When you run, you're constantly working your legs and arms, but to become a faster, stronger runner, that's where cross-training comes in. Cross-training can be high or low-intensity exercise and is different than just tacking on more miles. Engaging in cross-training will not only make you a better runner, it will also help you avoid injuries. Cross-training contained in this book will help to prevent injury by allowing your primary muscles the opportunity to rest and recover while providing your body with a good cardiovascular workout. Also, you will also be helping your supporting muscles to grow stronger, without having to do additional resistance training, which can increase your power, strength, and even speed in your primary sport. It will also explain to you how to add flexibility, agility, and stability into your overall fitness when regularly performing cross-training activities. Again, you'll be developing additional coordination and stability which will only help you to perform better and better. For example, runners use their hamstrings and calves the most, as these are the primary muscles used when running. For a runner, kickboxing may be the ultimate cross-training workout because it helps strengthen the core, promotes balance, and helps strengthen the underused muscles of the legs such as the quadriceps and abductors, which can make a difference when running a marathon.