阅读理解
The purpose of keeping fit is to avoid illness ,resist the mental and physical fatigue（疲劳）that make us more likely to contract infections ,and above all to feel more pleasure in being alive. Research has shown the value of a balanced diet ,fresh air, sunshine ,enough rest and some forms of regular exercise for everyone .If everyone followed this plan ,the number of people visiting the doctor and going to hospital would be considerably reduced.
Unfortunately, although most people acknowledge the importance of physical fitness, not enough of us put this into practice .We spend too much time watching others from the comfort of an armchair or a seat in the stand at a sports stadium.
One reason why people who are physically fit to live longer is that they do not put the heart under excessive strain（过度劳累）and so they reduce the risk of heart and blood vessel（血管）diseases .They also avoid suffering the consequences of weakened muscles ,which are the cause of a great deal of back and abdominal（腹部的）trouble.
About 640 muscles for about 45% of our body's weight, they must have the ability to store energy and be continually supplied with fuel by the blood .Sensible exercise, suited to each individual and preferably undertaken on the advice of a doctor, is the best insurance of meeting these requirements.
