Does Cardio Kill Your Gains?

by Joseph Munoz

Does cardio impact your ability to build muscle?

This is an important question to answer because there are many people who purposefully avoid any kind of cardiovascular exercise because they think that it will hinder their ability to get jacked.

While there is some truth that cardio may inhibit your ability to make gains in some situations, cardio does not inherently prevent you from gaining muscle in all situations. In fact, if you have a well-structured training program, performing cardio alongside your resistance training may help improve your gains rather than hinder them. For example, low-intensity forms of cardio like walking may help promote recovery, which can improve your ability to train hard in the gym and potentiate better muscle growth.

Nonetheless, there are certainly some situations in which cardio may inhibit your muscle-gaining progress, which we will discuss later in this article.

How the different goals of cardio and resistance training have led to the myth that you can’t do both

So, if cardio doesn’t inherently kill your gains, where does this idea even come from?

This idea comes from the fact that the adaptations caused by resistance training and cardio oppose each other. I’m going to explain this as simply as possible. The type of exercise you do stimulates adaptations in your body that will make you better at that specific type of exercise.

For example:

  • If you lift weights, your body will get better at lifting heavier weights.
  • If you run long distances, you’ll eventually be able to run longer distances.
  • And if you jump a lot, you’ll get better at jumping.

The adaptions that occur in your body following exercise are determined by specific mechanisms that happen inside different types of cells. For example, the main way that lifting weights causes muscle growth is by activating a protein called mTOR inside our muscle cells. mTOR is what we call the master regulator of muscle protein synthesis. mTOR activation is the same mechanism by which eating protein helps stimulate muscle growth as well.

On the other hand, long-distance cardiovascular exercise like running doesn’t promote muscle growth. Instead, it promotes better cardiovascular endurance by activating a protein called AMPK, which is involved in mitochondrial biogenesis. In other words, AMPK helps your muscle cells grow more mitochondria, which improves your ability to use oxygen and increases energy production during exercise, which results in better cardiovascular endurance.

So, lifting weights stimulates muscle growth by activating mTOR and cardio increases endurance by activating AMPK. However, AMPK also directly inhibits the activation of mTOR as shown in the image below. Since mTOR activation is necessary for building muscle, it’s speculated that cardio inhibits muscle growth because it stimulates AMPK, which directly inhibits mTOR.

 

Image source: mTOR/AMPK signaling in the brain

I mean, it makes sense. If you want to run marathons, it’s not helpful to have to carry around an additional 100 lbs of muscle, so your body isn’t going to promote a ton of muscle growth if you’re trying to be an elite endurance athlete.

What the clinical research say about doing cardio and resistance training together 

The issue is that many of these mechanistic studies often don’t pan out when we test them in controlled clinical studies.

For example, a 2022 meta-analysis analyzed data from over 15 concurrent training studies, which are studies that use both cardiovascular and resistance training interventions at the same time. The researchers found that concurrent training has, at best, small detrimental effects on lower body hypertrophy of type 1 muscle fibers. However, only the studies that used running as a cardiovascular intervention showed negative effects on the growth of type 1 muscle fibers, not those that used cycling instead of running.

Type 1 muscle fibers are slow-twitch muscle fibers, which are main muscle fibers used during long-distance endurance exercises like running. The authors speculated that the slight negative effects were not due to the fact that participants were doing cardio itself since cycling did not negatively affect muscle growth, but instead, it was an interference between running and resistance training. This simply means that the type 1 muscle fibers weren’t able to fully recover from training and the running was negatively impacting their ability to train hard during lifting because it was simply just too much total exercise.

Keep in mind, the participants in these studies are not well-trained and they probably don’t concurrently train on a regular basis. So, if you completely change up their exercise routine and have them start doing some running on top of some hard lifting, they’re probably going to be very sore for a while and probably won’t be able to lift as hard as they usually would if they weren’t running. So, the negative effects of running are not inherently due to cardio but instead, by the fact that the type 1 muscle fibers weren’t able to recover between sessions.

However, this doesn’t mean that cardio is bad for overall muscle growth. Type 1 fibers are just one type of muscle fiber of a couple that exist inside our muscles.

This other 2022 meta-analysis of over 43 controlled studies examined the effects of concurrent training on total muscle hypertrophy instead of specific muscle fibers and showed that doing cardio does not negatively impact total muscle hypertrophy in the lower or upper body. They further analyzed the effects of specific types of cardio, like running and cycling, and showed that neither running nor cycling negatively impact growth at the whole-muscle level.

Overall, the evidence is pretty clear that doing some cardio alongside your lifting isn’t inherently going to negatively affect your ability to grow muscle.

Can cardio have some negative effects on muscle growth?

There are some situations in which cardio may have some negative effects on building muscle. Those would include any situations where the cardio that you’re doing is impacting your performance in the gym.

