The psoas connects the low back to the upper thighs. It starts out at the lower vertebrae, runs through the pelvic bowl, over the hip, and connects at the top of the femur. It is one of the most powerful of what are considered to be the hip flexor muscles. The psoas is a central player in posture and can often be the culprit behind chronic lower back tightness or pain. In crossfit, weakness in the psoas can result in power loss in the low snatch position and instability in a squat.