The iliopsoas combines the iliacus from the iliac fossa and the psoas major from the lumbar vertebral bodies into a single powerful tendon inserting on the lesser trochanter, forming the most powerful hip flexor and the only muscle connecting the lumbar spine to the femur. Psoas tightness, overactivity, and trigger points are implicated in a large proportion of low back pain presentations because its lumbar vertebral origin means it compresses the lumbar spine on contraction.
| Origin | Iliac fossa, iliac crest, and anterior inferior iliac spine; Lateral surfaces and transverse processes of T12 and L1 through L5 vertebral bodies |
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| Insertion | Lesser trochanter of the femur via a combined tendon |
| Nerve Supply | Femoral nerve (L2, L3, L4) for iliacus; Ventral rami of L1, L2, L3 for psoas major |
| Blood Supply | Medial circumflex femoral artery; Iliolumbar artery |
| Actions | Flexion of the hip — the most powerful hip flexor; External rotation of the hip when acting at end range; Flexion of the lumbar spine when acting with the hip fixed (psoas major) |
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Its dual origin from the lumbar vertebral bodies and the iliac fossa means that when the hip is fixed it acts on the lumbar spine (increasing lordosis and compressing lumbar discs and facet joints), and when the spine is fixed it flexes the hip with enormous force for activities like kicking, sprinting, and sitting up.
Tight iliopsoas from prolonged sitting is a contributing factor in anterior pelvic tilt, lumbar hyperlordosis, and anterior hip pain in desk-working populations. The Thomas test assesses iliopsoas flexibility by passive hip extension with the opposite hip and knee held flexed. Psoas abscess from vertebral osteomyelitis or Crohn disease presents as a tender hip flexion contracture with fever, and is confirmed by CT showing a fluid collection within the psoas sheath.
The iliacus is palpable with deep pressure applied inside the iliac fossa medial to the ASIS. The psoas is not directly palpable externally. The combined iliopsoas tendon is palpable in the femoral triangle medial to the sartorius during resisted hip flexion.
Overuse degeneration at the lesser trochanter insertion producing anterior groin pain that worsens with resisted hip flexion and passive hip extension, common in runners, dancers, and cyclists.
Bursal inflammation between the iliopsoas tendon and the anterior hip capsule producing anterior groin swelling and pain, often communicating with the hip joint and requiring investigation of the underlying joint pathology.
