Teenagers may want to build muscle fast. To do that, they need calories. For some, calories do not come in quick and easy. Some have to eat their ass off to get those CLEAN calories in. Now what does cardio do for you? It burns calories, yes. Burning calories isn't really good for muscle building but it is good for other stuff. You might think that cardio is bad for you, but it really isn't. If you can do 30 minutes of cardio for day, it can be jogging, walking, or anything else, it will help your cadiovascular health.
Now don't get me wrong, when you are lifting weight, you are doing some sort of cardio, which is already helpful. But if you have time, why not add some 30 minute cardio 3-4 days a week. It could be walking to your gym, or even walking back from school.
Stay in school and stay healthy!
