Have you ever heard that if you do cardio, you'll lose all your gains? Yeah, me too! When I first started working out I would avoid doing cardio at all cost because that thought was really ingrained in my mind. But is that true? Is doing cardio that bad if your primary goal is to build muscle?
To understand why this MIGHT be the case, think of the strength-endurance spectrum. If you train for strength, you’ll have specific adaptations; if you train for endurance, you’ll have very different ones, because they're in pole opposites of the spectrum. This means that, whether you do one or the other, your body will adapt in different ways and via different pathways, generating different outcomes. Not only are these pathways different, they can even be mutually exclusive. This means that doing cardio can decrease the adaptations you get from strength training. This is called the interference effect.
Hypertrophy is not on this spectrum because it’s not a skill, it’s a physiological adaptation that can happen with different types of training, but the overall type of training (and therefore adaptation), is very similar to the strength side of the spectrum. This means that this effect, despite being stronger in strength training, also applies to muscle growth.
To mitigate or counteract it, consider the following:
Volume, frequency & duration – the more volume and frequency you do, the worst the interference effect.
Duration – The longer the cardio session, the worst the interference effect. Generally, sessions below 20 minutes have a small interference effect.
Intensity – choose lower-intensity modalities. It seems that higher-intensity cardio causes a higher interference effect. HIIT (High Intensity Interval Training), especially in the form of interval sprints, might be one of the worst options. The only real benefit is the time you save.
Schedule – from the worst to the best scenario you have:
doing the cardio session right before the training session.
doing the cardio session right after the training session.
doing the cardio session at least ~6 hours after the training session.
doing the cardio sessions in days where you don’t have training sessions.
Modality – most (but not all) of the effect is local, so if you don’t want any specific muscle groups bigger, rely on cardio that uses primarily those muscle groups. Besides that, choose modalities that don’t have a lot of impact (greater stress on the joints and connective tissue) and eccentric muscle contractions, which are more damaging and require more time to recover from. Assault bike, cycling, stair waking, elliptical or rowing are good options.
So, the interference effect is real, but is generally not a big deal if you're not doing tons of cardio. You can absolutely ally cardio to your strength train routine, without your gains being diminished. Just don't overdo it. Pay attention to the points above and, if your main goal is gaining muscle apply them to the best of your abilities, so that you can have the best gains possible.