March 19

Concurrent training and interference effect: Part 1.

Fat loss

Can I build muscle, get strong and get fit at the same time?

This is a question I get asked a lot in my varied roles as a personal trainer, strength and conditioning coach and performance nutritionist.

And the answer is a resounding YES! And the solution? Concurrent training.


Concurrent training

Simply put, concurrent training is the simultaneous development of strength and endurance qualities over the same training block.

For most athletes to perform at their best, it’s a necessity. And for anyone looking to “get fit, lean and healthy”, it’s the approach you should be taking.

If you manage this correctly, here’s some benefits:

  • Improved endurance performance via maximal strength gains
  • Positive impact on health markers via aerobic/ endurance training
  • Improved recovery within and between workouts
  • Body composition improvement

Clearly, training strength and endurance qualities together has some huge benefits for both athletes and recreational gym goers alike!

So, why do so many people believe it’s not possible to achieve strength and muscle gains alongside any form of endurance training?

Why do we hear irrational statements like “cardio makes you fat and weak”, and “don’t lift, you’ll get too bulky and slow”.

These are simply lacks of understanding of how the we develop muscle, fitness and performance. But there is one question that I’ll answer over this two part blog.

Will strength and endurance training cancel each other out?

Enter the “interference effect”.


The interference effect

The “interference effect” is the idea that developing strength and muscle mass at the same time as developing “fitness”, or more endurance based adaptations will actually lead to a blunted training response, particularly in the strength qualities vs performing them without endurance training.

And it absolutely has some merit.

The idea dates back to 1980. Dr Robert Hickson was the first to publish a study showing an apparent detrimental effect on strength and muscle development after adding endurance training to a strength training programme after noticing the effect on his own strength after taking up endurance training alongside it.

Interestingly, this seems to be a one-way relationship, and strength development is often seen as beneficial to endurance performance. So, for today, we’re going to focus on whether strength and muscle development is really inhibited by the addition of an endurance training stimulus, how this might happen, and how to best manage concurrent training.

And before we do, please hold in your mind a key point, that we’re talking mainly about a possible reduced IMPROVEMENT in strength and muscle mass with concurrent training, and not a REDUCTION in strength and muscle mass.


Why would endurance training reduce strength adaptations?

It seems to be generally accepted now in the minds of coaches and fitness enthusiasts that this reduced ability to adapt to strength/ power/ hypertrophy training is heavily linked to the cell signalling processes. Quick overview.

Different types of exercise create different demands on the body. These demands may be related to metabolic stress, loading, hormonal response, and more, but the point is that the specific demand will lead to a specific response and subsequent adaptation. This is the entire idea of “training”, and from an adaptation point of view, it starts with molecular signalling. These specific signals tell your body how to adapt (a “build more of this” type response).

With regards to signalling, the two key players are MTORC1 for resistance adaptations, and AMPK for endurance adaptations, and how these two pathways interact. The theory is, that AMPK (an endurance signalling protein) blunts MTORC1 response, leading to a reduced strength adaptation.


A couple of problems with this idea

First off, AMPK can be stimulated by resistance training, just as MTORC1 can be stimulated by endurance type training. So straight away, we start to see the issues that may arise here when we put these ideas in play in actual, living, breathing, and training humans.

There’s certainly more grey area than we like to admit.

Turns out, although in theory this would make sense as to why we’re seeing the reduced gains, it’s just not as clear cut as we’d like. At times, adding a form of endurance training can actually enhance strength and muscle gains, particularly with new exercisers.

And if you’re not already starting to question the signalling idea as the key player in this, then think about this point.

High intensity interval training causes a huge increase in AMPK activation, and this should absolutely impact MTORC1 response when performed in close succession to resistance training.

However, as it turns out, this form of training actually seems to reduce any potential interference effect, and doesn’t seem to show any reduction in whole muscle hypertrophy. So the AMPK/ MTORC1 interaction role in the interference effect is possibly less prominent than we once thought.

And yet, we absolutely still see an impact of concurrent training on strength and muscle development time and time again, particularly in more advanced exercisers. Endurance training CAN clearly interfere with strength development.


So, what else is going on in concurrent plans?

What else could be causing the reduced strength, power and muscle development?

There’re a few key avenues that seem to have a key role in why endurance training can reduce strength development. Considered together, they may explain the varied findings from studies in this area, and give a little more clarity on what we can do to maximise training outcome.


Total energy Intake

Muscle building 101 is to get out of severe energy stress. If you’re in a large energy deficit, you’re going to struggle to build muscle mass, and even recover from training. Not only is muscle and strength development impacted here, but so is performance across board.

Low energy intake can lead to other problems such as immune dysfunction and hormonal dysregulation.

And remember, energy intake is RELATIVE. So, if you’re close to the limit of energy intake, then add several hours of additional endurance training, this may be the point that you tip into an energy status so low that the addition of training has a negative impact.

When you add additional conditioning work, you create an additional energy demand. In some cases, this demand would easily interfere with strength and muscle gains.


Stimulus Quality

To build muscle, we need to train with sufficient intensity and volume.

If resistance sessions are following demanding endurance sessions without adequate refuelling and recovery time, then session quality will be impacted. Simply put, the resistance training quality is reduced following the endurance session.

The more demanding and damaging the endurance session, the more impact we’re likely to see on the strength session. Zoom out over a training block, and this could absolutely be a key reason why we see diminished strength gains with concurrent training approached.

Remember this key point: Fatigue is not the same as overload. Feeling at your limit with 60% of what you could do fresh is not the same as being at your limit with 100%, whether you have more in you or not.

Again, a simple idea, but very much something which demands consideration when putting training plans together.


Total Recovery

We’ve all heard of overtraining. Whether you choose to believe it or not, it exists, and is associated with muscle dysfunction and reduced performance, along with a host of other health and performance related conditions.

For the more advanced exerciser, a lack of focus on adequate nutrition and recovery with increasing demands of training concurrently may lead to an overall exhaustion that impacts health, training quality and recovery and adaptation alike. This can be hugely detrimental to any fitness, performance or body composition goals.


All of these are very valid considerations, and like everything in human performance, they don’t work in isolation. In addition to the signalling aspect, they probably all have a key role to play.

But on to the good news….

All can be helped by better training programme design and nutrition management.

Keep an eye out for part 2: How to minimise interference effect in concurrent training programmes.

Find that here. 


Josh Kennedy MSc, ASCC, CSCS


P.S. For more content, find the rest of the blogs HERE

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Key papers:

Skeletal Muscle Hypertrophy with Concurrent Exercise Training: Contrary Evidence for an Interference Effect, Murach and Bagley, Sports Medicine, 2016

Integrative Biology of Exercise, Hawley et al, Cell, 2014

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