Does Cardio Kill Your Gains?

One of the more bro-type questions of life is… should we do cardio?

Now, any true bro won’t even know what cardio is. But really, there has been cause for concern from the fitness community about cardio potentially hurting our muscle gains. A notion borne from a simple understanding of muscle physiology. To build muscle, we need resistance training to create an muscle-growing stimulus and we need more nutrients to feed said muscles.

With cardio, we’re trying to burn calories in order to burn more fat. So, on one hand, you’re trying to lose mass and burn more calories. On the other, you’re trying to gain mass and consume more calories. Not exactly complementary goals. And there’s also the matter of competing muscle fiber adaptations. Cardio, specifically steady-state cardio, stimulates type 1, endurance muscle fibers.Resistance training leans more into type 2 fibers. Trying to stimulate both simultaneously supposedly hinders maximal adaptation for both. Now, these potential gains-killing excuses to not do your cardio sounds fine and dandy, but do the results actually reflect these issues?

A quick dive into the scientific literature would suggest… maybe, but not as bad as some bros would think. Some studies do show that concurrent aerobic and resistance training has slightly worse muscle and strength adaptations when compared to resistance training alone.
However, other studies show that concurrent training didn’t impact muscle adaptations much at all. So, why the difference in findings?

The findings differ depending on how and what type of cardio is used in the study and what measures were observed. When we see cardio getting in the way of gains, it often applies to the gains of the lower body. This makes sense since most of the implemented cardio in studies involve leg-based exercises, like jogging, sprinting, and cycling.

Strength and hypertrophy measures for the upper body were largely unaffected. Another varying factor is time resting between sessions. A very clear trend shows that the more time you can rest between sessions, the less it will hurt muscle and strength adaptations. 24 hours seem to be the best outcome. Cardio intensity also plays a role. Pairing resistance training with low-intensity steady state cardio usually hurt muscle and strength gains. Interestingly enough, pairing resistance training with short-duration high intensity cardio, like HIIT, had little to no impact on strength and muscle gains.

This actually matches the competing adaptions theory mentioned earlier. Steady state cardio is type 1 fiber dominant, which competes with resistance training’s
type 2 fiber stimulation. HIIT, on the other hand, stimulates more type 2 fibers, sharing similar adaptations to resistance training. Other things to note:

Cycling versus running, where running, specifically high-intensity running, seems to be a slightly better option. Cardio duration matters as well, where the longer the cardio session, the more we see it hurt strength and muscle adaptations. And lastly, cardio frequency, where the more cardio sessions performed, the more it might hurt adaptations. This might very likely be a matter of overreaching, creating a constant stress level that is difficult for the body to overcome.

Now it’s clearer as to why cardio MIGHT, but not necessarily will, hurt your gains. And even if it did, when you think about the benefits of cardio, it’s clear you should do it regardless. It is cardio that improves our heart health and aerobic capacity, things that we sorely need for overall health and fitness. Just lifting weights won’t get you that. Also, when paired together, fat loss is much higher compared to either cardio or resistance training alone. So if you want to burn fat AND build muscle at the same time, concurrent training is the best option. These cardio pros, in my opinion, heavily outweigh the cons.

And now, with all this information, we can be smart about it to avoid the cons all together. Best practice is to do higher intensity cardio for a short duration, maybe no more than 20 minutes. Try to take at least a full day between your lifting and cardio sessions. If possible, avoid doing your cardio close to your leg days.And make sure you gauge how your body is responding to the additional cardio workload and adjust when needed to avoid overreaching and overtraining.

So, let’s strap on your running shoes as you would your lifting belts. Let’s get your cardio going.

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