The Interference Effect: Fact or Fiction?
The notion of the interference effect has limited too many people from building the fitness that they’re capable of building.
The reality is: it may not exist in the form you think it does.
So, my aim with this article is to give you a clearer picture of the situation, in layman’s terms, to help you be more confident in your training choices.
What is the interference effect?
This combination of strength and endurance training is known as concurrent training.
Other terms you may hear this described as include hybrid training or general physical preparation (GPP).
There are technically differences between these three terms, but for the sake of this article you can consider them synonymous.
Proponents of the existence of the interference effect will claim that such pursuits cannot be done.
At its core, the interference effect states that strength training activities and endurance training activities have negative effects on one another.
In some sources, you will see claims that the presence of one decreases the other. Or, more colloquially, that “cardio will kill your gains” or “lifting will make you bulky and slow.”
In other sources, you will see claims that the progress you see in one will be less with the presence of the other as opposed to the other’s absence.
Mechanistic reasoning for the interference effect’s existence
If you’ve read or watched any of my other work, you may know that I am against the idea of the interference effect’s existence in its original definition.
However, some of the thought processes behind it aren’t implausible.
For the sake of the debate, they are worth acknowledging.
The primary, and most plausible, reason that people claim the interference effect’s existence is the dichotomy of muscle fiber types.
Type 1 muscle fibers are classically known as the “endurance” fiber type. They can’t produce a high amount of force quickly, but they can continue to work for long periods of time.
Type 2 muscle fibers are classically known as the “strength & power” fiber type. They can produce a high amount of force quickly, but they cannot work for very long and fatigue quickly.
When you perform strength training or endurance training, a fiber-type “shift” may occur. Meaning, type 1 fibers might become type 2 as a result of strength training; or vice versa. [source]
There are other mechanistic or molecular claims you may see made in favor of the interference effect, but none are nearly as accepted in the research as the aforementioned point.
So, for the sake of this article, they will not be discussed.
Mechanistic reasoning against the interference effect’s existence
Despite how different strength & endurance training may seem, there aspects of each that have been shown to benefit one another.
First, a few studies have shown that improved cardiovascular fitness can improve a person’s capacity to build muscle; through increases in capillary density as well as increased satellite cell proliferation. [source]
Second, strength training has been demonstrated to improve both running economy and cycling economy. [source]
Running “economy” essentially refers to the amount of energy a person must spend with each step of their foot. A person with higher running economy will spend less energy with each step they take, and be able to go farther with any given amount of fueling.
So, from a mechanistic standpoint, it’s not a clear task to say whether there definitely is or isn’t an interference effect.
I also believed it was worth bringing these points up because arguments from a mechanistic point of view are what most people use to argue in favor of the interference effect’s existence.
Problem is: mechanistic thinking or data is NOT what you draw conclusions off of.
The reason is because you could find some singular mechanism to argue both ways; as illustrated above.
What matters is what the actual OUTCOME of concurrent training versus single mode training.
So, let’s discuss what that data says.
What ACTUALLY happens when you combine cardio + strength training?
A 2022 systematic review & meta-analysis from Schumann et al. looked at all of the evidence regarding the effects of concurrent training (strength + cardio) versus strength training alone on muscle size, strength, and power.
Their main findings were that there were no inherent effects of interference when strength training was combined with aerobic training compared to strength training alone when it comes to building muscle and strength.
They did find that maximal power (how fast you can produce force) was lower in the concurrent trainees, but the effect of this overall was small.
Through a number of subgroup analyses, they also found no evidence of an interference effect:
whether running or cycling was chosen as the aerobic exercise
whether the trainees were older or younger than 40 years
no matter how many times per week they trained aerobically
or whether the trainees were trained (high fitness) or untrained (low fitness)
It’s worth noting that among those who underwent concurrent training, better results in terms of maximal strength were seen when strength & cardio activities were separated by at least 3 hours.
So, if possible, this paper suggests that it is best to perform these trainings in different sessions as opposed to in the same session.
So, what about the effect on aerobic fitness?
Well, another 2022 systematic review & meta-analysis from Khalafi et al. answered this question.
This paper looked at the effect of concurrent training versus single mode training on the development of aerobic fitness.
In subjects aged 50+, they found no difference between the two forms of training.
Even though this paper didn’t look at younger subjects, it would be reasonable to infer that the result would be the same given that adaptability tends to be better in our youth.
As a final, less credible, and probably less applicable point of evidence, I think it’s worth taking into account what the athletes at the CrossFit games are capable of.
Not only have they built both their strength and aerobic fitness together, but they’ve built both to a higher level than most people who train only one will ever reach.
Yes, their life revolves around fitness and they probably have the ideal genetics for the job; but their existence at least nods to the idea building strength and endurance, even at a high level, can be done simultaneously.
As a last addition here, you can reasonably infer that at very advanced levels, it may become more difficult it will be to develop it may become to develop each simultaneously.
To be clear, when I say “advanced” in that statement, I am largely referring to the highest echelons of each domain.
In other words, you might not win the World’s championship of powerlifting or break the IronMan world record with a concurrent or hybrid athlete approach.
However, most people won’t get nearly advanced enough in their life for this to be a concern.
Just another reason to not worry too much about the interference effect.
Pseudo-interference
If I’ve done my job well with this article, you’re hopefully somewhat more convinced that you can build your strength and endurance together and rest assured you can still make all the progress in each that you desire.
Despite this, many of us know someone (or, have been that someone) who felt like they weren’t making the progress they wanted and attributed this to an interference effect.
Causes of this are what I like to refer to as “pseudo interferences” as it’s usually a result of some other variable.
The most notable example of this is bad programming.
Even though strength and endurance training don’t inherently inhibit one another, that doesn’t mean they can’t have such an effect.
The biggest variable in concurrent programming is properly managing fatigue throughout the week, and it isn’t as simple as doing a strength and running program at the same time.
I talk about this at length in the articles to be linked below.
Other sources of pseudo-interference are found in nutrition and sleep habits. Specifically:
Not sleeping enough; which may be obvious but is worth pointing out.
Not eating enough calories. Generally, a concurrent training program generally involves higher calorie expenditure. If you inadvertently put yourself in a calorie deficit, then that in of itself may limit strength or endurance gains.
Not proper fueling for endurance. For instance, endurance based activities tend to drain your carbohydrate stores more than strength training does. Failure to properly refuel these carbohydrate stores may inhibit recovery.
The overarching point of this section is the following.
If you or someone you know has experienced what was believed to be an interference effect, understand that it was more than likely some other, confounding variable.
Programming strength + endurance together
The purpose of this article was to provide the reasoning as to why the interference effect should not be a concern of yours.
If you would like help/guidance in regard to creatine a concurrent or hybrid program, the following articles may be of use:
