The main object most people have in mind by running is the prevention of heart attacks. The goal of running is to enhance cardiovascular fitness and stamina and significantly increase the pulse rate for a prolonged period of time. The goal of such exercise is a physiological one, a sustained increase in heart activity and oxygen consumption, rather than a specific physical goal. The measure of cardiovascular exercise is an appropriate increase in pulse rate (not fatigue, sweat, or muscle ache).

A cardiovascular fitness program as running involves three phases: a warm-up period, the actual exercise period -- during which the pulse rate is increased -- and a cool-down period. All three are important and should be tailored to the individual and the particular kind of exercise involved.

The actual pulse rate and amount of exercise will vary with each person according to age and physical condition. In general, the maximal heart rate for a healthy person is 220 minus his or her age. The exercise heart rate -- the pulse rate to be sustained during exercise -- is approximately 75 percent of this maximal rate.

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Exercise at less than 70 percent of the maximal pulse rate for a given individual loses some of its value in developing cardiovascular fitness. Exercise at more than 85 percent of the maximal heart rate introduces unnecessary stress and adds no benefits.

The actual amount of such exercise needed for benefit is a matter of debate among experts. In general, however, the recommendations suggest sustained exercise for at least 20 minutes at least three times per week. This program should be achieved gradually over a period of weeks, or possibly months for those who previously have not been physically active.

In general, a person under age 35 with no current health problems and no past history of disease can undertake an exercise program if it is done gradually and moderately. They should, of course, stop at the earliest sign of any problem: chest pain, dizziness, shortness of breath to the point of inability to speak while exercising, and so on.

Over age 35, hidden heart disease becomes more likely and a physician's checkup before undertaking an exercise program of running is highly recommended.

Dr. Robert F. Weiss is a sports podiatrist specializing in foot and ankle surgery. He was a member of the Medical Advisory Committee of the 1984 and 1988 Olympic Marathon Trials.