus to form iliopsoas, the strongest flexor of the hip. The name comes from a Greek word for the loins, and in meat animals the same muscle is the tenderloin.

Its origin has two layers. The superficial layer comes from the sides of the bodies of T12 to L4 and the discs between them, together with tendinous arches over the waists of the vertebrae. The deep layer arises from the front of the transverse processes of all five lumbar vertebrae. The lumbar plexus forms between the two layers, so its branches emerge from the edges and front of the muscle. Covered by iliac fascia, iliopsoas passes through the muscular lacuna under the inguinal ligament and ends on the lesser trochanter, with bursae separating its tendon from the hip capsule and from the bone.

With the trunk fixed, psoas major flexes the thigh, as in lifting the knee to climb a step. With the thigh fixed it does the reverse, bending the trunk and pelvis forwards, which is its role in a sit-up from lying. Acting on one side it bends the lumbar spine sideways. Through the lesser trochanter it may add slight lateral rotation of the thigh. Branches straight from the L1 to L3 anterior rami supply it, and its blood comes from the lumbar arteries and the iliolumbar artery.

Its relations make it clinically important. The kidney, ureter and lumbar sympathetic trunk lie on or beside it, and needles for a lumbar sympathetic block often pass through it. Infection from the spine or abdomen can track down its sheath as a psoas abscess, appearing as a swelling in the groin. Pain on stretching the hip into extension, the psoas sign, can point to inflammation next to the muscle, as in appendicitis. A smaller psoas minor lies in front of it in some people.

Attachments, action and supply

Origin

Sides of the bodies of T12 to L4 and their discs, and the transverse processes of L1 to L5.

Insertion

Lesser trochanter of the femur, through the iliopsoas tendon shared with iliacus.

Action

Flexes the hip; flexes the trunk on the thigh and bends the lumbar spine to the same side.

Nerve supply

Anterior rami of L1, L2 and L3.

Blood supply

Lumbar arteries and the iliolumbar artery, with branches of the external iliac and femoral arteries lower down.