Psoas major muscle

Origin and insertion

Psoas major is a long muscle located immediately lateral to the vertebral column. It has an extensive origin from the:

• Transverse processes of all the lumbar vertebrae
• Vertebral bodies of T12-L5
• Intervening intervertebral discs 

The fibres of psoas major extend inferolaterally through the lesser pelvis towards the thigh. They pass along the pelvic brim and inferior to the inguinal ligament towards their merged insertion in the anterior thigh. Along its course, the lateral-most fibres of psoas major fuse with the fibres of the iliacus muscle to form the iliopsoas muscle. Iliopsoas passes deep to the inguinal ligament and anterior to the joint capsule of the hip to insert onto the lesser trochanter of the femur. 

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Relations

Superiorly, psoas major lies posterior to the diaphragm. Quadratus lumborum is located lateral to the muscle. In the abdomen, psoas major is closely related to a number of retroperitoneal structures. The inferior vena cava is located medial to the right psoas major muscle. On the left, the abdominal aorta lies medial to psoas major. The sympathetic trunk and aortic lymph nodes are also located medially.  Other retroperitoneal structures related to the muscle are the kidneys and ureters, the gonadal vessels and the genitofemoral nerve. 

Because of its location immediately lateral to the vertebral column, the roots of the lumbar plexus are embedded in the belly of psoas major as they exit the vertebral canal. The plexus forms in the muscle with its branches emerging from its lateral border.

In the thigh, psoas major contributes to the floor of the femoral triangle. Its tendon (iliopsoas tendon) lies deep to the fascia lata, sartorius, rectus femoris and deep femoral artery. It is separated from the joint capsule of the hip by the iliac bursa. Pectineus and the femoral vein are medial while the femoral nerve is lateral to the muscle.

Innervation

Psoas major is innervated by branches of the lumbar plexus. These are the anterior branches of spinal nerves L1-L3.

Blood supply

The psoas major muscle is supplied mainly by the iliolumbar branch of the internal iliac artery. The lumbar branches of the aorta, the obturator branch of the internal iliac artery as well as branches of the external iliac and femoral arteries also contribute to the blood supply

Venous drainage of the psoas major muscle mirrors the arterial supply. It drains back into the femoral, external iliac and internal iliac veins as well as directly into the inferior vena cava.

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Function

The actions of psoas major are flexion and lateral rotation of the thigh at the hip. It also flexes the trunk at the hip and flexes the trunk laterally. Psoas major is also a postural muscle. During standing, it stabilizes the vertebral column by maintaining normal lumbar lordosis and indirect thoracic kyphosis.

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