Changing hormones. Childbirth. A stunted metabolism. Gravity. If any or all of the above have caused your belly to become a rounder, puffier version of your stomach of yesteryear, we have good news. You don't have to order a lifetime supply of Spanx, and you don't need to work out harder or longer to get a flatter, stronger core -- just smarter.

We partnered with yoga guru Kimberly Fowler to create Flat Belly Yoga!, the master of core-conditioning programs for age 40-plus. It uses juiced-up yoga routines -- basic poses combined with light weight lifting—to transform your midsection.

"Yoga is known for its relaxation and flexibility benefits, but it's also a full-body workout that strengthens and lengthens your core muscles. When you add light weights, you'll amp up yoga's belly-firming power by boosting calorie burn, revving metabolism, and building calorie-burning muscle mass, all of which add up to a leaner, tighter core," says Fowler, 55. She started experimenting with light weights and yoga to keep her own body in tip-top shape, and she now shares the hybrid workout with more than 500 students a day at YAS Fitness Centers, her studios in California.

These souped-up moves are gentle, but you'll feel them going to work immediately on your core, especially the transverse abdominis. "This muscle acts like a natural girdle -- think Spanx. Strengthening it will automatically give your belly a flatter appearance," says Fowler.

Aim to do this practice 6 times a week. Do the routine without weights until you're comfortable with the poses. Focus on your breath and take your time; doing a pose correctly requires more core strength than doing it fast and sloppily. Stay connected to your core throughout each pose, keeping your abs engaged. For faster results, repeat the main sequence 2 or 3 times.

Ready for a strong, slender stomach? You'll need a yoga mat and a set of 3-pound dumbbells.

Adapted from Flat Belly Yoga! by Kimberly Fowler and the editors of Prevention, copyright 2013 Rodale Inc. Available wherever books are sold.

Story continues below