OLD WAY
Do moderateintensity cardio to burn fat.

NEW THINK
Boost the intensity– but not too often.

While exercising at a moderate level does melt calories, the harder you push, the more calories and fat you'll blast. But vigorous sessions are physically demanding, so you shouldn't do them every day. To get a high calorie burn you can maintain, spend most of your workout time at your lactate threshold, or LT, which is your fastest sustainable aerobic speed. Depending on your fitness level, that can be anywhere from 60 to 80 percent of your maximum heart rate (go to here to see how to estimate your max heart rate) —or a rate of perceived exertion (RPE) of about 7. Exceed this intensity and fatigue sets in rapidly. Research has shown that the highest rate of fat burn occurs at or slightly below your LT. "Working out at this level teaches your muscles to use more fat during exercise," says Todd A. Astorino, Ph.D., an assistant professor of kinesiology at California State University San Marcos.

Here's how to do it: After a 5- to 10-minute warm-up, exercise for 15 to 20 minutes at your LT. (You might have to experiment to find a level that feels hard but sustainable.) Finish with a 5- to 10-minute cooldown. If you can’t maintain your LT for 15 to 20 minutes, break up your workout into shorter segments with rests: Do 5 minutes at your LT, rest for 1 minute, and repeat 3 times. Build up to 20 solid minutes.