Read time: 4 minutes

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Most of you don’t know this.

But back in the summer of 2012, I began the long journey towards my PhD.

The topic?

Something very different to what I focus on now.

It had more to do with how to - dare I say it…”optimise” training.

And specifically, the combination of both resistance training and aerobic training.

You see, since the early 1980‘s we’ve known combined or “concurrent” training can blunt some of our gym gains.

In other words, the benefits from one type of exercise are less when combined with another exercise type (than when done alone).

But how big is this effect, and does it really matter for the everyday exerciser?

Here’s the truth about the so-called “interference effect”:

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Concurrent training and the interference effect

Concurrent training is the combination of aerobic training and resistance training.

And not necessarily within the same session, either.

Any time you’re doing both within the same week, you’re doing concurrent training.

For many athletes, it’s important to be “fit” AND strong or fast - all while reducing the risk of injury.

This makes concurrent training essential for most athletes - and recommended for everyday people alike.

But the problem with concurrent training is there can be negative interactions between different “types” of exercise.

Studies have shown that relative to resistance training alone, concurrent training can blunt gains in:

• Power
• Strength
• Muscle mass

What’s also interesting is aerobic training adaptations (like VO2max) don’t seem to be affected.

So let’s look at an example.

The data below is from one of my PhD studies in young healthy males.

It shows the change in leg press strength after 8 weeks of resistance training alone (black) or when the same program was combined with aerobic training (cycling) in the form of:

• High-intensity interval training or HIIT (green)
• Moderate-intensity continuous training or MICT (purple)

In either case, the aerobic training was done immediately before each resistance training session.

As you can see, those who did concurrent training didn’t get as strong as those who did weights alone.

So what explains this “interference effect”, and is it something we have to worry about?

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What explains the interference effect?

There’s a couple of theories.

One is the molecular signals activated in muscle by endurance training “switch off” those driving muscle growth with resistance training.

But studies show these signals are not blunted with concurrent training - and if anything, combining aerobic and resistance training gives muscle-building pathways an extra boost.

The other theory is carry-over fatigue from endurance training simply “ruins” strength training sessions.

There’s little doubt strenuous aerobic training limits the amount of weight you can lift, how fast you can move, or how many sets you can get through.

Over the long run, these things could blunt the gains in strength, power, or muscle mass.

But it’s important to mention studies don’t always show any interference effect with concurrent training.

So why might this be the case?

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The “interference” is not always with you

It’s simplistic to think aerobic training will always kill your gains in the gym.

The truth is, it depends on many things.

There are so many variables at play:

• The type of aerobic training (running, cycling, rowing, swimming)
• The order of aerobic and resistance training
• The recovery length between sessions
• The amount (or volume) of training
• The intensity of training

The degree of interference depends on how these variables are configured in any given training program.

And when the interference effect does occur, some outcomes are affected more than others.

Adaptations linked to force output, like strength or power, are often blunted more than changes in muscle mass.

We found strength and power gains suffered much more than muscle (lean) mass when resistance training was combined with HIIT:

This fits with the “fatigue” theory of the interference effect, as carry-over fatigue would affect muscle function gains more than muscle mass (by limiting the loads lifted, or lifting speeds).

And it suggests if you want to avoid the interference effect, you should limit fatigue before your resistance training sessions.

But just how important is this interference effect, anyway?

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Should I care about the interference effect?

For the vast majority, the answer is no.

The truth is, interference effects with concurrent training are very small (if they’re present at all).

And they probably only occur with reasonably high amounts of training.

Both of these things mean interference effects are not meaningful for most people.

If you’re doing enough exercise to blunt some of your gains, you’re probably getting enough benefit anyway.

Sure, athletes need the extra 1% to perform at their peak.

These individuals probably want to tweak their programs to ensure the different types of training are as complimentary as possible.

But for most of us mere-mortals, the advice remains to:

Do both aerobic and resistance training if you can.

And if you can’t, just stick with one.

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Both exercise types are best for health benefits

Unless you’re a powerlifter or an elite athlete, getting a little less strong with concurrent training won’t be a problem.

Doing either aerobic or resistance training can reduce the risk of mortality and many chronic diseases.

But the health benefits are even greater if we combine aerobic and resistance training.

So instead of worrying about different types of exercise interfering with one another, we should be more concerned with the impact of not combining them.

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As always, thanks for being here.

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Until next week,

Jackson

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IMPORTANT:

The information contained herein is of general nature only and does not constitute personal advice. You should not act on any information without considering your personal needs, circumstances, and objectives. Any exercise program may result in injury. We recommend you obtain advice specific to your circumstances from an appropriate health professional before starting any exercise program.