Exercise and physical fitness are obviously necessary for athletes, soldiers, firemen, and all those whose jobs require high levels of physical performance. Then why should a sedentary person living in a comfortable, industrialized society exercise?

Medical and health professionals have determined that everyone, depending on the individual’s metabolism, has a minimum level of physical activity that must be maintained to prevent serious physical deterioration. The human body and all of its parts, like any living organism, must be used or they atrophy. The loss of structure and function that occurs when a broken arm is immobilized in a cast clearly demonstrates what happens when body parts are not exercised and used. 

Basic survival once required the output of enormous physical energy by people on many levels of society. Modern technology has simplified life’s physical demands in various ways. Machines------from washing machines to automobiles and elaborate industrial equipment------have reduced the amount of labor required of people.

In more primitive times most individuals burned up the calories gained from the food they consumed through the rigors of their daily activities. This is no longer true for most people, especially those living in industrialized nations.

Many people have retained their capacity for physical work. Even those who have been physically inactive for a long time can restore lost physical capability with just a month or two of daily physical training. People who exercise and reach their near maximum physical capability can maintain it by exercising vigorously on alternate days.

Physical fitness and exercise are important for good physical and mental health, including weight control. Exercise helps the individual develop and maintain a strong self-image and a sense of emotional balance. As a person gets older, exercise becomes more important because after age 30 the heart’s blood pumping capacity declines at a rate of about 8 percent each decade.

Exercise is also important for children. Vigorous physical activity aids in a child’s overall development so he or she reaches optimal size and functional capacities in adulthood. Current research shows exercise can reduce the accumulation of low-density lipoprotein (LDL) and cholesterol on artery walls of children and adults. Excess cholesterol can increase the risk of heart disease. There is, however, no evidence that exercise prolongs life. Former athletes do not live longer than non-athletes, nor are they saved from heart disease. The benefits of exercise cannot be sustained for more than a few months or years without continued exercise. Even Olympic-level athletes will regress rapidly to pre-training levels once they stop exercising. The amount of activity necessary for fitness varies from person to person. Age, physical structure, health, and gender are contributing factors.

Most common forms of exercise, such as bicycling and swimming, rarely cause serious injury. But contact sports, such as football and judo, can cause wear on the joints that can lead to articulation disease, or joint problems. The problem for most beginners is over-exercise. Many people experience stiffness after the first day of exercise, but this is harmless and does not last long. People who are overweight, past middle age, or who suffer from heart disease should consult a physician before starting any exercise program. Sometimes children who are involved in competitive sports suffer from muscle and joint problems.

运动员、士兵、消防员等职业对从业人员的身体健康状况有着严格的要求，因而，他们当然有必要锻炼身体，保持强健的体魄。可是一个居住在舒适的工业社会中坐着工作的人为什么也要锻炼呢？