Aerobic exercise is commonly referred to as cardio and includes things like jogging and biking. The importance of aerobic exercises in your quest to become buff is twofold. First, if you're eating as you should to gain muscle, you might be gaining some weight. Cardio will help keep the unwanted type of weight off and allow your new muscles to show. Second, and most important, when you're engaged in aerobic exercises your heart is going to be working at a higher rate and will be pushing blood through your body faster. Your muscles will be getting more blood and oxygen, helping them rebuild [source: Casillo].
Short bursts of speed will help you get the most out of your cardio routine. Think interval training. If you're going for a run, jog for a minute and then sprint for a minute. Work up to a total of about 30 minutes of running.