A、Because their hearts have nothing to worry about. B、Because their heart and blood vessel are strong enough. C、Because their mu

The purpose of keeping fit is to avoid iii health, resist the mental and physical fatigue 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, adequate 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 live longer is that they do not put the heart under excessive strain and so they reduce the risk of heart and blood vessel diseases. They also avoid suffering the consequences of weakened muscles, which are. the cause of a great deal of back and stomach trouble.
    About 640 muscles account 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 for meeting these requirements.