| Psoas major muscle | |
|---|---|
 Position of psoas major (shown in red) | |
 The psoas major and nearby muscles | |
| Details | |
| Origin | Transverse processes of T12–L4 and the lateral aspects of thediscs between them |
| Insertion | In thelesser trochanter of thefemur |
| Artery | lumbar branch of iliolumbar artery |
| Nerve | Lumbar plexus via anterior branches of L1–L3 nerves |
| Actions | Flexion in the hip joint |
| Antagonist | Gluteus maximus |
| Identifiers | |
| Latin | musculus psoas major |
| TA98 | A04.7.02.004 |
| TA2 | 2595 |
| FMA | 18060 |
| Anatomical terms of muscle | |
Thepsoas major (/ˈsoʊ.əs/ or/ˈsoʊ.æs/; fromAncient Greek:ψόᾱ,romanized: psóā,lit. 'muscles of the loins') is a longfusiform muscle located in the lateral lumbar region between thevertebral column and the brim of thelesser pelvis. It joins theiliacus muscle to form theiliopsoas. In animals, this muscle is equivalent to thetenderloin.
The psoas major is divided into a superficial and a deep part. The deep part originates from thetransverse processes oflumbar vertebrae L1–L5. The superficial part originates from the lateral surfaces of the lastthoracic vertebra, lumbar vertebrae L1–L4, and the neighboringintervertebral discs. Thelumbar plexus lies between the two layers.[1]
Together, the iliacus muscle and the psoas major form the iliopsoas, which is surrounded by theiliac fascia. The iliopsoas runs across theiliopubic eminence through themuscular lacuna to its insertion on thelesser trochanter of the femur. Theiliopectineal bursa separates the tendon of the iliopsoas muscle from the external surface of thehip-joint capsule at the level of theiliopubic eminence.[2] The iliac subtendinous bursa lies between the lesser trochanter and the attachment of the iliopsoas.[1]
Innervation of the psoas major is through the anterior rami of L1 to L3 nerves.[3]
In fewer than 50 percent of human subjects,[1] the psoas major is accompanied by thepsoas minor muscle.
One study using autopsy data found that the psoas major muscle is substantially thicker in men of African descent than in Caucasian men, and that the occurrence of the psoas minor is also ethnically variant, being present in most of the white subjects and absent in most of the black subjects.[4]
In mice, it is mostly afast-twitching, type II muscle,[5] while in humans it combines slow- and fast-twitching fibers.[6]
The psoas major joins the upper body and the lower body, the axial to the appendicular skeleton, the inside to the outside, and the back to the front.[7] As part of the iliopsoas, psoas major contributes toflexion in the hip joint. On the lumbar spine, unilateral contraction bends the trunk laterally, while bilateral contraction raises the trunk from itssupine position.[8] In addition, attachment to the lesser trochanter, located on theposteromedial aspect of thefemur, causes lateral rotation and weak adduction of the hip.
It forms part of a group of muscles called thehip flexors, whose action is primarily to lift the upper leg towards the body when the body is fixed or to pull the body towards the leg when the leg is fixed.
For example, when one performs asit-up that brings the torso (including the lower back) away from the ground and towards the front of the leg, the hip flexors (including the iliopsoas) will flex the spine upon the pelvis.
Owing to the frontal attachment on the vertebrae, rotation of the spine will stretch the psoas.
Tightness of the psoas can result in spasms orlower-back pain by compressing the lumbar discs.[9] A hypertonic and inflamed psoas can lead to irritation and entrapment of theilioinguinal and theiliohypogastric nerves, resulting in a sensation of heat or water running down the front of the thigh.
Psoas can be palpated with active flexion of the hip. A positive psoas contracture test and pain with palpation reported by the patient indicate clinical significance. Care should be taken around the abdominal organs, especially the colon when palpating deeply.
The appearance of a protruding belly can visually indicate a hypertonic psoas, which pulls the spine forward while pushing the abdominal contents outward.[10]
The psoas lies posterolateral to the lumbarsympathetic ganglia, and the needle tip will often pass through the psoas major during a lumbar sympathetic block.
Thegenitofemoral nerve is formed in the midsection of the psoas muscle by the union of branches from the anterior rami of L1 and L2 nerve roots. The nerve then courses inferiorly within the psoas muscle and finally "pierces" the muscle and emerges on the anterior surface of the psoas distally. The nerve then traverses theretroperitoneum, descending over the anterior surface of the psoas.[11]
This article incorporates text in thepublic domain frompage 467 of the 20th edition ofGray's Anatomy(1918)
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