It's a sad truth: The older we get, the harder it is to lose weight — and that's because our metabolism gets sluggish.

In fact, fitness legend Denise Austin says that every 7 years — especially after menopause — our metabolism slows down by about 7 percent. Yowza!

Because of this, it's super important to do muscle-conditioning exercises.

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"The more muscles you have," Denise says, "The more calories you burn!"

And if you don't think you have enough time or energy, think again!

"You can do leg squats while blow-drying your hair," Denise suggests. Or you can exercise your triceps while you're chatting on the phone, she adds.

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Denise's favorite, though? You can also build muscle in your belly anytime, anywhere, by clasping your hands behind your neck and lifting your knees up one at a time.

"All kidding aside, that's the most dangerous fat," Rach says of belly fat.

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"Because it's closest to your heart," Denise continues.

Watch Denise's simple, yet effective, ab exercise in the video above!