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.

Your cardio questions: Fitness experts weigh in on seven common conundrums

How much cardio do I really need?

It depends on your goals, says Glenn Gaesser, Ph.D., an exercise physiologist at the University of Virginia in Charlottesville. For weight loss, at least 200 minutes of moderate-intensity cardio a week may be optimal, he says. In one recent study published in the Journal of the American Medical Association, obese women on calorie-controlled diets who exercised at least 200 minutes each week lost nearly 14 percent of their total body weight, while those who accumulated fewer than 150 minutes of exercise reduced their weight by less than 5 percent. "When it comes to shedding weight, total calories expended is most important, and this is a product of duration and intensity," Gaesser adds. "Since longer duration can usually be tolerated better than higher intensity, duration should be emphasized."

To boost cardiovascular fitness, on the other hand, you'll need to elevate your heart rate, working at higher intensities and, therefore, taking more time off between difficult workouts; 25-45 minutes of moderate- to high-intensity cardio four to five times a week should do the trick.

Are long, slow, steady workouts better for burning fat?

No. It is true that lower-intensity aerobic workouts burn a slightly higher percentage of calories from fat (versus carbohydrate), while burning lots of carbs during higher-intensity exercise usually leads to more fat being burned after exercise. However, when it comes to shedding body fat, it doesn't matter what form the calories are stored in or how quickly or slowly you burn them, Gaesser says. The only thing that counts is the total number of calories burned. Steady-state workouts take longer than higher-intensity workouts to get you to the same total, although you may be able to exercise longer at a lower intensity. "Ultimately, it doesn't matter whether you burn your calories long and slow or short and quick," says Cedric X. Bryant, Ph.D., chief exercise physiologist for the American Council on Exercise in San Diego.

Is there one cardio machine that's most effective for burning calories?

Only if you work at a higher intensity or stay on a particular machine for more time. The harder and longer you work, the more calories you'll burn, regardless of the type of workout. That said, you may be apt to exert more effort, and thus burn more calories, on certain pieces of equipment. According to a landmark 1996 study, machines such as the treadmill and stair climber raised heart rate more than machines such as the cross-country skier, rowing ergometer and stationary bike. These results could be related to familiarity with the activity demanded by the machine (i.e., for most people, walking, running and climbing stairs are more familiar than skiing and rowing). However, using the same machine day after day just because it could burn a few more calories may not be the smartest idea. "If your program has variety, you're less likely to become bored," Bryant says. "It also helps to prevent overuse injuries and can keep your body from adapting, so you may even see an increase in caloric expenditure." As Roberts says, "Once you've adapted, you stop improving. If you're not making progress, it's definitely time to change."

How do I know which preprogrammed cardio workout to choose?

It's really a matter of personal preference, Bryant says. Such preset workouts are essentially a marketing tool developed by equipment manufacturers. Interval workouts that include recurring changes in intensity may be slightly better from a calorie-burning standpoint, he notes. But for the most part, it boils down to how hard you work, which you can control by pushing a button to change the level or resistance, or by speeding up your pace.

Is breaking up my cardio into shorter bouts really as effective as doing one longer workout?

Yes, at least as far as weight loss and general fitness are concerned. "Studies show that the effects of exercise are cumulative," Bryant says. "It all adds up, like loose change in your pocket." The same may not be true for performance, however. So if you're preparing for a race or other athletic event, try to do each scheduled training session in its entirety rather than splitting it up into smaller segments.

I do an hour of cardio almost every day, but I'm not losing any weight. What's wrong?

Aerobic exercise is just one part of the weight-loss equation, Bryant notes. Good nutrition is crucial when you're attempting to slim down. In addition to regular cardio workouts, do resistance training to build muscle, as this can help to keep your metabolism from slowing down (a common consequence of dieting). You also can boost your calorie burn (and hence your weight loss) by trying a more challenging activity or by interval training (alternating several minutes of high- and low-intensity work). Finally, don't use the scale as your only measure. If your clothes are fitting better or you're feeling healthier and more energetic, you're doing great, Bryant concludes.

I hate cardio exercise. Any suggestions for sticking with it?

Despite what you may think, cardio doesn't have to be unpleasant or monotonous, Bryant says. Pursue activities that feel more like play, such as hiking, swimming, a dance class or various sports. During the week, keep mixing it up -- try a group class or a circuit workout. Last but not least, during easier, steady-state workouts, read a magazine or listen to music.

Keep track of how you're doing with our Calories Burned tracker!