问答题 For years, the prescription for maintaining health hearts had been vigorous exercise—running, swimming, dancing—whatever it took to get the heat rate up and keep it there for 20 to 30 minutes at least three times a week. But that message changed. A panel of exercise researchers convened by the Centers for Disease Control and Prevention and the American College of Sports Medicine reported that people needn't exercise vigorously to improve their health. The American Heart Association has weighed in with similar recommendations. But despite this apparent consensus, there is considerable disagreement in the exercise research community about whether the recommendations are amply supported by scientific data. Policy-makers caught in the middle of this disagreement are in a difficult position. It's a classic dilemma confronting health experts in areas ranging from mammography (哺乳动物学) to diet, where the scientific data are not clear-cut.