Physical activity and health: need to define the required stimulus.

Exercise is frequently recommended as part of a comprehensive program of prevention, treatment or rehabilitation of chronic degenerative disease. General guidelines on prescribing exercise are based more on the characteristics of exercise required to improve physical performance than on preventing disease. Most exercise regimens are evaluated according to their effect on aerobic power or endurance. Health benefits of exercise may occur in conjunction with an improvement in physical performance capacity, but some benefits appear to be achieved by exercise that normally does not lead to improved physical fitness. Health benefits may occur as a result of repeated acute responses to exercise (but without producing a training effect) and by frequent performance of low intensity exercise (inadequate for increasing fitness). Psychological benefits may also be derived by the process of exercising. The persons who benefit most from an increase in exercise are the very inactive because the detrimental health-related consequences of extreme inactivity are rapidly reversed. There is less evidence that an increase in exercise by the already active person results in significant health benefits.