Psoas major muscle

The psoas major muscle is a deep, elongated, and fusiform muscle located in the lumbar and pelvic regions. It lies dorsolaterally to the psoas minor and extends from the lumbar spine to the proximal part of the femur. It joins closely with the iliacus muscle to form the iliopsoas, the main flexor of the hip.

Origin: The psoas major originates from the ventral surface of the lumbar vertebrae and their transverse processes.

Insertion: The psoas major ends in a common tendon with the iliacus muscle. Together, they form the iliopsoas muscle, whose single tendon inserts on the lesser trochanter of the femur.

Relations: Cranially, the ventral surface of the psoas major is in contact with the pleura in the thoracolumbar region. In the abdomen, it is covered by the psoas minor and the fascia iliaca, which separate it from the peritoneum and neighboring organs. Caudally, the muscle passes under the inguinal arch, near the external iliac vessels. Its dorsal surface rests against the lumbar vertebrae, their transverse processes, and the quadratus lumborum and intertransversarii muscles. The medial border allows passage of the lumbar vessels and branches of the sympathetic system. In the iliac region, it is partially enveloped by the iliacus muscle.

Action: The psoas major acts in synergy with the iliacus muscle to flex the hip and bring the thigh toward the abdomen. It also contributes to external rotation and adduction of the femur.

Innervation: The muscle is innervated by the ventral branches of the lumbar nerves, mainly from L2 to L4. The femoral nerve gives additional motor branches to the terminal part of the muscle.

Vascularization: The vascular supply is provided by branches of the lumbar arteries, the internal iliac arteries, and by branches of the deep circumflex iliac arteries with their accompanying veins. These vessels run along the muscle throughout its course to its femoral insertion.