I'm not sure I agree with that. The "fat burning zone" is a simplistic idea. At low heart rates, you are burning a high percentage of fat. As you increase the heart rate the ratio of fat to carbs being burned shifts. At a sprint you are burning predominately carbs. But that's not necessarilly bad. Studies show that working at 90% of max heart rate stimulates Growth Hormone, which leads to accelerated fat loss for several hours after the exercise is over.