Psoas major muscle
The psoas major is a long and large muscle of the pelvic wall that joins with the iliacus and forms the iliopsoas. It is located on each side of the lumbar spine, stretching between the lower thoracic and lumbar vertebrae and the femur.
Origin: superficial part - lateral sides of the vertebral bodies T12 - L4 and the intervertebral discs between them; deep part - transverse processes of the L1 - L5 vertebrae.
Insertion: lesser trochanter of the femur.
Action: flexion and external rotation of the thigh. If the leg is fixed in position, this muscle also participates in flexion of the trunk.
Innervation: anterior rami of the lumbar spinal nerves (L1 - L3) via muscular branches of the lumbar plexus.