Even women who are otherwise slender and fit can end up with a belly pooch due to underworked lower abdominal muscles. This workout will tap into the deep abdominal muscles—the transverse abdominis—that pull in your waistline like a corset. Do these moves one after another with no rest in between. Then repeat the circuit so you're performing it a total of two times. Excerpted from The Women's Health Big Book of 15-Minute Workouts. Get your copy today!