Research Spotlight: The interference effect is getting less scary by the day

Research Spotlight articles share concise breakdowns of interesting studies. The study reviewed is "Compatibility of Concurrent Aerobic and Strength Training for Skeletal Muscle Size and Function: An Updated Systematic Review and Meta-Analysis" by Schumann et al.

With concurrent training, you’re always trying to balance and manipulate your strength training and endurance training to mitigate the impact of the dreaded interference effect (a reduction in the rate of strength gains, power/velocity gains, and hypertrophy observed when adding endurance training to a resistance training program). However, the research regarding concurrent training and the interference effect has been shifting over time.

The first study in the area by Hickson in 1980 found that concurrent training led to considerably smaller strength gains than resistance training alone. By 2012, there was enough research to warrant a meta-analysis; this meta-analysis suggested that concurrent training led to smaller strength gains, less muscle growth, and smaller improvements in power output and explosive strength than resistance training alone. More recently, a 2021 meta-analysis (reviewed in MASS here) broke things down further, separating studies by the training status of the subjects and the timing of the resistance training and endurance training sessions (trained versus untrained subjects, and studies where endurance and resistance training were performed in the same training session versus different training sessions). That meta-analysis suggested that, at least for strength development, there’s no significant interference effect for untrained subjects, nor is there any interference effect when trained subjects split their endurance and resistance training into separate training sessions. Thus, in the intervening years since 1980, the balance of evidence has shifted considerably – we used to be concerned that the interference effect would have a fairly large, fairly consistent negative effect for virtually anyone who wanted to gain strength and build muscle while also doing some endurance training. Now, it appears that the interference effect should only be a small concern for some people, some of the time (and only in situations where they have to perform their endurance and resistance training in the same session). But will this trend continue?

Well, if you’re a fan of concurrent training, a brand new meta-analysis should give you even less reason to be concerned about the interference effect. Schumann and colleagues started by identifying all of the studies that met the following inclusion criteria:

  1. The studies needed to include a training intervention lasting at least four weeks.
  2. The studies needed to include groups completing identical resistance training programs, with one group performing only resistance training, and at least one group performing additional aerobic training.
  3. The studies needed to include measures of maximal strength, explosive strength, and/or muscle hypertrophy.
  4. The exercises used to assess performance needed to be specific to the resistance training the subjects performed.

The researchers identified 43 studies with a total of 1,090 subjects that met their inclusion criteria, including 37 studies measuring maximal strength, 18 studies measuring explosive strength, and 15 studies assessing hypertrophy.

They found that concurrent training did not lead to significantly smaller strength gains than resistance training alone (Figure 1; Standardized Mean Difference [SMD] = -0.06; p = 0.45), nor did it lead to significantly less hypertrophy (Figure 3; SMD = -0.01; p = 0.92). However, concurrent training did lead to significantly smaller improvements in explosive strength than resistance training alone (Figure 3; SMD = -0.28; p = 0.007).

The researchers also performed a series of subanalyses that can be found here. For strength, they found that the modality of endurance training (running versus cycling), the weekly frequency of endurance training, the training status of the subjects (“active” versus untrained; they didn’t run a subgroup analysis on specifically resistance-trained subjects), the age of the subjects (18-40 years old versus >40 years old), and the timing of resistance and endurance training sessions (performing both in the same training session versus different sessions) all failed to significantly modify the effect. Of note, however, the researchers didn’t run a subanalysis investigating the impact of total endurance training duration. For hypertrophy, it’s a similar story: endurance training frequency, training status, and the timing of resistance and endurance training sessions all failed to significantly modify the effect. In other words, this meta-analysis suggests that the interference effect doesn’t really exist in any generalizable sense for strength and hypertrophy outcomes – it’s only “real” and noteworthy for measures of power output and explosive strength.

Overall, this meta-analysis doesn’t necessarily affect my recommendations regarding concurrent training to any large extent, but I do think it recontextualizes this body of research. Previously, the default assumption was that the interference effect generally mattered quite a bit, and that it was the goal of a coach to find the exact right mix of training variables to mitigate the interference effect to the greatest extent possible. However, I think the overall balance of evidence now suggests that the interference effect isn’t that big of a deal, and you probably don’t need to be that concerned about it most of the time.

To be clear, I don’t necessarily endorse the position that would be implied by a literal and expansive interpretation of this study’s findings: I absolutely think that if your endurance training volume, frequency, and/or intensity is high enough, it can have a negative impact on your muscle growth and strength development. It’s always important to keep context in mind when research findings seem to contradict common sense. Most concurrent training studies don’t involve resistance training protocols that push subjects to their absolute limits in an effort to maximize rates of hypertrophy and strength gains, nor do they put subjects through an endurance training protocol that might be typical of a runner attempting to qualify for the Boston marathon. Your capacity to recover from training is finite, so the introduction of a non-trivial amount of endurance training will necessitate some level of resistance training volume below the maximal amount you could theoretically tolerate (and possibly/probably below the amount of training volume that would theoretically maximize your rate of muscle growth and/or strength gains). However, I also think that, in general, “we” (referring to myself and the “evidence-based” fitness community in general) may have previously been a bit too concerned about the interference effect.

As more and more research on the subject is published, I’m becoming more and more convinced that the interference effect shouldn’t be a major concern for most people, most of the time. However, there are a few groups of people who probably need to be a bit more careful:

  1. If your capacity to recover from training is significantly diminished (due to poor sleep, high levels of psychogenic stress, or a large calorie deficit), you may not be able to handle a substantial amount of simultaneous endurance and resistance training.
  2. If you’re already stressing your capacity to recover from a given volume of endurance training, you may struggle to add in a significant amount of resistance training (and benefit from it).
  3. If you’re already stressing your capacity to recover from a given volume of resistance training, you should be careful about adding in a large amount of endurance training, or ramping up endurance training volume too quickly.
  4. Most importantly, if you have major goals related to explosive strength or power output (for example, improving your jumping ability), endurance training will likely reduce your rate of progress.

Now, I realize that a lot of readers probably fall into the third group above. However, I also suspect that >80% of people who do some sort of endurance or resistance training can combine both without compromising their strength and hypertrophy results. And that’s really my main point: Rather than framing the interference effect (for strength and hypertrophy) as the likely outcome of concurrent training that is challenging to mitigate, it may be more appropriate to frame it as a relatively uncommon phenomenon that is unlikely to impact training outcomes unless someone is already really pushing their limits (or attempting to push their limits) in multiple capacities at once.

Finally, I’d just like to acknowledge that most of this article has been written with strength and hypertrophy-related goals in mind. However, it’s worth reiterating that endurance training does seem to consistently and significantly affect explosive performance. So, for example, a powerlifter may not notice any negative effects from jogging a few times per week, but a thrower or high jumper probably would. Or, in the context of team sports, intensive conditioning work could reduce the explosiveness and agility of athletes. If your main goal is to maximize physical capacities related to power output, speed, or explosiveness, it wouldn’t be a bad idea to limit endurance training to whatever extent is feasible.

This Research Spotlight was originally published in MASS Research Review. Subscribe to MASS to get a monthly publication with breakdowns of recent exercise and nutrition studies.

Credit: Graphics by Kat Whitfield.

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