For people who aspire to be better physically, be it as a runner, a swimmer, a bicyclist or a weight lifter, they need to choose how to train. For many, it may make sense to jump in and train with a single focus. Let’s take running for example. You want to be the best runner you can be, so you may focus all of your time and energy hitting the path, running shoes on. An alternative approach is to take advantage of cross-training and help develop strength body-wide, instead of merely focusing on one aspect.
If you cross-train you work with the body as a whole. Why would doing high-impact aerobics help a runner? Aerobics is a cardiovascular workout. While it may work the legs some, it focuses more on the cardiovascular system. By strengthening the heart and increasing lung capacity, a runner is better able to use their legs to go further and faster. Make sense? Same with training for any sport or activity—by developing the body as a whole, all the systems strengthen and work together.