On many treadmills, ellipiticals and other cardio machines at the gym, you'll find a chart that gives target heart rates for working out in the "fat-burning zone" and "cardio zone." Many of these machines also offer pre-programmed workouts for each. If you've ever used these routines or tried to hit these target heart rates for both zones, you've probably noticed that exercising in the fat-burning zone is easier than working out in the cardio zone. Does that mean you're burning more fat working out at a lower intensity? Michelle Hering, M.S., Best Life fitness expert, explains these settings and tells you how to get the most calorie-burning benefits out of your workouts. Read her comments below.

Question: Is calorie burn affected by how hard you work out? So if I work out at a 4 or 5 on the perceived exertion scale will I burn more or less than a 7 or 8?

Michelle Herring: The simple truth is the harder you work out, the more total calories you burn. So, using the rate of perceived exertion (RPE), if you're working out at a 7 or 8 (which is roughly where the cardio zone would put you), you'd be burning more calories than exercising at a 4 or 5 (where the fat-burning zone would put you.) But it can get a little confusing becuase you would expect to burn more fat and calories in the fat-burning zone.

The idea of a fat-burning zone came about when scientists discovered the different sources of energy that we use in order to keep up with external stress, such as exercise. From a physiologic standpoint, the body uses a greater percentage of calories from fat when working out at the lower end of the RPE scale, however you burn fewer total calories at this level.

To give you an example, let's say you were to take a walk, which would burn 100 calories. Maybe 75 percent of those calories are from fat. If you chose a higher intensity exercise, you could burn 200 calories with about 125 of those coming from fat. The percentage of fat calories burned may drop (walking burned 75 percent while the higher intensity workout burned 62.5 percent), but you burn more total calories, and therefore more total fat calories at the higher intensity.

The bottom line is for sustained weight loss, a person must burn more calories than they are taking in. Putting too much emphasis on burning fat calories alone will not help. Calories are calories and the ultimate goal is to burn as many total calories as possible. That means you'd be better off trying to work out in the cardio zone instead.