Iliopsoas

Psoas major originates along the lateral surfaces of the vertebral bodies of TXII and LI-LIV and their associated invertebral discs. Psoas minor, present in only some 50 per cent of the population, originates at the transverse processes of LI-LV. Iliacus originates in the Iliac fossa of the pelvis.

Psoas major unites with iliacus at the level of the inguinal ligament and crosses the hip joint to insert on the lesser trochanter. Psoas minor inserts at the iliopectineal arch, the thickened band at the iliac fascia which separates the muscular lacuna from the vascular lacuna.

Flexion of hip

Femoral nerve, L1, L2

Medial femoral circumflex artery, iliolumbar artery

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Properties:

Strongest of the hip flexors (rectus femoris, sartorius, and tensor fasciae latae), iliopsoas is important for standing, walking, and running.

It is, however, a typical posture muscle dominated by slow-twitch red type 1 fibers, and is therefore susceptible to pathological shortening or contracture (especially in older people with a sedentary lifestyle) and requires regular stretching to maintain normal tone. Such shortening can lead to increased anterior pelvic tilt and lumbar lordosis (unilateral shortening), and limitation of hip extension (bilateral weakness).

The iliopsoas muscle is covered by the iliopsoas fascia, which begins as a strong tube-shaped psoas fascia, which surround the psoas major muscle as it passes under the medial arcuate ligament, Together with the iliac fascia, it continues down to the inguinal ligament where it forms the iliopectineal arch which separates the muscular and vascular lacunae.

Owing to its proximal attachments, a pus-filled abscess, as may occur in lumbar tuberculosis, may drain inferiorly into the upper medial thigh and present as a swelling in the region.