"GET SOME EXERCISE" has long been one of the standard prescriptions from doctors to people whose sedentary ways may be leading them toward coronary disease. The advice was never seen as being much more than a medical establishment hunch that heart attacks could be avoided through regular exercise, the more intense the better. Earlier this week, a report to the American Heart Association suggested that the conventional wisdom may be soundly based after all: Strenuous sports like swimming, tennis, running, cycling and mountain climbing, it said, have definite protective effects against heart attacks.

The study, which involved 17,000 Harvard alumni aged 35 to 75, says that the protection afforded by strenuous exercise extends even to those who are overweight or who have high blood pressure. Among the Harvard men, those whose exercise burned fewer than 2,000 calories a week had a 64 per cent greater risk of heart attacks than alumni who used up more than 2,000 calories a week. (A squash player or a runner expends more than 660 calories an hour).

The findings come at a moment when the death rate from heart and blood-vessel disease is falling sharply. In 1973, 1,062,000 deaths from cardiovascular disorders were recorded. In 1975, with a larger population, only 975,000 such deaths occurred. Last July, an official of the National Heart Lung and Blood Institute said that "great changes" are occurring in the lifestyles of American men and women. Besides the increase in such high-exercise sports as tennis and jogging, he noted that dietary changes are also contributing to the decreases in cardiovascular deaths.

What the Harvard study confirms is that the so-called mysteries of the human body are not so complex after all. In this instance, the heart is a muscle that becomes stronger the more strenuously it is flexed; it needs fewer beats to pump more blood with less effort. As for inducing people to take up strenuous exercises, the Harvard study offers only persuasive advice. The final word - "get off the sofa and into the sweatsuit" - can be uttered only by a person's restless voice from within. Few people take up hard exercise with the idea of avoiding a possible coronary in the distant future. They are more likely to start sweating it out because of the immediate rewards - perhaps the emotional delights of testing one's stamina or the competitive pleasures of doing in one's 40s and 50s what others in the neighborhood gave up doing in their 20s. If the result is motion, the motive doesn't matter - although we can think of worse ones than protection against a heart attack.