Iliopsoas is the inner hip muscle whose function is to flex the thigh at the hip joint. It is a large muscle composed of the union of two individual muscles; iliacus and psoas major. It takes its name from the combination of the names of these two muscles. Iliopsoas is the chief flexor of the hip joint.
Iliopsoas has an extensive origin, with the fibres converging on a localised insertion point in the proximal thigh. This article will outline the morphology of the iliopsoas muscle and its origins and insertions. It will then describe the action and neurovascular supply of the muscle, as well as some relevant clinical features.
Psoas Major: Vertebral bodies of T12-L4, Intervertebral discs between T12-L4, costal processes of L1-L5 vertebrae
Iliacus: Iliac fossa
|Insertion||Lesser trochanter of femur|
Psoas Major: Anterior rami of spinal nerves L1-L3
Iliacus: femoral nerve (L2-L4)
Flexion of the thigh at the hip; flexion of the trunk at the hip, external rotation of the thigh at the hip; lateral flexion of the trunk (psoas major only)
|Blood supply||Iliolumbar artery mainly with contributions from the obturator, external iliac, femoral arteries|
All of the important anatomical facts about the iliopsoas muscle will be described in this article.
- Origin and insertion
- Blood supply
- Clinical relations
- Related diagrams and images
Iliopsoas is the combination of the iliacus and psoas major muscles. The bellies of the muscles lie in the posterior abdominal wall and greater pelvis. They merge together as they pass deep to the inguinal ligament into the anterior compartment of the thigh. As well as being the chief flexor of the hip joint, iliopsoas is also a stabiliser of the trunk because of its expansive attachments.
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Origin and insertion
Psoas major is a long muscle located lateral to the vertebral column. It is found in the space between the bodies and spinous processes of the vertebrae. It has an extensive proximal attachment to the sides of the T12-L4 vertebrae and the intervertebral discs separating them. It is also attached to the transverse processes of all of the lumbar vertebrae. Variation is very common in the origin of psoas major. The muscle descends along the pelvic brim. It enters the anterior thigh by passing posterior to the inguinal ligament.
Iliacus is a triangular shaped muscle. It arises mainly from the superior two-thirds of the iliac fossa, the iliac crest and the lateral sacrum. It also originates from the anterior sacroiliac and iliolumbar ligaments that connect these parts of the pelvis and vertebral column. It passes anteriorly in the iliac fossa as far as the anterior superior and anterior inferior iliac spines.
The fibres of iliacus merge with the most lateral fibres of psoas major. The newly formed iliopsoas muscle passes anterior to the capsule of the hip joint towards its insertion 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 innervation of iliopsoas comes from the lumbar plexus. Psoas major is innervated by the ventral rami of the lumbar spinal nerves, mainly L1 and L2, but with some contribution from L3. Iliacus is innervated by branches of the femoral nerve.
Why not learn more about the lumbar plexus using these helpful resources?
Both the iliacus and psoas major portions of iliopsoas receive the same blood supply. The iliolumbar branch of the internal iliac artery is the main supply to the muscle, with contributions from the lumbar arteries from the aorta, the obturator branch of the internal iliac artery, as well as branches of the external iliac and femoral arteries.
Venous drainage mirrors the arterial supply, draining back into the femoral, external iliac and internal iliac veins, as well as directly into the inferior vena cava.
Iliopsoas flexes and externally rotates the thigh at the hip joint. It also flexes the trunk at the hip joint, such as in the motion of a ‘sit-ups’ exercise. The psoas major part of iliopsoas laterally flexes the trunk. Iliopsoas is also a postural muscle. In the symmetrical upright stance, iliopsoas maintains normal lumbar lordosis during standing, and indirectly the compensatory kyphosis of the thoracic vertebral column. Iliopsoas is the antagonist of the gluteus maximus muscle in the gluteal region and the hamstring muscles (biceps femoris, semitendinosus and semimembranosus) in the posterior thigh.
The most superior end of the psoas major muscle lies posterior to the diaphragm. In the abdomen, it is closely related anterolaterally to a number of retroperitoneal structures, including the kidneys and ureters, gonadal vessels, and the genitofemoral nerve. Medially, the right psoas major muscle is related to the inferior vena cava and the left psoas major muscle is related to the abdominal aorta. The sympathetic trunk and aortic lymph nodes are also located medially.
As psoas major lies in the groove between the vertebral bodies and transverse processes, the lumbar plexus is embedded in the belly of psoas major as its roots emerge from the intervertebral foramina and pass directly into the muscle. The plexus is formed in the muscle, and the branches of the plexus emerge from the lateral border of the muscle.
Anterior to the iliacus muscle is the iliac fascia, the lateral femoral cutaneous nerve, the cecum (on the right), and the descending colon (on the left). Posteriorly is the iliac fossa, and medially is the psoas major and the femoral nerve. Posteriorly is the joint capsule of the hip, separated from the muscle by the iliac bursa.
In the thigh, the fascia lata, sartorius, rectus femoris, and the deep femoral artery are anterior to the tendon of iliopsoas. The capsule of the hip joint and iliac bursa are posterior, pectineus and the femoral vein lie medially, and the femoral nerve is located laterally.
Iliopsoas syndrome is a term applied to conditions affecting the iliopsoas muscle. These conditions may include iliopsoas tendonitis, which is inflammation of the iliopsoas tendon, or iliopsoas bursitis, or inflammation of the iliac bursa separating the tendon from the joint capsule of the hip. These types of injuries are common injuries from activities that involve repeated flexion of the hip joint, such as gymnastics, athletics and dancing.
The typical symptom of iliopsoas injury is pain in the hip region that worsens with movement of the joint. The muscle can be tested by flexing the hip joint in the supine position against resistance, with the hip and knee joints flexed. Resting the joint as well as physical therapy and anti-inflammatories are the typical method of treatment for iliopsoas tendonitis or bursitis. Injury can be best avoided by gradually building strength of the muscle at a steady pace using exercises that target the iliopsoas muscle.