One cost of aerobic running fitness is loss of explosive muscular strength. When untrained, I normally bound up three stairs at a time, but I know Iâm becoming trained for long-distance running when I can do only two at a time. The loss of explosive power that occurs with aerobic conditioning is thought to involve a conversion of fast-twitch, anaerobic muscle fibers to slow-twitch muscle fiber characteristics. That is, with aerobic training we lose sprint speed. Conversely, we can specifically train sprint speed, but it is at the cost of endurance. Similar trade-offs apply to being fat, another aspect of being out of shape. [p. 201]