whether you only bear the time (or the patience) to do one abdominal exercise,make it this highly effective move - the kneeling roll-out with an exercise ball. It strengthens your core while toning your abs and glutes; you'll feel it after only a few reps. Trust me. Related: Skip the Crunches and Try These 25 Ab-Sculpting MovesKneel with the ball in front of you and lean into the ball as you area your forearms on the ball. You should already feel your abs working here, so be sure to pull your navel toward your spine. Push your arms into the ball as you roll it absent as far as you can without compromising your form - breaking at the waist or over-arching your back. Hold this plank-like position for two to three seconds. Slowly return to the starting position by contracting your abs and rolling the ball back toward your knees. That completes one rep. Do three sets of eight to 10 reps.
Make this more challenging by kneeling upright with your hands on the ball as the starting position. From there, and roll the ball out as far as you can,then return to kneeling without breaking at the hips.
Source: popsugar.com