MYTH BUSTED · STRENGTH TRAINING MYTHS · 9 MIN READ
The ‘Interference Effect’ Gym Folklore Won’t Let Go Of — And What Four Decades Of Research Actually Found
I’ve lost count of how many people at the gym have told me they’re skipping cardio entirely because it’ll ‘eat into their gains.’ It’s one of the most repeated pieces of gym folklore there is, and it traces back to one real study from 1980 that the fitness industry has been quietly over-generalising ever since.
The original finding was real. What happened after it is the more interesting part: forty-five years of follow-up research have made the effect look smaller and smaller every time someone runs a bigger, better-controlled study.
The most recent large-scale meta-analysis on this, pooling 43 studies, found essentially no overall interference effect on muscle hypertrophy at all. Not a small effect. Statistically indistinguishable from zero.
Concurrent aerobic and strength training showed no meaningful interference for hypertrophy (SMD −0.01, p = 0.919) across 43 pooled studies.
— Schumann et al., Sports Medicine, 2022
The term ‘interference effect’ comes from a 1980 study by Dr Robert Hickson, who found that untrained men doing combined strength and endurance training gained strength more slowly than men doing strength training alone. It was a real, published finding, and it kicked off decades of research trying to figure out why — and how big the effect actually is outside that one specific study.
The proposed mechanism was molecular: endurance training activates a cellular pathway (AMPK) that can, in theory, blunt the signalling pathway (mTOR) responsible for muscle protein synthesis after resistance training — the two forms of exercise supposedly sending the body conflicting instructions at the same time.
That mechanism is real at the molecular level. What’s become clear since 1980 is that its practical, real-world effect on how much muscle and strength you actually build is far smaller and far more conditional than the original study, and forty years of gym folklore built on top of it, ever suggested.
A 2012 meta-analysis of 21 studies (422 effect sizes) found a real interference effect for both strength and hypertrophy — but only when the endurance training was running. Cycling, in the same analysis, did not cause meaningful interference at all. Modality mattered more than the mere presence of cardio.
A 2022 updated meta-analysis, pooling 43 studies with far more data than the original, found no overall interference effect on hypertrophy whatsoever (SMD −0.01, statistically indistinguishable from zero), and reported that whole-muscle mass and maximal strength development were not compromised by concurrent training. The one adaptation that showed some attenuation was explosive, power-based strength — not size, not maximal strength.
A separate meta-analysis looking specifically at the muscle-fibre level (rather than whole-muscle size) did find a small negative effect on fibre hypertrophy with concurrent training — but again, it was more pronounced with running than cycling, and none of training frequency, training status, or exercise order within a session made a meaningful difference to the result.
The consistent thread across every major meta-analysis on this topic is the same: running-based cardio shows more interference than cycling or other low-impact modalities, particularly for slow-twitch (type I) muscle fibres. The mechanism isn’t fully settled, but the repeated eccentric impact and metabolic demand of running appear to compete more directly with resistance training recovery than the more controlled, lower-impact demand of cycling or rowing.
Volume and frequency matter too. The studies that do find meaningful interference tend to involve high volumes of endurance work stacked closely alongside frequent heavy lifting, without enough recovery between the two. Moderate cardio — two to four sessions a week, sensible duration — sits well within the range the newer meta-analyses found produced little to no measurable cost to strength or size.
Cardio doesn’t compete with your gains. Unmanaged volume and zero recovery time does — and that applies whether the extra volume comes from running or from lifting.
The interference effect shows up most consistently with running. Low-impact cardio modalities carry meaningfully less risk to muscle size and strength in the pooled research.
If your schedule allows it, spacing resistance and endurance sessions by several hours, or onto different days entirely, reduces any residual fatigue carrying over between the two.
Where sessions must be combined, research on exercise order found a resistance-then-endurance sequence was favourable for dynamic strength outcomes specifically, with no clear disadvantage for hypertrophy either way.
Two to four sessions a week at a sensible duration sits comfortably within the range the current evidence shows causes little to no measurable interference with strength or muscle growth.
No. The best current evidence — larger, more recent, and better controlled than the 1980 study the myth is built on — shows muscle size and maximal strength holding up just fine alongside a sensible amount of concurrent cardio.
If you’re chasing elite-level explosive power specifically, there’s a small, real trade-off worth knowing about. For essentially everyone else training for size, strength, and general health, the far bigger risk to your gains is skipping the cardiovascular training that keeps the rest of your body working well enough to keep lifting at all.
Your heart is training for the same goal as your muscles.
BOTH.
• Hickson RC (1980). Interference of strength development by simultaneously training for strength and endurance. European Journal of Applied Physiology, 45(2–3).
• Wilson JM, et al. (2012). Concurrent training: A meta-analysis examining interference of aerobic and resistance exercise. Journal of Strength and Conditioning Research, 26(8).
• Schumann M, et al. (2022). Compatibility of concurrent aerobic and strength training for skeletal muscle size and function: An updated systematic review and meta-analysis. Sports Medicine, 52(3).
• The effects of concurrent aerobic and strength training on muscle fibre hypertrophy: A systematic review and meta-analysis. Sports Medicine, 2022.
• The role of intra-session exercise sequence in the interference effect: A systematic review with meta-analysis. Sports Medicine.
• Development of maximal dynamic strength during concurrent resistance and endurance training in untrained, moderately trained, and trained individuals: A systematic review and meta-analysis. Sports Medicine, 51(5), 2021.
Does cardio really reduce muscle growth?
The best current evidence, from a 2022 meta-analysis of 43 studies, found no meaningful overall interference between cardio and muscle hypertrophy. Earlier, smaller studies found more of an effect, but largely when the cardio in question was running.
Is running worse for gains than cycling?
Yes, consistently across multiple meta-analyses. Running shows more interference with strength and hypertrophy than cycling or other low-impact cardio, particularly for slow-twitch muscle fibres.
Should I do cardio before or after lifting?
Research on exercise order found lifting before cardio in the same session was favourable specifically for dynamic strength development, with no clear disadvantage to muscle size either way.
How much cardio is too much for building muscle?
The interference effect becomes more likely with high volumes of endurance training stacked closely against frequent heavy lifting without adequate recovery. Moderate cardio — two to four sessions a week — falls well within the range shown to cause little to no measurable cost.
Does cardio affect strength the same way it affects muscle size?
Largely no. The clearest, most consistent finding across recent meta-analyses is that maximal strength and whole-muscle size hold up well with concurrent training; explosive, power-based strength shows the most consistent (small) attenuation.
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