Psoas major and the iliopsoas muscle: the deep hip flexor of the lower back

What it is

A long muscle of the posterior abdominal wall that forms the main part of iliopsoas.

Location

Originates from the sides of the T12 to L4 vertebral bodies and the intervening discs, and from the transverse processes of the lumbar vertebrae, and inserts onto the lesser trochanter of the femur together with iliacus.

Function

Flexes the thigh at the hip, and flexes the trunk when the thigh is fixed.

Body system

Muscular · paired (left and right)

FMA ID

FMA22342 (Foundational Model of Anatomy)

Key facts

• Innervation: anterior rami of L1–L3
• Blood supply: lumbar and iliolumbar arteries
• The lumbar plexus lies within its substance

Clinical relevance

Psoas abscess can track along the muscle from spinal infection.

Psoas major is a muscle of the posterior abdominal wall that lies alongside the lumbar vertebrae. From there it runs downwards and laterally, passes deep to the inguinal ligament and inserts on the lesser trochanter of the femur. (TeachMeAnatomy Encyclopaedia: Psoas major.)

It has two layers of origin. The superficial layer arises from the sides of the vertebral bodies and the intervertebral discs from T12 to L4 (and the adjacent vertebral bodies); the deep layer arises from the transverse processes of L1 to L5. Radiopaedia summarises the origin as the vertebral bodies, discs and transverse processes of T12 to L5. (Radiopaedia: Psoas major muscle.)

Sources differ slightly on the lowest vertebral body. Kenhub's text gives the bodies of T12 to L4 plus the discs between them and the transverse processes of all lumbar vertebrae, while its summary box says T12 and all five lumbar vertebrae. (Kenhub: Iliopsoas muscle.)

Iliopsoas is the largest and strongest inner hip muscle, running from the vertebral column to the upper femur. It is formed by psoas major and iliacus, whose fibres converge into a common tendon that inserts on the lesser trochanter. (Kenhub: Iliopsoas muscle.)

Iliacus arises from the upper two-thirds of the iliac fossa, the inner lip of the iliac crest, the side of the sacrum and the ventral sacroiliac and iliolumbar ligaments. It lies in the hollow of the iliac fossa, lateral to psoas major. (Radiopaedia: Iliacus muscle.)

Below the inguinal ligament the iliopsoas runs in front of the hip joint capsule and forms part of the floor of the femoral triangle. A subtendinous iliac bursa separates the tendon from the hip joint and sometimes communicates with the joint cavity. (Kenhub: Iliopsoas muscle.)

TeachMeAnatomy lists the floor of the femoral triangle as pectineus, iliopsoas and adductor longus. The triangle contains the femoral nerve, artery and vein, from lateral to medial. (TeachMeAnatomy: The Femoral Triangle.)

Psoas minor is a small, inconstant muscle of the posterior abdominal wall. It arises from the bodies of T12 and L1 and the disc between them, and its long tendon passes behind the inguinal ligament to attach to the iliopubic eminence of the pelvis. It is present in about 60% of people (range 50 to 67%), is supplied by L1, and is a weak flexor of the lumbar spine. (Radiopaedia: Psoas minor muscle.)

Iliopsoas is the main flexor of the hip, producing powerful flexion of the thigh. When both psoas muscles contract, they flex the trunk at the hips. Contraction of one psoas major bends the trunk to the same side (lateral flexion). (Kenhub: Iliopsoas muscle.)

The University of Washington atlas describes the action of both psoas and iliacus in one phrase: they flex the torso and thigh with respect to each other. Which part moves depends on which end is fixed. (University of Washington Musculoskeletal Atlas: Psoas.)

Iliacus also helps to rotate the thigh laterally (externally). (Radiopaedia: Iliacus muscle.)

Tightness of the hip flexors is often checked with the modified Thomas test: the person lies back on the edge of a table hugging one knee while the other leg hangs down. If the hanging thigh cannot reach horizontal with the pelvis neutral, the hip flexors are considered short. (Eimiller K et al. Reliability of Goniometric Techniques for Measuring Hip Flexor Length Using the Modified Thomas Test. Int J Sports Phys Ther. 2024.)

Psoas major is supplied by direct branches of the anterior rami of L1 to L3 from the lumbar plexus. Iliacus is supplied by a muscular branch of the femoral nerve. (University of Washington Musculoskeletal Atlas: Psoas.)

