Psoas major muscle
|Psoas major muscle|
|The psoas major and nearby muscles|
|Horizontal disposition of the peritoneum in the lower part of the abdomen. (Psoas major labeled at bottom left.)|
|Latin||Musculus psoas major|
|Gray's||subject #127 467|
|Origin||Transverse processes of T12-L5 and the lateral aspects of the discs between them|
|Insertion||In the lesser trochanter of the femur|
|Artery||lumbar branch of iliolumbar artery|
|Nerve||Lumbar plexus via anterior branches of L1-L3 nerves|
|Actions||Flexes and rotates laterally thigh|
The psoas major (//) is a long fusiform muscle located on the side of the lumbar region of the vertebral column and brim of the lesser pelvis. It joins the iliacus muscle to form the iliopsoas. In less than 50 percent of human subjects, the psoas major is accompanied by the psoas minor. In mice, it is mostly a fast-twitching, type II muscle, while in human it combines slow and fast-twitching fibers.
The psoas major is divided into a superficial and deep part. The deep part originates from the transverse processes of lumbar vertebrae I-V. The superficial part originates from the lateral surfaces of the last thoracic vertebra, lumbar vertebrae I-IV, and from neighboring invertebral discs. The lumbar plexus lies between the two layers.
Joined by the iliacus, psoas major forms the iliopsoas which is surrounded by the iliac fascia. The iliopsoas runs across the iliopubic eminence through the muscular lacuna to its insertion on the lesser trochanter of the femur. The iliopectineal bursa separates the tendon from the iliopsoas muscle from the external surface of the hip joint capsule at the level of the iliopubic eminence. The iliac subtendinous bursa lies between the lesser trochanter and the attachment of the iliopsoas.
Innervation of the psoas major is through the anterior rami of L1 to L4.
As part of the iliopsoas, psoas major contributes to flexion and external rotation in the hip joint. On the lumbar spine, unilateral contraction bends the trunk laterally, while bilateral contraction raises the trunk from its supine position.
It forms part of a group of muscles called the hip 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 doing a situp 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.
Due to the frontal attachment on the vertebrae, rotation of the spine will stretch the psoas.
Tightness of the psoas can result in lower back pain by compressing the lumbar discs.
See also 
- Platzer (2004), p 234
- Nunes, MT; Bianco, AC; Migala, A; Agostini, B; Hasselbach, W (1985). "Thyroxine induced transformation in sarcoplasmic reticulum of rabbit soleus and psoas muscles". Zeitschrift fur Naturforschung. Section C: Biosciences 40 (9–10): 726–34. PMID 2934902.
- Arbanas, Juraj; Starcevic Klasan, Gordana; Nikolic, Marina; Jerkovic, Romana; Miljanovic, Ivo; Malnar, Daniela (2009). "Fibre type composition of the human psoas major muscle with regard to the level of its origin". Journal of Anatomy 215 (6): 636–41. doi:10.1111/j.1469-7580.2009.01155.x. PMID 19930517.
- Bojsen-Møller, Finn; Simonsen, Erik B.; Tranum-Jensen, Jørgen (2001). Bevægeapparatets anatomi [Anatomy of the Locomotive Apparatus] (in Danish) (12th ed.). pp. 261–266. ISBN 978-87-628-0307-7.
- Thieme Atlas of Anatomy (2006), p 422
- Akuthota, et all(2008). p 40
- Platzer, Werner (2004). Color Atlas of Human Anatomy, Vol. 1: Locomotor System (5th ed.). Thieme. ISBN 3-13-533305-1.
- Thieme Atlas of Anatomy: General Anatomy and Musculoskeletal System. Thieme. 2006. ISBN 1-58890-419-9.
- Akuthota, Venu; Ferreiro, Andrea; Moore, Tamara; Fredericson, Michael (2008). "Core Stability Exercise Principles". Current Sports Medicine Reports (American College of Sports Medicine) 7 (1): 39–44. doi:10.1097/01.CSMR.0000308663.13278.69. PMID 18296944. Retrieved 26 March 2011.
Additional images 
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- -1261436848 at GPnotebook
- LUC psmj
- SUNY Labs 40:16-0101 - "Posterior Abdominal Wall: Muscles of the Posterior Abdominal Wall"
- SUNY Anatomy Image 8916
- Cross section at UV pelvis/pelvis-e12-2
- Psoas Stretches for Back Pain
- Psoas Muscles and Abdominal Exercises for Back Pain