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
Human bodies vary in the amount of physical stress they can handle comfortably.