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Psoas Major – Functional Anatomy

Here you will find anatomy details on the psoas major. It fits into a larger group of muscles, the iliopsoas complex. There is a grid of related posts at the end of this post.


The anatomy of psoas major is complex, and its functions are fodder for endless bodywork discussions.


Psoas major originates from two sections.

  • The anterior bellies originate from the bodies and intervertebral discs of T12-L4.
  • The posterior portion originates from the transverse processes of L1-L5.


  • Both sections of psoas major blend with the fibers of the iliacus, and cross the pubic bone. This combined muscle inserts on the lesser trochanter of the femur.


  • primarily seen as a hip flexor
  • flexes the trunk laterally.

Wikipedia entry for psoas major.

Lateral view of psoas major showing:

  • posterior section attaching to transverse processes of L1-L5
  • anterior section attaching to bodies of T12-L4
  • bend of iliopsoas over the pubic bone and femoral head when standing.

Attachment Details

It has two sections. One originates on vertebral bodies or T12-L4, and the other originates on the transverse processes of L1-L5. Nerve roots from the lumbar vertebrae pass between these sections. They blend and then combine with the iliacus before inserting on the femur.

Functional Considerations

Psoas major varies a great deal in size and shape from one cadaver to another. This muscle traps the coxal bone and sacrum between its attachments.

The other functions of psoas major can get complicated and are argued extensively among researchers, physiologists, and clinical practitioners. For example, Most bodyworkers see it as extending the lumbar spine and compressing the lower lumbar vertebrae. However, when the lumbar is in a reverse curve, the upper sections can, paradoxically, flex the lumbar spine.

The postural and ambulatory functions of psoas major are even more complicated. Many factors contribute to the timing of which sections of abdominal muscle contract. The balance of the muscles that surround the iliopsoas complex impacts the balance of tension for each section.

Viscerosomatic reflexes complicate this even further.

Studies show that the iliopsoas lifts off the femoral head at about 14 degrees of hip flexion.

Anomalies, Etc.

Reports say that variations in the psoas major are as rare in most studies. However, dissections mention psoas quartus and psoas tertius. These are separate muscles originating from the transverse processes of the lower vertebrae and the quadratus lumborum. These anomalies insert at the lesser trochanter.

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