Iliacus
The pelvic half of the iliopsoas — a fan of muscle lining the inside of your hip bone.
- Type
- Muscle
- Region
- Hip
- Standard name
- m. iliacus
- Origin
- Upper two-thirds of the iliac fossa — the broad inner hollow of the pelvic bone
- Insertion
- Lesser trochanter of the femur, via the common iliopsoas tendon
- Actions
- hip flexion
- Verify
- TA2 2594 TA98 A04.7.02.003
What it is
A broad, flat, triangular muscle that lines the inner surface of the ilium, filling the shallow bowl called the iliac fossa. Its fibres converge downwards and join the tendon of psoas major to insert together on the lesser trochanter.
Unlike psoas major, it has no attachment to the spine at all. It runs pelvis to femur, which means it acts on one joint only.
What it does
Flexes the hip. That is the whole list.
That simplicity is actually useful for understanding the pair: iliacus is a pure hip flexor, so anything iliopsoas does at the spine must be coming from psoas major. The two halves of the combined muscle are not equivalent.
What loads it
The same movements that load iliopsoas as a whole — raising the knee against resistance, running, climbing, holding the legs elevated. It cannot be loaded independently of psoas major, since they share an insertion tendon.
What is commonly got wrong about it
That it is interchangeable with psoas. They are routinely treated as one thing, and for hip flexion that is fair. For anything involving the lower back it is not, because iliacus does not reach the spine.
That hip flexor stretching targets it specifically. A stretch that extends the hip lengthens both halves, plus rectus femoris, plus the front of the hip capsule. Nothing about the position selects one structure.