Intensity

Our cells, muscular as well as other, have the ability to work both without oxygen (anaerobically) and with oxygen (aerobically). In order to work aerobically, the intensity of the activity must be such that the circulatory system can deliver oxygenated blood to the working cells. If the activity is too intense, the amount of oxygen may not be enough for the cells to work completely in the aerobic system; they must meet part of the energy needs without oxygen. This process produces lactic acid, and the body cannot sustain this level of activity for very long.

I often refer to track events to demonstrate the difference between the two systems. A quarter-miler (who runs one lap of the track) runs at a much greater speed than a miler, but he cannot maintain that speed for very long. If a miler starts a race at the same speed as the quarter-miler, he must slow down during the second lap—otherwise he cannot continue. You can demonstrate this phenomenon yourself. Raise your arms to shoulder level (don t do this if you have shoulder problems) and start making small, rapid circles. Not only will your heart and breathing rates rise rapidly, but you will also start to feel a burning sensation in your shoulder muscles and you will eventually stop. If you want to keep going, you must decrease the intensity by slowing down the speed of the circles and lowering your arms. Your breathing and heart rate went up to try to increase the amount of oxygen to the working muscles; when you slowed down the activity your circulatory system was able to meet the needs of those muscles, and you could keep going.

The rise in heart rate is directly related to the intensity of an activity —the more intense the activity, the higher the heart rate, at least up to a point. You have a resting heart rate (which is like your baseline) and a maximal heart rate (which is the highest your heart rate can go). Maximal heart rate is related to age, and the most common formula for estimating maximal heart rate is to subtract one s age in years from 220. Because of the linear relationship between heart rate and exercise intensity, you can assess your exercise intensity by monitoring your heart rate.

The American College of Sports Medicine (ACSM) has developed exercise guidelines for the healthy adult based on numerous studies (Pollock et al. 1998; ACSM 2000). The recommended intensity for aerobic exercise, in order to get a training response, is at 50 to 85 percent of a person s heart rate reserve (HRR). Heart rate reserve is a method of adjusting one s estimated maximal heart rate using one s resting heart rate. The sidebar shows the formula for finding your heart rate reserve, called the Karvonen formula, and includes an example of estimating intensity. The heart rate to aim for during exercise is usually called the target heart rate. I recommend using the Karvonen formula because it gives you a more appropriate heart rate if you have a resting heart rate that is very low or very high.

Assessing Your Intensity

Setting Your Intensity