The cardiovascular system responds beautifully to a slow, steady buildup in exercise frequency and intensity. It does not react well to sudden demands--such as a person who never exercised suddenly shoveling snow or running 3 miles. A recent study confirmed age-old anecdotal reports of unaccustomed exercise causing heart failure.

The study was simple, but telling. Researchers in the United States and Germany each interviewed about 1,000 patients hospitalized for heart attacks, asking them what they were doing in the hour before the attack and what their exercise habits were before the attack. They also questioned the same number of people who had not had heart attacks about their activities during the same hours as the ill people. The people with heart attacks were much more likely to have been engaging in unaccustomed strenuous exercise. But the study also turned up good news for those who exercise regularly: Although sedentary people have a two to sixfold increase risk of cardiac arrest while exercising than when not, people in shape have little or no excess risk while exercising.

How much exercise is enough to benefit the circulatory system? To achieve the benefits of exercise, the heart rate must be elevated to 70% to 85% of its "theoretical maximum" for at least half an hour three times a week. You can calculate your theoretical maximum by subtracting your age from 220. If you are 18 years old, your theoretical maximum is 202 beats per minute. Seventy to 85% of this value is 141 to 172 beats per minute. Some good activities for raising the heart rate are tennis, skating, skiing, handball, vigorous dancing, hockey, basketball, biking and fast walking.

It is wise to consult a physician before starting an exercise program. People over the age of 30 are advised to have a stress test, which is an electrocardiogram taken while the subject is exercising. (The standard electrocardiogram is taken at rest.) An arrhythmia that appears only during exercise may indicate heart disease that has not yet produced symptoms.

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