Many people seem to view cardio as the "gains killer" as if any muscle automatically would vanish if this form of training was mingled with a muscle building program. Athletes and exercisers have been known to have 2 seasons, an in-season and and off-season. For those who desire to build a better physique, or even compete, the in-season is typically where fat is lost and the off-season is where muscle is built. The off-season for a lot of people doesn't include cardio training due to it's reputation as a potential muscle catabolizer. So is it really all that bad? Before you cut out your cardio completely, I would recommend you continue reading. When calories are in a surplus, it is very difficult to eat away your muscle so long as you are sleeping enough and your weight training includes basic principles like overload and progressions. Also, there is no reason to believe that some moderate amount of cardio training will do any damage to your "gains". Now that we got that out of the way, can it actually be beneficial for building muscle?
The short answer is, yes, when it is done correctly. During your muscle building cycles, I would recommend 2-3 high intensity interval sessions per week at 20 minutes per session. I would recommend a 3:1 rest:work ratio for these sessions. For example, you could walk for 90 seconds and then sprint for 30 and repeat. One thing that is important to keep in mind is that your high intensity bouts need to be 100% intensity, balls to the wall! This type of training will actually increase satellite cell production which in turn means more muscle! Thats right, cardio training can actually help you build muscle. So why not do more? It is also important to understand that the intensity with these sessions will actually cause muscle breakdown and thus will require some recovery time. Also, you don't want to be so sore from your HIIT training that it hinders your weight training sessions. I would recommend switching off between the stationary bicycle, "deadmill" pushes, row machine, sprinting and stairs. So there you have it. Do your cardio and do it right to maximize your GAINS!