You know that cardio workouts are crucial for a healthy heart and lungs and that they burn calories and fat, lower stress and boost energy. But what are the factors that make for the best possible sweat session? Is one machine better than another? How hard should you push yourself? Do you need to do more cardio, or less?

With so much confusion, it's no wonder a lot of us simply climb on our favorite machine (or the one we loathe the least), press the same old buttons and sink into a comfortable routine, inevitably undercutting our results. To the rescue: this exclusive Smart Cardio plan, designed to help you get the most from your workouts.

Created by certified trainer Keli Roberts, group fitness manager at Equinox in Pasadena, Calif., and a competitive cyclist, these mega-effective programs each focus on a specific goal. We'll show you which type of aerobic training is best for losing weight, boosting endurance and staying motivated. If you still have questions about all things cardio, turn the page for enlightening answers.

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