Smarten up your cardio: which machine blasts the most calories? How hard do you need to exercise? We've got the answers, plus three workouts to help y

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

fitness experts weigh in on seven common conundrums.

q How much cardio do I really need?

a 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.

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

a 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.

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

a 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."

q How do I know which preprogrammed cardio workout to choose?

a 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.