Lie in prone (facedown) position, with your weight supported on your forearms. Contract your abs and try to keep your body in line; don't sag in the midsection. Hold the contraction for as long as possible. To increase intensity, walk your elbows slightly forward and hold a little longer. This isometric exercise works all abdominal muscles. Hold the isometric contraction for 10 seconds, walk the elbows forward, and hold for 10 seconds, then relax. Do as many sets as possible.