Traditional "cardio", where you do long bouts (45 minutes or more) on a treadmill or an elliptical machine at a moderate pace has always been the recommendation of doctors and trainers. Why? Because at a moderate pace your body is using stored body fat for energy. You'll often see this referred to as the "fat burning zone." Well, isn't that what we're trying to achieve? Have I finally lost my mind?

Your body is a pretty complicated engine, so when the fat is being burned, it says "well, I better save up some fat for the next time we go through this." So, what does it do? It uses your lean muscle tissue to help preserve fat. Now, that doesn't sound right, does it?

Confused? It's really quite easy. You don't exercise to see the results that happen to your body during exercise - all of the magic actually happens after, not during, exercise. So, while you rest and recover, your body is burning the fat. Yeah, baby!

Intervals are short bursts of exercise followed by recovery, but your body doesn't use fat as its energy source. During interval training, your body will call on carbohydrates which are stored in your muscles. The effort that your body needs to go through to replace the carbohydrates stored in your muscle is much more intense, therefore it needs to call on the stored body fat for up to 24 hours after you're done. So, you would rather burn fat for 45 minutes or 24 hours?

Back to oxygen debt - this is how you can judge the proper intensity of your interval training. Simply put, your interval needs to be of a high enough intensity that, when you stop and start recovery, that you are left panting. This means that your body is asking for more oxygen than you currently can provide.

You can also monitor yourself with a heart rate monitor. If your interval was intense, your heart rate will actually bump up when you stop before heading downward.

You'll be revving up your fat burning engine like never before in 20 minutes or less. At some point I'l share some of my favorite interval workouts with you. But for now, even if you're just walking outside, crank up your intensity for 30 seconds, followed by 60 seconds of recovery at a slower pace for 20 minutes total.