"Come on, Jill, run faster -- we only have three more weeks to fit into our bikinis!"

"No way, Sarah! If I run faster, I will move out of my fat-burning zone, and I need to get rid of that excess fat!"

Who's right? Who will burn more fat, who will burn more calories and who will lose more weight?

We've all seen the cardio equipment in the gym. The ellipticals, Stairmasters, treadmills and bikes all have some way of gauging how hard you are working based on your heart rate. Basically, according to the machines, a lower heart rate places you in the "fat-burning zone," while a higher heart rate puts you in the "cardiovascular zone." Why would you want to work harder to achieve that higher heart rate when the machine is telling you this will move you out of the fat-burning zone?

Let's examine the truths behind these "zones" and what they mean for your workout.

The fat-burning zone is defined by a lower-intensity workout (meaning you are not working as hard). Jill is working in her fat-burning zone, so 60 percent of the calories she is burning are coming from fat stores. But because she is working at a lower intensity, she is burning fewer calories overall.

The cardiovascular zone is defined by a higher-intensity workout (meaning you are working harder). Sarah is working in her cardiovascular zone, so 30 percent of the calories she is burning are coming from fat stores. Sarah is working at a higher intensity, though, so she is burning more calories overall.

Now let's look at how many total calories and how many calories from fat each girl is burning.

Let's say each girl weighs the same and is working on the same piece of cardio equipment for the same length of time. If Jill, working at low intensity, burns 100 calories, 60 percent of them -- or 60 calories -- would come from fat. If Sarah, working at a higher intensity, burns 400 calories in the same amount of time, 30 percent of them -- or 120 calories -- would come from fat. (Note: These numbers are an approximation to illustrate the concept. Actual numbers will vary.)

So, even though a higher percentage of Jill's calories came from fat, Sarah still burned more fat calories. Sarah also burned more calories overall.

What is the end result of the two girls' workouts? Sarah will burn more calories, burn more fat calories and lose more weight. Jill could burn more fat calories if she wanted to, but she would have to continue working out twice as long as Sarah.

The main indicator for weight loss is not fat calories, though. The true indicator for weight control is burning more calories than you consume. And the most efficient use of time would be to exercise at a higher intensity to burn those calories quickly.

Now, I am not advocating that everyone must participate in high-intensity workouts at all times. Low-intensity workouts do have their place.

For instance, low-intensity workouts can be used on recovery days, or to change your routine and avoid getting burned out on one particular method of exercising. Another advantage of a low-intensity workout is that you are less likely to get injured or re-injured.

Whatever exercise you decide to participate in, though, don't let the machines in the gym tell you how good of a workout you are receiving. Use your own judgment and monitoring to gauge your workouts.