There's a range of exercise intensities and under some circumstances you may benefit more if you exercise at the low end of the range. But how do you know if you're in the range?

There has been a major change since I wrote my original book "Fit or Fat?" in 1977. At that time the intensity of exercise was largely based on the formula 220 minus your age = maximum heart rate.

In those days we thought that this formula applied to a large percentage of the population, as much as 86 percent. We now know it applies to only about 60 percent. Approximately 15 percent of the population have hearts that beat considerably slower than the predicted maximum and another 15 percent have hearts that beat much faster. This doesn't mean there's anything wrong with these hearts or that they are abnormal. It just means they aren't "average".

Let's say your heart beats faster than average during exercise. If you're thirty years old, you would expect your heart to beat 220 minus 30, or 190 beats a minute if you exercise at maximum. But yours goes 210. It's as if you have a Kawasaki heart: it's built very well but it's made to function at a high RPM. When you exercise, your heart goes much faster than all the charts say it should, and your aerobics instructor is afraid you're going to die any minute. You're not - you just have heart that beats very fast and is therefore off the chart.

Another 10 percent of the population (I'm only guessing at this number) is taking medication that affects heart rate. These people's hearts may fit the predicted formula, but the medication artificially depresses their heart rate during exercise. Pulse monitoring as a measure of exercise intensity is not reliable for this group either.

If you add together the 15 percent of people with slow-beating hearts, the 15 percent with fast-beating hearts, and the approximately 10 percent on some kind of medication, you have 40 percent of the population for which the 220 minus age formula becomes completely useless. Only about 60 percent of the population finds the formula to be a useful one.

Let me tell you a newer method. The new approach is simply to use common sense. When you are doing aerobic exercise, keep in mind the basic intention of the exercise. You're not trying to burn a lot of calories. You're saying to your body, "Please adapt to this so that tomorrow I can exercise better than I did today."

What you are really after is an adaptation phenomenon, since the body seems to adapt to whatever treatment it receives. It adapts to hard, intense exercise by changing muscles so that they burn sugar well and fat poorly. Slow, gentle exercise, on the other hand, turns muscles into fat-burning machines. It's the time you spend urging your body to change that really matters. The body adapts beautifully to steady pressure, just as teeth can be moved by the gentle, steady pressure of braces. I see men who run like crazy around the local track, proud that they can cover a mile in six minutes flat, and then wonder why they still must fight a bulging waistline. Such exertions are as effective in weight control as trying to move teeth with a hammer. Run slower and longer and let your body adjust.

With this in mind, you need to adjust the intensity of your exercise; your pace should be comfortable enough that you can continue beyond the minimum twelve minutes without feeling fatigued. You should be breathing deeply but not gasping. Some people call this the "perceived level of exertion," while others simply use what they call the "talk test." Say you are jogging with a friend. Is he able to talk, but you are not? Each of you should be able to talk a little bit, but neither of you should be able to sing an aria. For fun, try singing "God Bless America". If you can't get beyond the first word without gasping, you're exercising too hard. On the other hand, if you get past "land that I love" before you need your first breath, you should speed up.

When I'm teaching people how to exercise, I use the "talk to me" test. It's the same basic idea. If you're on a stationary bicycle, I say, "Can you talk to me? What's your name? Where do you live? What's your phone number?" If you can't talk to me without huffing and puffing and groaning, I know that the bicycle tension is too tight or you're pedaling too fast. Either one means you are doing anaerobic exercise. Your muscles are working without oxygen.

Exercising according to your perceived level of exertion or by using the talk test just boils down to using common sense. As you exercise, think to yourself, "Am I doing something so gentle and easy that I can go on for twenty, twenty-five, thirty minutes? Will my body change tonight as a result of this, or am I exercising too slowly or too fast?" If you are able to talk haltingly and are breathing deeply but comfortably, then you are almost certainly within the training zone. There's nothing wrong with occasionally exercising outside your training zone, but it doesn't fit the definition of aerobics.

Once you have found a comfortable exercise intensity, try taking your pulse. For 60 percent of you, the pulse you get should be between 65 and 80 percent of the formula 220 minus your age (maximum heart rate). But maybe you belong in the other 40 percent. Stick with the comfortable, common-sense intensity. We would probably find that the heart rate you get while exercising at that comfortable rate is 65 to 80 percent of your true underlying maximum heart rate as determined on a treadmill. Even though it doesn't fit everyone, we still recommend heart-rate monitoring of exercise as a useful tool.

For more on heart rate training see Smart Exercise.

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