Does cardio kill gains in strength and size?

By Coach Hunter Bennett

One of the most common questions in the fitness is industry is “will cardio kill my gains.”

This question comes from the long-held belief that doing aerobic exercise will reduce increases in strength and size from resistance training.

But this may not be the case.

Concurrent training and the interference effect

Before we dive into the question, I should explain where this thought process comes from.

And it originates with concurrent training.

Concurrent training refers to performing aerobic and strength training simultaneously, to improve both aerobic capacity, and muscle strength and/or size.

This type of training is recommended to maximise the health benefits of exercise. It is also essential for athletes who need a combination of these qualities (i.e., team sport athletes).

However, because the adaptations that come from aerobic and strength training are opposites, it is often thought that performing one will detract from the other.

This phenomenon has been coined the “interference effect.”

But does it really need to be feared…

Does cardio kill gains in muscle size?

It is well-known that adding strength training into a cardio regime can improve aerobic performance.

But when it comes to adding aerobic exercise into a strength training regime, it is thought that it will reduce gains in muscle size.

Although this may not be the case.

A recent meta-analysis (a study combining the results of multiple other studies) answered this question by pooling the results of 15 studies. They found adding aerobic exercise to a weight training program can slightlyreduce gains in size compared to weight training alone.

However, further exploration revealed something interesting.

When the studies were separated into different exercise modalities, they found that running was the only exercise type to significantly reduce gains in muscle size. Conversely, cycling had no negative impact.

While this may seem surprising, it makes sense.

Because running involves impact, it induces more fatigue than cycling (which is no-impact). This fatigue can limit recovery, and by extension, training adaptations.

Pretty wild really.

But what about strength?

Does cardio kill gains in strength?

Similarly, another recent meta-analysis answered this question using very similar methods.

Combining the results 43 studies, they found adding aerobic exercise to a strength training program had no significant impact on muscle strength compared to strength training alone. However, it did a appear to have a small impact on explosive power.

Interestingly, the effect on power was more pronounced when aerobic and power training were performed in the same session, compared to if they were separated by at least three hours.

Now, reading this you might wonder if the interference effect is even a real thing – and I assure you it is, but it all comes down to context.

Can cardio kill gains?

The totality of the literature suggests that adding small amounts of aerobic training to a strength program will have little (i.e., zero) impact on strength and size. But that doesn’t mean that the interference effect doesn’t exist.

Several studies have set out to demonstrate whether the interference effect exists by designing protocols that try and induce the interference effect.

One famous 1980 study provides a good example.

The researchers recruited subjects and split them into one of three groups.

• A strength group that did five strength sessions per week
• An endurance group that did six aerobic sessions per week
• A concurrent group that did both programs (nine total session per week)

All groups trained for 10 weeks, and the results were interesting, to say the least.

The strength only group saw no improvements in aerobic fitness. The endurance only group saw no increases in muscle strength.

Interestingly, the aerobic and concurrent groups saw the same increases in aerobic fitness.

And strength?

The rate of strength gain was similar between the strength and concurrent groups for the first few weeks. But by the end of the ten weeks, the strength group saw much larger strength gains than the concurrent group.

So, in summary, the inference effect does exist.

But that doesn’t mean cardio will kill your gains.

In this study, the concurrent training group was doing 6 sessions (~45m each) of aerobic exercise per week and still saw some increases in strength. This suggests that the interference effect probably isn’t something you need to worry about that much.

Especially if you are only doing a couple of 30-minute aerobic session per week.

How to make sure cardio doesn’t kill your gains

Looking at the above research, we can start to paint a picture regarding when cardio might start to kill your gains:

1. You do high volumes of cardio (like, really high volumes)
2. Your cardio is done in the same session as your strength work
3. Running is your main form of cardio

With this in mind, we can also start to make some recommendations around making sure cardio doesn’t kill your gains.

1. Try and separate cardio and weight sessions by ~24 hours if possible
2. If they must be performed on the same day, separate them by ~3 hours
3. If they must be performed in the same session, do your weight training first
4. Choose low impact modalities (cycling, rowing, ski-erg, etc.) over running

It is also important to note that your total training dose matters.

If you are performing 2-3 strength sessions per week, you can perform more cardio than if you were performing 5-6 strength sessions.

Similarly, if you are performing 2-3 aerobic sessions per week, this will have practically zero impact on your gains in strength and size. However, if you get that up to 5-6 sessions, it might start having a slight impact.

Can cardio improve my gains?

Before I finish up this article, I wanted to spin this topic on its head.

I wanted to discuss how cardio might have the ability to improve your gains.

Doing cardio – even in small amounts – will increase your aerobic fitness.

Having higher levels of aerobic fitness has been show to improve your ability to recover. This means faster recovery between sets, and between sessions.

As such, increasing your aerobic fitness might improve your gym gains by:

1. Increasing training intensity across a session
2. Enhancing recovery between sessions

All of which allows you to perform more reps at a higher intensity, leading to larger long-term adaptations.

Does cardio kill gains: key points

Based on the literature, we know that the interference effect is a real thing.

But we also know that cardio has little-to-no impact on strength and hypertrophy adaptations. Especially if you follow the recommendations above.

More importantly, doing some cardio and increasing your fitness may increase your gains (as well as make you live longer…).

So why not chuck some in your routine?