Kneel and grasp the handles of an ab wheel, holding them directly beneath your shoulders. Without moving your knees, brace your core and roll the wheel forward and to the right as far as you can without letting your hips sag. Pause, and return to the starting position. Do three sets of 10 reps per side. Why It Works: The rollout starts with the stretching of your abs and obliques and ends with their forceful contraction. "That leads to a good muscle-damage response," says Craig Ballantyne, C.T.T., creator of Turbulence Training. "Your body then repairs the damage by packing on muscle, making you stronger."