Human bodies vary in the amount of physical stress they can handle comfortably. Some organ damage results from any exercise that is more stressful than one's body can handle comfortably. Younger athletes can exercise harder before their bodies begin to put any energy into preventing organ damage than older athletes can. Any physical stress will improve fitness as long as the body does not have to put most of its energy into preventing organ damage.
The body responds to the physical stress of moderate exercise by improving its physical condition. But one cannot increase the severity of exercise indefinitely and still improve one' s physical fitness. This is because, under excessive stress, the body puts most of its energy into preventing organ damage. The ideal exercise is one that is just slightly more stressful than one' s body can handle comfortably. Of course, the ideal intensity of exercise varies from person to person.
Which one of the following is most strongly supported by the information above?