Iliopsoas muscle

Psoas major

Psoas major is a thick and powerful muscle situated on the posterior abdominal wall, lateral to the vertebral column. It originates from the transverse processes of all of the lumbar vertebrae, the anterolateral surfaces of the bodies of T12-L5 vertebrae and the interposed intervertebral discs. The muscle passes inferiorly towards the pelvic brim.

Before crossing the pelvic brim and entering the anterior thigh, the lateral fibers of the psoas major join the fibers from the iliacus to form the large iliopsoas muscle. The merged fibers then pass posterior to the inguinal ligament and anterior to the capsule of the hip joint. The inferior portion of the iliopsoas below the inguinal ligament forms a part of the floor of the femoral triangle. The iliopsoas finally converges into a tendon that inserts onto the lesser trochanter of the femur.

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The psoas major is innervated by the anterior rami of the lumbar spinal nerves, mainly L1 and L2, with some contributions from L3 and L4. Its blood supply is derived mainly from the iliolumbar branch of the internal iliac artery, with contributions from the obturator and lumbar arteries, as well as branches of the external iliac and femoral arteries.

Iliacus

The iliacus is a triangular-shaped muscle that arises from the superior two-thirds of the iliac fossa, the iliac crest and the lateral aspect of the sacrum. Some additional muscle fibers originate from the anterior sacroiliac and iliolumbar ligaments that connect the pelvis to the vertebral column. The origin of the iliacus spans anteriorly in the iliac fossa as far as the anterior superior and anterior inferior iliac spines. Here, the iliacus receives a tiny slip of muscle stemming from the upper part of the hip joint capsule known as the iliocapsularis.

The fibres of iliacus then merge with the most lateral fibres of psoas major to form the iliopsoas muscle, that passes anterior to the capsule of the hip joint to insert onto the lesser trochanter of the femur. The iliopsoas tendon is separated from the hip joint by a large subtendinous iliac bursa, which sometimes communicates with the joint cavity.

The iliacus is innervated by branches of the femoral nerve (L2, L3). It receives the same blood supply as the psoas major, derived from the branches of the iliolumbar, lumbar, obturator, external iliac and femoral arteries.

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Function

The iliopsoas muscle is the strongest flexor of the hip joint. Simultaneous contraction of the psoas major and iliacus muscles produces a powerful flexion of the thigh at the hip joint. However, psoas major can independently act on its attachment on the lumbar spine when its distal end is fixed. Thereby, bilateral contraction of the psoas major produces a flexion of the trunk at the hip joint, while unilateral contraction results in lateral flexion of the trunk.

In the symmetrical upright stance, iliopsoas maintains normal lumbar lordosis during standing, and, indirectly, the compensatory kyphosis of the thoracic vertebral column. Iliopsoas acts as the antagonist of the gluteus maximus muscle and the hamstring muscles (biceps femoris, semitendinosus and semimembranosus).

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