Iliopsoas

Musculus iliopsoas

Also known as: hip flexor, psoas, iliacus and psoas

The composite of psoas major and iliacus, which share a tendon inserting on the lesser trochanter. It is the most powerful flexor of the hip.

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Overview

Psoas major arises from the lumbar spine and iliacus from the iliac fossa. They converge beneath the inguinal ligament, passing through the muscular lacuna lateral to the femoral vessels, and insert together on the lesser trochanter of the femur. The iliopsoas tendon crosses the front of the hip joint, separated from it by the iliopectineal (iliopsoas) bursa, which communicates with the joint in ≈15% of people. In the femoral triangle, iliopsoas forms the lateral part of the floor, with pectineus medially.

Iliopsoas is the chief flexor of the hip. It brings the thigh forward in walking and running and raises the trunk from lying (sit-ups) when the femur is fixed. Psoas also helps stabilise the lumbar spine and laterally flexes it. A weak lateral rotator role is described but is debated. Psoas is supplied directly by lumbar anterior rami (L1–L3) and iliacus by the femoral nerve (L2–L3).

Location: Posterior abdominal wall and iliac fossa, passing under the inguinal ligament to the anterior thigh.

Parts & subdivisions

Psoas major
T12–L5 vertebral bodies, discs and transverse processes → lesser trochanter.
Iliacus
Iliac fossa → lesser trochanter (with the psoas tendon).
Psoas minor
Inconstant (absent in ≈40%). T12–L1 → iliopubic eminence. Weak flexor of the trunk.

Anatomy

Origin: Psoas major: sides of the T12–L5 vertebral bodies, intervertebral discs and the transverse processes of the lumbar vertebrae. Iliacus: iliac crest, iliac fossa, ala of the sacrum and the anterior sacroiliac ligaments.
Insertion: Lesser trochanter of the femur (and the shaft just below it).
Action: Flexion of the hip; flexion of the trunk on the fixed femur; lateral flexion of the lumbar spine (psoas).
Innervation: Psoas major: anterior rami of L1–L3. Iliacus: femoral nerve (L2–L3).
Nerve roots: L1–L3
Arterial supply: Lumbar arteries (psoas); iliolumbar artery (iliacus branch); deep circumflex iliac artery; femoral artery branches.
Relations: The lumbar plexus forms within psoas. The femoral nerve emerges at its lateral border and the genitofemoral nerve on its anterior surface. It lies behind the kidney, ureter, caecum (right) and sigmoid colon (left).

Function

Flexes the thigh at the hip (most powerful hip flexor)
Flexes the trunk on the thigh (sitting up)
Stabilises and laterally flexes the lumbar spine

Clinical significance

Psoas sign

Pain on passive extension of the right hip (or on active flexion against resistance) suggests irritation of the psoas by an inflamed retrocaecal appendix or by a psoas abscess.

Psoas abscess

Infection within the psoas sheath, classically tuberculosis of the spine (Pott disease) or now more often from Crohn disease, diverticulitis or haematogenous S. aureus. It tracks down beneath the inguinal ligament and presents as a groin mass, with a flexed hip and a limp.

Testing hip flexion

Flexion of the hip against resistance tests L1–L2. The Thomas test reveals a fixed flexion deformity (tight iliopsoas): when the opposite hip is fully flexed, the tested thigh lifts off the couch.

Compiled from standard anatomy and physiology references — see the Muscular System sources. For education only; not medical advice.