For example, if you’re doing 30 minutes of intense cardio before lifting, you’re probably not going to perform as well when you lift since you pre-fatigued yourself doing cardio. You’re probably not going to be able to lift as much weight on your lifting exercises and you’re probably not going to be able to do as many hard sets since you’re tired, all of which will result in less muscle growth from that session.

If your main goal is to build muscle, you should perform your resistance training first and then follow it up with cardio if you so desire.

Another situation in which cardio may negatively affect your performance is if you do so much cardio that you’re unable to properly recover from your training. Think of the total amount of exercise you can recover from as a full tank of gas. Every type of exercise that you do will need some of that gas in order for you to recover from it properly so lifting some weights will require gas to recover and running will require some gas to recover.

Now if you're doing so much running that it’s using up all the gas and there's not that much leftover to recover from your resistance training then your performance in the gym is going to be impacted even if the cardio that you're doing is not on the same days that you're doing your resistance training. Most recreational athletes that lift four or five times per week and maybe do cardio two or three times per week probably don't have to worry about this because they're simply not doing so much exercise that they can't recover. However, if you're a competitive bodybuilder and you're trying to squeeze every drop of muscle you can from your training, if you do a good amount of intense cardio on top of your lifting, it may have some slight negative effects on your ability to maximally build muscle.

Another important factor to consider is that if gaining muscle is your main goal and you have a limited amount of both time and effort, then any time spent doing cardio is theoretically time that you could have spent doing additional resistance training. For example, we could argue that if you lift for an hour and do 20 minutes of cardio afterward, you could have spent those 20 minutes doing an additional two or three sets of resistance training which should result in better gains if you could recover properly. Also, keep in mind that exercise is not the only thing that takes up “gas” and impacts recovery. Other variables influence recovery as well. For example, if you're super stressed out from work and you're not getting good sleep and you're not eating well all of those things are also going to impact your ability to recover AKA there's going to be less gas in the tank.

In general, if there’s ever a situation where the cardio that you’re doing is impacting your ability to recover from lifting and subsequently impacts your performance in the gym, then it’s likely that you’re not making the best gains possible. This is probably why you'll never see a huge bodybuilder who is an elite long-distance runner or a marathoner who's extremely jacked. It's because the amount of training that they have to do that is specific to their sport to be good at it it's so much that it doesn't really allow them to make progress in other areas of their fitness.

Are there ways that cardio may be good for your gains?

The negative impacts of cardio on health only really occur when you're doing a ton of cardio at a pretty high intensity. For most people it's realistic to balance some amount of cardio with resistance training even if your goal is to maximize muscle growth and actually doing some cardio may be beneficial for making gains.

For example, walking is an example of cardio that can be helpful for potentially improving gains. Walking is a very low-intensity form of cardio that doesn't really negatively impact your recovery. In fact, it may actually improve recovery because walking helps increase blood flow to the lower body which increases nutrient delivery to different muscles and helps clear waste products that accumulate following exercise – all of which helps you recover quicker between training sessions.

Additionally doing some low to moderate-intensity state cardio on a regular basis will help improve your cardiovascular health and work capacity so that you can handle more total work in the gym which again should result in better muscle growth. There's nothing worse than doing a set of eight to ten repetitions on the back squat and being completely out of breath for ten minutes. If that's you, then doing some cardio to improve your work capacity will allow you to perform better in the gym which again should have positive effects on muscle growth.

In other words, some types of cardio can actually increase the amount of gas that you have in the gas tank.

For this reason, it's not so easy to give a straightforward answer and say that cardio kills your gains. It's the total amount and the intensity of the cardio that you're doing that will determine whether that cardio is positively or negatively impacting your ability to build muscle.

We can argue that if you're running several miles out of pretty high intensity every single day yes, you might not be able to maximally build muscle. That also doesn't mean that you can't build muscle at all. You can definitely still make good progress in the gym doing substantial amounts of cardio, but it just won't be as much as if you were doing less cardio and you were focusing more on your lifting.

On the other hand, even for people who want to absolutely maximize their ability to build muscle and the only thing they care about is getting jacked doing some cardio is beneficial. As I mentioned previously, going on long walks and doing some moderate-intensity cardio on your off days will help improve your work capacity and your ability to recover from training which should actually improve your ability to train hard and build muscle.

Conclusion

Cardio does not inherently inhibit your ability to build muscle. Some cardio is likely beneficial for everyone, even those who want to build a muscular physique. All forms of intense exercise require you to recover from them appropriately. If the cardio that you’re doing is negatively affecting your ability to recover from your resistance training and thus, your performance is suffering, then you may need to reprioritize. But otherwise, there’s not enough evidence to suggest that you can’t benefit from including walking, cycling, running, or other heart-healthy activities into your fitness routine.

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