Psoas: Two muscles of the lower back. There are two psoas muscles on each side of the back. The larger of the two is called the psoas major and the smaller the psoas minor. The psoas major originates at the spine, around the bottom of the rib cage, and runs down to the thighbone (the femur). The psoas major acts to flex the hip. The psoas minor also originates at the spine, around the bottom of the rib cage, but it runs down to the bony pelvis. The psoas minor acts to flex the lower (lumbar) spine.