Home » Psoas Major – Functional Anatomy

Psoas Major – Functional Anatomy

Psoas major varies a great deal in size and shape. It originates in the last thoracic and all lumbar vertebrae and inserts on the lesser trochanter of the femur. This muscle traps the coxal bone and sacrum between its attachments.

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 together and then combine with the iliacus before inserting on the femur.

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.

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

Variations in psoas major are reported as rare in literature, Dissections mention psoas quartus and psoas tertius, which are separate muscles originating on the transverse processes of lower vertebrae and the quadratus lumborum. These anomalies insert at the lesser trochanter.

Origin – Psoas major usually 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.

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

Function – Psoas major is primarily seen as a hip flexor but also flexes the trunk laterally.

The other functions of psoas major can get complicated and are argued extensively among researchers, physiologists, and clinical practitioners. For example, it is primarily seen as contributing 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.

Wikipedia entry for psoas major.

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