The zone is the intensity of exercise that safely produces the results you want. If you exercise below the zone, you risk not getting results. If you exercise above the zone, you risk not being able to continue your exercise or, worse, you risk injury. The zone is the perfect intensity level at which to perform your cardiovascular exercise -- between 70 percent and 80 percent of your maximum ability.

 How do you know when you're in the zone? There are essentially two ways: taking your heart rate and monitoring your level of perceived exertion. I prefer monitoring perceived exertion because there are a number of shortcomings in using your heart rate to monitor your exercise intensity.

First, your heart rate doesn't always directly reflect how hard you're working or how much oxygen you're consuming. A number of factors beyond your control can throw this relationship off, such as your emotional state, what you're thinking, medications you're taking, the temperature and the altitude you are in and your caffeine consumption. This could mean that although you're exercising at your prescribed target heart rate, you may be working either too hard or not hard enough.

Second, the formula to find your range (220 minus your age multiplied by 75 percent and then plus or minus 5 heart beats) is only an estimation of your heart rate and is accurate for only about a third of the population.

Third, obtaining an accurate maximum heart rate requires a maximum treadmill test and most people do not want to incur the cost and inconvenience of doing this.

Finally, have you ever taken your heart rate during exercise? Even if you're one of the lucky people whose target heart rate can be accurately calculated by the formula, trying to measure it during your workout can be next to impossible. You have to stop exercising, find your pulse and count the number of heartbeats all in a matter of seconds so your heart rate doesn't slow down too much. Many people couldn't find their pulse even if they were given an hour to do it!

You can see why I prefer perceived exertion, a method that requires you to pay close attention to your body and what's happening to it. Perceived exertion is a subjective rating of how hard you're working during exercise, which is based primarily on your breathing. You simply evaluate -- using a scale from zero to 10 -- how hard you're working based on how short of breath you are. Your optimum zone is anywhere between a seven and eight. I want you to exercise consistently at level seven when you first start out. Later, you can stay at seven or move up to working at level eight, if and when you're ready.