When the level of conditioning decreases, so too does the body’s ability to recover which obviously negatively impacts muscle gain. Another advantage to performing low-intensity cardio training during the bulking phase is that it requires a lot less effort to maintain a certain level of cardiovascular conditioning than it does to reach it again from a state of being unconditioned.
An added benefit of doing aerobic training is that it could potentially improve mass gaining results during a bulking phase with overall caloric partitioning. It has been proven that the most effective form of caloric partitioning that has ever been tried is lifting weights.
Regular muscle contractions increase the uptake of nutrients into skeletal muscle
This basically means that when you’re lifting weights, fewer excess calories get deposited as fat cells. It is still debatable how big the impact of low or moderate intensity cardio will have on this, though it certainly won’t be counterproductive if performed at a reasonable level and will most likely help in the long-term too. Additionally, there’s also the issue of keeping the fat-burning processes active and staying lean during the bulking phase. Cardio is not even the most effective way to lose fat to begin with, however it helps create a caloric deficit, provided that you don’t eat more to compensate for it.
In any case, a good reason why you might want to keep cardio in your workout program is that if you are currently in the bulking phase, you will most likely want to lean out eventually. It has been proven time and time again that if you want to get the fastest rate of muscle growth possible you have to allow some fat gain at the same time, and it’s only logical that this accumulated fat will have to be lost through proper dieting. So, if we conclude that this is the best possible method, then you might as well try to gain the least amount of fat possible.
One of the reasons why steady state cardio is not commonly considered effective when it comes to losing fat is because very often during a bulking phase or a de-load period where the lifter usually eats a lot of food, the body loses its efficiency at using fat as an energy source. If this resting period is followed by cardio and dieting, it can take the body several weeks to be able to provide enough amounts of fat as an energy source and it is during this period that muscle tissue is sacrificed.
This partly explains why when first starting out a new diet, it can a couple of weeks before you start getting noticeable results. The scenario above tends to be even more pronounced when the lifter has done no cardio at all during the bulking phase or during the deload phase.