Cardio Kills Gains: True or False? How & Why?

Is it a myth that cardio kills your gains? Can you mix cardio and weights without putting the brakes on muscle growth? Here’s everything you need to know.

Does Cardio Really Ruin Your Gains?

Cardio doesn’t have a negative impact on muscle growth, just as long as you don’t do too much of it.

Most research shows that cardio interferes more with the development of power than it does strength or muscle mass [1].

In fact, cardiovascular exercise may actually help rather than hurt your gains, just as long as it’s used in the right way.

More about that in a moment.

First, I want to take a closer look at how and why too much cardio, running in particular, can hurt your gains.

How Cardio Can Interfere With Gains in Muscle Size and Strength

Cardio certainly has the potential to put the brakes on muscle growth, which it does in a number of ways.

First, cardio can compromise the quality of a lifting session, which in turn reduces the strength of the muscle-building stimulus generated by a given workout. 

What exactly do I mean by that?

Let’s say you go to the gym, do a hard high-intensity interval training (HIIT) session on the bike, then train your legs immediately afterwards.

Chances are it’s not going to go well. 

You won’t be able to do as many reps, or lift as much weight, because of the residual fatigue from the HIIT.

As a result, the intensity of the growth signals sent to your muscles will be weaker than they otherwise would be, which in turn means a slower rate of muscle gain [2]. 

Cardio can also interfere with your ability to recover from lifting weights. 

Let’s say that you’re doing a full-body workout three days a week, lifting weights on Monday, Wednesday and Friday.

Adding cardio on several of your rest days means that some of the resources used by your muscles to recover and grow are now spread more thinly than they were before.

Cardio workouts can also mess around with some of the molecular signaling pathways involved in hypertrophy [3]. 

That is, the strength of the “make me bigger” signal being sent to muscle fibers in the hours and days after training is going to be reduced, slowing the rate at which muscle protein is synthesized.

Is Cardio Bad for Muscle Growth?

However, saying that cardio has the potential to hurt your gains doesn’t necessarily mean that it will, and the extent to which cardio interferes with hypertrophy depends on a number of factors, with the main ones being:

• The proximity of the cardio to your lifting sessions.
• The order in which the training is done.
• The type of cardio you’re doing (i.e. jogging vs cycling).
• How often it’s done.
• How hard it is.
• How long your cardio sessions last.
• How well you recover from it

The Benefits of Combining Cardio and Weights

Cardiovascular exercise has the potential to help rather than hurt your gains if it’s used in the right way.

For one, some types of cardio may help recovery by promoting blood flow to the muscles without causing further damage. 

Cycling at a low-to-moderate intensity for 20-30 minutes the day after a heavy leg day in the weight room, for example, will often reduce post-exercise muscle soreness, and may accelerate the rate at which muscle damage is repaired.

There’s also some interesting research to show that differences in capillary density go some way towards explaining why some people make gains faster than others.

Capillaries are tiny blood vessels, which deliver oxygen, nutrients and hormones to muscle cells, clear the metabolites that build up during a workout, as well as helping with the repair and recovery process from one workout to the next.

In one review, researchers report that men with a higher capillary density saw faster gains in muscle mass than individuals with a lower capillary density after six months of resistance training [4].

And one of the best ways to go about boosting capillary density is to do some cardio.

Will Running Cause You to Lose Muscle Mass?

Running won’t cause you to lose muscle mass. However, it’s my experience that running has much greater potential for interfering with your gains than most other types of cardio.

For one, when I go for a run, my heart rate rarely drops below 75 percent of its maximum. Most of the time, it’s nearer 80 or 85 percent.

Training at a relatively high intensity isn’t a problem if your cardio workouts are short and/or infrequent. But too much hard cardio can have an adverse effect on your ability to recover from and adapt to resistance exercise.

Running also tends to cause a lot more muscle damage than something like swimming, cycling, or even incline treadmill walking. And that muscle damage needs to be repaired from and adapted to.

There’s also a lot more wear and tear on your joints. The day after a big run, for example, I can feel a level of soreness in my hips, ankles and knees that I don’t get with low-impact cardio such as cycling.

I can’t say that running will or won’t hurt your gains, because it depends on the total amount of running you’re doing, how hard you’re training, and your current level of cardiovascular fitness.

And if you’re doing concurrent training because you’re trying to build muscle and improve your cardiovascular fitness, there may well be a tradeoff to be made. 

And by that, I mean accepting a slower rate of muscle growth as the price to be paid for improving your cardiovascular fitness at the same time.

But of all the different forms of cardio out there, running is the one I think has the greatest potential for impeding recovery and interfering with hypertrophy [5, 6].

Does Lifting After Cardio Slow Muscle Growth?

Does lifting after cardio slow muscle growth? That depends a lot on what type of cardio you’re doing, and how long you’re doing it for.

Using low-intensity steady-state cardio as part of a general warm-up for weight training has the potential to improve your performance in the gym. That’s because a warm muscle tends to perform better than a cold muscle, as well as being more resilient to injury.

But it’s a different story with more intense and/or longer duration cardio.

In one study, lifting performance during the high pull, squat, bench press, deadlift and push press was adversely affected when lifting was performed after cardio, with high-intensity interval exercise leading to the largest decline in exercise performance [7]. 

Over time, this dip in performance is likely going to mean slower gains in strength and size.

How Much Cardio When Building Muscle?

There are no strict rules that lay out exactly how much cardio you should do and when you should do it. But I do have some general guidelines that will minimize the extent to which cardio interferes with muscle growth.

1. First, cap the amount of moderate- to high-intensity cardio you do to a couple of hours a week. 

2. Avoid any intense cardio immediately before lifting weights. You’re better off doing it once the heavy training is out of the way, or even on a separate day. 

For example, if you lift weights on Monday, Wednesday and Friday, you could do some cardio on Tuesday, Thursday and Saturday.

As long as you don’t go overboard on the duration and/or intensity, doing some cardio on your off days won’t ruin your gains.

3. Some light cardio as part of a general warm-up is fine. And by a general warm-up, I’m talking about 10-20 minutes of light cardio (around 55% of your maximum heart rate) on an indoor bike or rowing machine.

4. I’d also suggest that you focus mainly on low-impact cardio, such as cycling, swimming, climbing stairs, rowing or even incline treadmill walking, rather than running.

How To Do Cardio Without Slowing Muscle Growth

Let me give you a few examples of how to put it all together.

The first option is to do some form of low-intensity steady-state cardio (also known as LISS), such as walking, every day. 

In most cases, LISS won’t interfere with your recovery from a bout of weight-training, nor will it impair your performance when you hit the weights later on that day.

While walking isn’t going to have much of an impact on your cardiovascular fitness (unless you’re completely out of shape), it is going to change your health for the better. 

A brisk walk first thing in the morning is also a great way to clear your head and set you up for the day.

Option two is 30-40 minutes of moderate-intensity (60-70% of maximum heart rate) cycling 2-3 times a week, on the days you don’t train with weights. 

If I had to choose a single best form of cardio for muscle gain, it would be cycling.

With cycling, there’s far less muscle damage and joint stress compared to running, which means there’s less potential to interfere with muscle growth.

Interestingly, the addition of cycling to a resistance training program has been shown to improve rather than impair gains in muscle mass [8]. 

In fact, there are various studies to show that cycling alone can stimulate muscle growth in the lower body, albeit in untrained novices rather than seasoned lifters [9, 10, 11, 12].