Psoas major anatomy: The psoas major is a deep muscle that originates from the transverse processes, vertebral bodies, and intervertebral discs of T12 to L5. It travels downward through the pelvic cavity, passes deep to the inguinal ligament, and inserts into the lesser trochanter of the femur. Along its course, it lies adjacent to the lumbar plexus, ureter, and abdominal aorta. It’s innervated by branches of the lumbar plexus, primarily from spinal nerves L1 to L3, and occasionally L4. Function