Psoas major

The psoas major (Latin: musculus psoas major) is a long and large muscle of the pelvis that is located lateral to the lumbar spine. It stretches between the thoracic and lumbar vertebrae and femur. Together with the iliacus muscle, psoas major forms the iliopsoas muscle. It belongs to the anterior compartment of the pelvic muscles and is one of the posterior abdominal wall muscles.

Origin

The psoas major muscle is composed of two parts - superficial and deep. The superficial part arises from the lateral sides of the vertebral bodies of the twelfth thoracic (Th12) and upper four lumbar vertebrae (L1 - L4) and their intervertebral discs. The deep part originates from the transverse process of the lumbar vertebrae L1 - L5.

Insertion

The psoas major merges with the iliacus muscle and forms a common tendon that inserts on the lesser trochanter of the femur.

Action

The psoas major muscle provides flexion of the thigh and trunk. Also, it assists in the external rotation of the thigh.

Innervation

The psoas major is innervated by the anterior rami of the lumbar spinal nerves (L1 - L3).

Blood supply

The psoas major muscle receives arterial blood supply from the iliolumbar artery - a branch of the internal iliac artery. Also, it is supplied by the lumbar branches of the abdominal aorta, muscular branches of the common iliac artery and deep circumflex iliac artery arising from the external iliac artery.