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Iliopsoas muscle

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Anatomy and functions of the iliopsoas muscle shown with 3D model animation.
Iliopsoas muscle (Musculus iliopsoas)

Iliopsoas is a large compound muscle of the inner hip composed of the iliacus and psoas major muscle. Aside from the iliopsoas, other muscles of the inner hip include the psoas minor, obturator externus, obturator internus, superior gemellus, inferior gemellus, piriformis and quadratus femoris.

The iliopsoas has an extensive origin, with the psoas major portion originating from the twelfth thoracic and all five lumbar vertebrae, and the iliacus portion arising mainly from the iliac fossa of the pelvis. Their fibers merge into a single muscle belly around the pelvic brim. The iliopsoas ends in a tendon that inserts to the lesser trochanter of femur.

The iliopsoas muscle is the main flexor of the hip joint. The joint actions of psoas major and iliacus produce the flexion of the hip; however, due to its proximal attachments, the psoas major can produce additional movements of the lumbar vertebral column.

This article will discuss the anatomy and function of the iliopsoas muscle.

Key facts about the iliopsoas muscle
Definition Iliopsoas is the largest and strongest inner hip muscle extending from the vertebral column to the proximal femur
Muscular components Iliacus, psoas major
Innervation Iliacus: femoral nerve (L2-L4)
Psoas majo
r: anterior rami of spinal nerves (L1-L3)
Blood supply Iliolumbar, obturator, external iliac and femoral arteries
Function Flexion and external rotation of the thigh at the hip joint; flexion of the trunk at the hip, lateral flexion of the trunk (psoas major only)
  1. Psoas major
  2. Iliacus
  3. Function
  4. Sources
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Psoas major

Psoas major (Musculus 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.

Test your knowledge with this quiz on the muscles of the hip and thigh in different difficulty levels!

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 muscle (Musculus 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.

Want a quick overview of all the muscles in the body? Try our muscle anatomy charts that contain the attachments, innervation and function of all muscles organized in neat tables.


Contraction of the iliopsoas muscle

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).

Learn everything about the anatomy and function of the muscles of the hip and thigh with our video tutorials, articles, quizzes and labeled diagrams.

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