Iliacus (Iliopsoas / Hip Flexor)

Origin, Insertion, Action & Nerve

Attachments explained

The Iliacus (Iliopsoas / Hip Flexor) is defined first by where it attaches. Its origin is the more fixed anchor, usually the proximal or more stable end that stays put during contraction, while its insertion is the more mobile point that is pulled toward the origin when the muscle shortens.

Action & function

When the Iliacus (Iliopsoas / Hip Flexor) contracts, it produces the following movements: Flexion of hip; assists flexion of trunk.

As part of the Gluteal & Hip group of the Lower Limb, it works alongside neighbouring muscles to generate smooth, coordinated movement and to stabilise the structures it acts on.

Nerve supply & clinical relevance

The Iliacus (Iliopsoas / Hip Flexor) receives its nerve supply from the Femoral nerve (L2, L3). This nerve carries fibres from spinal segment(s) L2, L3.

Because a muscle can only contract when its nerve is intact, injury to the Femoral nerve or to its spinal roots (L2, L3) can weaken or paralyse the Iliacus (Iliopsoas / Hip Flexor), impairing the movements it normally produces (flexion of hip). This is why knowing the innervation is central to localising nerve lesions in clinical practice.

How to study the Iliacus (Iliopsoas / Hip Flexor) (the OIANS method)

Study the iliacus as the pelvic partner of the psoas major, since the two blend into the iliopsoas and share both the lesser trochanter insertion and the powerful hip-flexion action. Picture the iliacus filling the iliac fossa and sweeping its fibres onto the psoas tendon, so learning the pair together covers most of what you need.

Fix the point of difference: the iliacus is supplied by the femoral nerve (L2, L3), whereas the psoas major receives lumbar anterior rami directly. Holding those two nerve supplies side by side is the reliable way to keep the two halves of the iliopsoas distinct on exams.

Frequently asked questions

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