The iliacus is the flat triangular muscle that fills the iliac fossa, joining the psoas major to form the iliopsoas as they pass under the inguinal ligament. While the psoas major has a long moment arm for hip flexion from standing, the iliacus is particularly active in the last 30 to 40 degrees of hip flexion from neutral, the range used in sitting up from lying. Its broad iliac fossa origin gives it a large cross-sectional area making it a powerful hip flexor in that range.
| Origin | Iliac fossa, iliac crest, and anterior inferior iliac spine |
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| Insertion | Lesser trochanter via the iliopsoas tendon alongside the psoas major |
| Nerve Supply | Femoral nerve (L2, L3, L4) |
| Blood Supply | Iliolumbar artery |
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Unlike the psoas major which also acts on the lumbar spine, the iliacus acts purely on the hip joint, making it the preferred hip flexor in contexts where lumbar spine loading must be minimised, such as in hip flexor rehabilitation after lumbar surgery.
Iliacus haematoma from anticoagulant therapy, trauma, or haemophilia produces hip flexion contracture and femoral nerve compression as the expanding haematoma compresses the nerve against the iliacus fascia, the iliopsoas haematoma syndrome. CT confirms the diagnosis and size. Iliacus muscle tears during sprinting produce deep groin pain with hip flexion weakness distinct from hip flexor tendon injuries.
The iliacus is palpable with deep pressure inside the iliac crest pressing posteroinferiorly into the iliac fossa with the hip slightly flexed and externally rotated to relax the overlying abdominal wall muscles.
Iliacus compartment bleeding from anticoagulation or trauma producing femoral nerve compression with quadriceps weakness and anterior thigh numbness, managed with anticoagulation reversal and rarely surgical decompression.
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