Psoas major is a long fusiform muscle located on the side of the lumbar region of the vertebral column. In about 50% of individuals, the psoas joins the iliacus muscle to form the iliopsoas. Sometimes it is accompanied by the psoas minor muscle, which is absent in 40% individuals.
Origin of psoas major muscle:
The psoas major arises from the following structures of the twelfth thoracic to fifth lumbar vertebrae.
- Roots of transverse processes
- Sides of vertebral bodies
- Intervertebral discs
Insertion of psoas major muscle:
The fibers of the muscle run downward and laterally and leave the abdomen to enter the thigh by passing behind the inguinal ligament. The muscle is inserted into the lesser trochanter of femur.
The psoas muscle is enclosed in a fibrous sheath derived from the lumbar fascia. Above, the sheath is thickened to from the medial arcuate ligament.
Blood supply of psoas major muscle:
The psoas major muscle receives its blood supply from the lumbar branch of iliolumbar artery.
Nerve supply to psoas major muscle:
Psoas major is innervated by the lumbar plexus via anterior branches of L1-L3 nerves.
Actions of Psoas major muscle:
As part of the iliopsoas, it contributes to flexion and external rotation of the hip joint on the trunk. If the thigh is fixed, it flexes the trunk on the thigh as seen in sitting up from a lying position.
The psoas major muscle forms a part of a group of muscles known as hip flexors. They primarily lift the upper leg towards the body when the body is fixed or pull the body towards the leg when the leg is fixed.