Iliopsoas

The iliopsoas is a composite muscle formed by the psoas major and the iliacus. It is the primary flexor of the hip and plays a key role in stabilizing the pelvis and lumbar spine during movement.

Anatomy and attachments

The psoas major arises from the transverse processes, bodies, and intervertebral discs of the T12 to L5 vertebrae. The iliacus originates from the iliac fossa and the internal lip of the iliac crest. The two muscles merge to form the iliopsoas tendon, which inserts on the lesser trochanter of the femur. The iliopsoas thus crosses the hip joint from a deep, anterior position and is separated from the inguinal ligament by the femoral nerve and vessels.

Innervation and blood supply

The psoas major is innervated by branches of the ventral rami of L1–L3. The iliacus is primarily innervated by the femoral nerve (L2–L4). Blood supply comes from lumbar and iliolumbar branches, with contributions from the iliac and femoral arteries.

Function

As the main hip flexor, the iliopsoas facilitates flexion of the thigh at the hip and, when the thigh is fixed, assists in flexing the trunk. It also helps stabilize the pelvis and lumbar spine during standing and locomotion, and works with other hip flexors such as the rectus femoris and sartorius.

Clinical relevance

Tightness, strain, or tendinopathy of the iliopsoas can cause hip, groin, or anterior thigh pain and may contribute to snapping hip syndrome. Clinical tests include the Thomas test and hamstring/hip flexion maneuvers; pain on resisted hip flexion can indicate iliopsoas involvement. Variations such as a psoas minor are present in some individuals.