Iliacus muscle

At a glance

English

Iliacus muscle

Latin (TA2)

Musculus iliacus

System

Muscle

Region

Lower limb

Side

Paired, left and right

About the iliacus muscle

Iliacus is a fan-shaped muscle that fills the shallow bowl on the inner side of the ilium, the iliac fossa. Below it merges with psoas major to form iliopsoas, the strongest flexor of the hip, and the two are often described together. Of the pair, iliacus is the lateral, flatter part.

Its fibres arise from the upper two-thirds of the iliac fossa, the inner lip of the iliac crest, the front of the sacroiliac and iliolumbar ligaments and the ala of the sacrum, with some fibres from near the anterior inferior iliac spine and the hip capsule. They converge downwards, most joining the lateral side of the psoas tendon. The combined muscle passes beneath the inguinal ligament through the muscular compartment, the lacuna musculorum, and over the iliopubic eminence and the front of the hip joint, separated from it by a large bursa. It ends on the femur's lesser trochanter, with many iliacus fibres attaching directly to the bone in front of and just below the trochanter.

Iliacus is covered by the iliac fascia, which separates it from the abdominal contents; on the right the caecum and appendix lie in front of it, on the left the sigmoid colon. The femoral nerve runs down in the groove between iliacus and psoas, and supplies iliacus, while psoas takes branches directly from the lumbar plexus.

With psoas it flexes the thigh at the hip, as in lifting the knee when climbing or running. With the thigh fixed, it bends the pelvis and trunk forwards, as in sitting up from lying. Psoas crosses the spine as well, but iliacus acts on the hip joint alone. Bleeding into the muscle, for example in people on anticoagulants or with haemophilia, can press on the femoral nerve and cause thigh weakness and numbness. Infection or an abscess spreading within its fascia may track down into the groin.

Attachments, action and supply

Origin

Upper two-thirds of the iliac fossa, inner lip of the iliac crest, ala of the sacrum and the anterior sacroiliac and iliolumbar ligaments.

Insertion

Lesser trochanter of the femur, through the psoas tendon and directly on the bone just below it.

Action

Flexes the thigh at the hip; with the thigh fixed, flexes the pelvis and trunk forward.

Nerve supply

Femoral nerve (L2, L3).

Blood supply

Iliac branch of the iliolumbar artery, deep circumflex iliac artery, obturator artery and branches of the femoral artery.