Root values vary between sources. Kenhub's text says psoas major receives mainly L1 and L2, with some L3 and L4; Radiopaedia gives L1 to L3, mainly L2. For iliacus, Radiopaedia gives femoral nerve fibres from L2 and L3, while TeachMeAnatomy gives L2 to L4. (Radiopaedia: Psoas major muscle.)

Psoas major receives blood from several levels: lumbar arteries above, the iliolumbar artery (its main artery), deep circumflex iliac and external iliac arteries in the middle, and branches of the femoral artery below. (Radiopaedia: Psoas major muscle.)

Iliacus is mainly supplied by the iliolumbar artery, a branch of the internal iliac artery, with smaller contributions from the femoral, obturator and deep circumflex iliac arteries. (Radiopaedia: Iliacus muscle.)

The lumbar plexus forms within the substance of psoas major, so its branches emerge from the muscle at predictable points. The genitofemoral nerve appears on its front surface. The iliohypogastric, ilioinguinal, lateral femoral cutaneous and femoral nerves emerge from its lateral border. The obturator nerve and the lumbosacral trunk emerge from its medial border. Lumbar vessels and sympathetic rami pass behind it. (Radiopaedia: Psoas major muscle.)

Below the psoas, the femoral nerve runs between the psoas tendon and the iliacus, beneath the strong iliacus fascia. This relationship matters clinically, as the next section explains. (Kong WK et al. Femoral Neuropathy due to Iliacus Muscle Hematoma in a Patient on Warfarin Therapy. J Korean Neurosurg Soc. 2012.)

Bleeding into the iliacus (an iliacus haematoma) is a recognised cause of severe pain in the groin, hip and thigh. It occurs mostly in people taking anticoagulants or with clotting disorders. Because the iliacus fascia does not stretch, the blood raises pressure in the compartment and compresses the femoral nerve against the psoas tendon, causing quadriceps weakness and sometimes a reduced knee jerk. (Kong WK et al. Femoral Neuropathy due to Iliacus Muscle Hematoma in a Patient on Warfarin Therapy. J Korean Neurosurg Soc. 2012.)

A psoas abscess is a collection of pus in the iliopsoas compartment. A primary abscess spreads through the blood from a distant infection; a secondary abscess spreads directly from a nearby source such as the spine, bowel (Crohn's disease, appendicitis) or kidney. The classic triad of fever, back pain and limp is rarely complete, and pain on resisted hip flexion or on hyperextending the hip can point to iliopsoas inflammation. CT is the reference test, and treatment is drainage plus antibiotics. (Tasci T, Zencirci B. A female presenting with prolonged fever, weakness, and pain in the bilateral pelvic region: a case report. Cases J. 2009.)

The fascia around psoas is what retains pus within the muscle in a psoas abscess. Radiopaedia also lists psoas haematoma as a related condition and notes that asymmetry between the two psoas muscles is usually not clinically significant. (Radiopaedia: Psoas major muscle.)

In internal snapping hip, the iliopsoas tendon catches as the hip moves, usually producing an audible pop. The femoral head seems to be the structure that most often hinders the tendon, and the pectineal eminence of the pelvis can also catch it. Conservative care (rest, stretching, physiotherapy) is first line; persistent painful cases can be treated by iliopsoas release or lengthening. (Walker P et al. Snapping Hip Syndrome: A Comprehensive Update. Orthop Rev. 2021.)

Psoas major arises from the spine (T12 to L5) and is supplied directly by branches of the lumbar plexus (L1 to L3). Iliacus arises from the pelvis (the upper two-thirds of the iliac fossa, the iliac crest and the side of the sacrum), lies lateral to psoas major and is supplied by the femoral nerve. Both insert on the lesser trochanter of the femur. (University of Washington Musculoskeletal Atlas: Iliacus.)

TeachMeAnatomy summarises their actions: psoas major flexes the thigh at the hip and laterally flexes the vertebral column, while iliacus flexes and laterally rotates the thigh at the hip. (TeachMeAnatomy Encyclopaedia: Psoas major.)

The nearest named structures in the 3D model, measured from the centre of each part:

• Descending colon · digestive
• Ureter · urinary
• Appendicular artery · cardiovascular
• Mesoappendix · digestive
• Anterior cecal artery · cardiovascular
• Sigmoid vein · cardiovascular
• Ileocecal junction · digestive
• Sigmoid artery · cardiovascular
• Appendix · digestive
• Common iliac artery · cardiovascular