The sport of triathlon is an interesting example of cross-training in action. In a triathlon, contestants take part in three activities, one after the other: swimming, biking, and running. Swimming is aerobic, improves your flexibility, uses most of the muscles in your body (improving muscular strength and endurance), and is not weight bearing (giving your skeletal system a rest from the pounding effect of running). Biking is aerobic, uses the hip and leg muscles as well as some of the upper body, works your balancing abilities, and, like swimming, is not weight bearing. Running is aerobic, uses the hip and leg muscles, and is weight bearing, but it is also jarring.

Even triathletes, who are extraordinarily fit, have to cross-train and do activities other than swimming, biking, and running. This occurs because, as you can see from the descriptions above, their sport (triathlon is considered one sport comprising three activities) doesn't include enough strength-training and flexibility. In fact, most triathletes make time for weightlifting and stretching, at the very least. This makes them better athletes and cuts down on their risk of injury.