People who cross-train have one main activity, such as running or yoga, but they complement this activity with other exercises. When done properly, cross-training gives the body everything it needs in terms of exercise and fitness. Elite athletes cross-train. For example, a football player's yoga practice makes him less vulnerable to injury when he plays on Sundays, a dancer's weight training makes her stronger for a performance, and a swimmer's walking helps her build bone. This chapter will give you an introduction to cross-training, as well as offer two programs you can try.