From a recently published study:
This study assumes that individuals choose to exercise primarily to extend their lives. These individuals recognize that this gain in longevity comes at the cost of hours lost to exercise. Given that many previous studies have found that consumers have fairly high discount rates, the image of the American as couch potato, rather than being disparaging, can be seen as evidence for the traditional economic assumption of a rational consumer.The paper is by Kemper W. Moreland. He assumes that the main purpose of exercising is to extend one´s years of life. He explains:
This assumption may appear extreme, but we note that many of the other supposed benefits of running wilt under scrutiny. Runners occasionally say that they enjoy running, but casual observation does not reveal many smiling runners. People might exercise to lose weight, but eating less usually works better; see Pontzer et al. (2012) and Thomas et al. (2012). Runners speak of endorphins, nature's natural pain reliever, but the makers of Tylenol and Advil do not fear that the next 10K run will reduce the demand for their products. In addition, we note that there are also costs associated with running: the Centers for Disease Control (2002) lists aerobic exercise as one of the top injury activities for men and women over 20. For convenience and tractability, we avoid these ancillary costs and benefits by assuming that longevity serves as the sole benefit from running (or equivalently, that any additional benefits are offset by costs).As you might have, I have also met people who have suffered serious injuries when working out, and in some cases they have to stop exercising completely. Almost everybody advises to exercise but the costs are not usually discussed, and because of that we might tend to overestimate the net gains. One of the interesting contributions of Moreland´s paper is that it mentions the costs, and includes some of them into his analysis.