The psoas major is a long muscle of the pelvic wall that joins with the iliacus and forms the iliopsoas.

Origin: the lateral sides of the 12th thoracic vertebra (T12) and first four lumbar vertebrae (L1 - L4), and their intervertebral discs; transverse processes of the L1 - L5 vertebrae.

Insertion: the lesser trochanter of the femur.

Action: flexion of the hip joint and external rotation of the thigh. If the leg is fixed in position, this muscle also participates in flexion of the trunk.

Innervation: muscular branches of the lumbar plexus.