阅读下面的短文，根据要求完成下列小题。 America is a country on the move．In unheard-of numbers, people of all ages are exercising their way to better health．According to the latest figures, 59 percent of American adults exercise regularly－up 12 percent from just two years ago and more than double the figure of 25 years ago．Even non-exercisers believe they would be more attractive and confident if they were more active． It is hard not to get the message．The virtues of physical fitness are shown on magazine covers, postage stamps, and television ads for everything from beauty soaps to travel books．Exercise as a part of daily life did not catch on until the late 1960s when research by military doctors began to show the health benefits of doing regular physical exercises．Growing publicity for races held in American cities helped fuel a strong interest in the ancient sport of runningAlthough Americans have also discovered other forms of exercise(such as walking and swimming), running ________． As the popularity of exercise continues to increase．so does scientific evidence of its health benefits．The key to fitness is exercising the major muscle group vigorously enough to approximately double the heart rate and keep it doubled for 20 to 30 minutes at a time．Doing such physical exercises three times or more a week will produce considerable improvements in physical health in about three months． | 
