[填空题]Physical fitness is the result of many factors—good medical care, proper nutrition, adequate rest and relaxation, and sensible personal habit. But these are never enough. An essential factor is regularly physical activity-exercise for a body that needs it to function well.
All of us know that ordinary movements—things as running, jumping, and bending— made possible by muscles. Muscles also do many other things. They suck air into your lungs, push food along your digestive tract, and tighten your blood vessels to raise blood pressure when you need more pressure for an emergency. Your heart is a muscular pump.
When your muscles are not used, or are not used enough, they do not develop and grow proper. They soon become flabby and weak, shrink in size, and lose the ability to do the jobs they were mean to do. Recent studies showed that the average high school student spends fifteen to thirty hours a week watching television and only two hours a week in organizing play or ex