Low Back Pain Could Be a Tight or Shortened Psoas. (pronounced: So-as). The Psoas is a hip flexor located deep in the abdomen attaching the femur to the pelvis and lumbar spine. The lumbosacral nerve plexus runs through it. Prolonged sitting and lack of exercise can shorten the Psoas. When we stand after sitting for a while, the shortened Psoas pulls the lumbar spine forward into increased lordosis (sway back). A stiff, achy or painful low back & sciatic pain is often a result.