Cardio should really be done regardless if you're bulking, cutting, or maintaining just for cardiovascular health but it has some extra advantages when done while bulking. For just cardiovascular health doing some sort of cardio for 10-20 minutes 3-4 times a week is sufficient, this could be running outside/on a treadmill, jump roping etc. If you have a hard time getting enough calories doing some cardio can be great to spike your appetite or if you like to eat a lot of food you can afford to eat more if you throw in some cardio every once in a while. I would recommend not using cardio machines and just running outside or doing other activities but you could use machines if you really want to. Circuit style workouts/conditioning workouts are great too. Personally, I jump rope around 20 minutes a couple times a week and that works great. You don't ever want to be that guy who starts panting heavily after walking up one flight of stairs, so do your dam cardio.
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