Engaging in a variety of aerobic training methods reduces the chance of acquiring an overuse injury. At the same time, it conditions muscle groups that might be neglected when focusing on just one form of cardio. Think about it: In an explosive sport like BMX, you don't want the slight build of a marathon runner, nor the larger upper body and slimmer legs of a swimmer. If the aerobic exercise itself isn't your sport, then you want to stay aerobically fit through a variety of training types, while maintaining a balanced physique.
Nyquist says he has completed two Olympic-length triathlons through the Lava Man series. It's not surprising that he noticed a massive increase in his in-event stamina while training for them. "Each event [running, biking, swimming] had its own benefit. Swimming got me in amazing shape. The only downside was that I was really losing body weight – I need some of that armor for when I fall!" he says. The goal is to strike a balance between the benefits of establishing an aerobic base and keeping the muscle mass required for a particular sport.