The hip-flexors are made up of the psoas major and iliacus muscles which are known collectively as the iliopsoas and the quadriceps. These flexors have an enormous effect on the body’s structure, especially its posture. If too tight (and/or over-strong) they tend to lift or lurch the body from a flat (supine) position to sitting upright. In order to preferably ‘roll’ the spine into an upright position, these muscles need to be controlled by producing more mid and lower abdominal; strength.