MYTH BUSTED · STRENGTH TRAINING MYTHS
The 'Interference Effect' Gym Folklore Won't Let Go Of — And What Four Decades Of Research Actually Found
The short answer: for most people, no — normal amounts of cardio don't meaningfully compromise muscle growth or maximal strength. But the 'interference effect' isn't a total myth either. It's real, small, and outcome-specific — explosive strength and power seem more sensitive than muscle size or maximal strength, and the details of how you combine training matter more than a blanket rule either way.
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. Later research has shown that the interference effect is smaller and more outcome-specific than gym folklore suggests — it depends heavily on which quality you're measuring: whole-muscle size, maximal strength, muscle-fibre-level growth, or explosive power.
A major 2022 updated meta-analysis pooling 43 studies found no detectable overall difference in whole-muscle hypertrophy between concurrent training and resistance training alone.
Concurrent aerobic and strength training showed no detectable overall difference in whole-muscle hypertrophy (SMD −0.01, p = 0.919) across 43 pooled studies. Maximal strength wasn't significantly compromised either — but explosive strength showed a small attenuation.
— 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.
Acute endurance and resistance exercise can activate partly different molecular signalling pathways, and AMPK–mTOR interactions have been proposed as one mechanism behind interference. But acute signalling doesn't translate cleanly into a predictable long-term loss of muscle or strength — fatigue, training volume, modality, timing, nutrition and training status all matter too.
What's become clear since 1980 is that the practical, real-world effect on how much muscle and strength you actually build is smaller and more outcome-specific than the original study, and forty-five years of gym folklore built on top of it, ever suggested.
An influential 2012 meta-analysis of 21 studies (422 effect sizes) found significant decrements in both strength and hypertrophy when the endurance training was running — but concurrent cycling didn't show the same pattern in that dataset. Endurance training frequency and duration were also negatively associated with some outcomes. It was an important early signal, but not the last word.
A 2022 updated meta-analysis, pooling 43 studies with far more data, found no detectable overall difference in whole-muscle hypertrophy (SMD −0.01) and reported that maximal strength wasn't significantly compromised either. Explosive, power-based strength showed a small attenuation — the one adaptation that did move. Notably, this analysis didn't find a significant moderator effect for cycling vs running, more than 5 vs fewer weekly sessions, training status, or age on the main pooled outcomes.
A separate meta-analysis looking specifically at the muscle-fibre level (rather than whole-muscle size) found a small negative effect on fibre hypertrophy with concurrent training, with preliminary evidence that running may show more interference than cycling, at least for type-I fibres.
A 2024 meta-analysis adds another layer: small lower-body strength interference was detected in males but not females in the pooled data — highlighting that sex and training context may matter, and that this literature doesn't collapse into one universal effect. Female participants remain underrepresented in parts of the concurrent-training research.
Whole-Muscle Hypertrophy
Major 2022 pooled analysis: no significant overall interference.
Maximal Strength
Major 2022 pooled analysis: no significant overall interference; newer analyses suggest small subgroup-specific lower-body effects may exist.
Muscle-Fibre Hypertrophy
Separate 2022 analysis: small negative effect; preliminary running/type-I fibre signal.
Explosive Strength / Power
Most consistent interference signal; same-session concurrent work appears more problematic.
The 2012 meta-analysis found more interference with running than cycling, and a 2022 fibre-level analysis found preliminary evidence of a larger running effect specifically for type-I muscle fibres. But the updated 2022 whole-muscle and maximal-strength meta-analysis didn't find a significant running-vs-cycling moderator effect for those outcomes. Running isn't automatically 'bad for gains' — the picture is more mixed than a single confident rule suggests.
Running does involve repeated eccentric loading and can create substantial lower-body fatigue for some people, in a way that a controlled cycling session often doesn't. If running leaves you meaningfully more fatigued than cycling would, a lower-impact modality can be easier to recover from — that's a genuinely useful personal observation, not proof that running suppresses growth for everyone.
High mileage stacked on top of high lower-body lifting volume can be a real programming challenge — but that's a recovery-management problem, not proof that running itself is incompatible with building muscle. Running can coexist with lifting for most people training both.
The myth is too simple. The science is more useful — it just depends on what you're actually training for.
General Health + Fitness
Keep both. There's no strong need to over-optimise order or scheduling.
Muscle / Hypertrophy Priority
Keep cardio recoverable, watch lower-body fatigue, don't make every session hard, eat and recover adequately.
Maximal Strength Priority
Heavy strength first when combined in a session; manage fatigue before key lower-body work; cardio remains possible.
Power / Sprint / Jump Priority
The clearest interference signal in the research — avoid stacking hard endurance and explosive sessions when practical.
Endurance Priority
Keep strength training, but let it complement your key endurance sessions rather than compete with them.
If strength is the priority in a combined session, lift first. If endurance performance is the priority, doing endurance first may make more sense. Put the quality you care most about first, while you're fresh.
Separating hard sessions reduces acute fatigue overlap and is especially sensible when both sessions are demanding, or when power is important. It doesn't guarantee zero interference, but it's a reasonable default when your schedule allows it.
There's no single weekly cardio limit that protects gains — the relevant dose depends on session duration, intensity, modality, training status, lifting volume, energy intake and recovery, all together. The practical risk rises when total training stress exceeds what you can actually recover from.
Adding endurance work increases energy expenditure — if food intake doesn't roughly keep pace, chronic energy deficiency can make lean-mass gain harder and recovery worse.
Signals worth paying attention to: key lifting loads or reps repeatedly falling, unusual persistent soreness, worsening sleep or recovery, endurance session quality collapsing, or motivation and training quality dropping off. Adjust volume or scheduling based on those, not on a number you read somewhere.
Adding endurance work increases how much energy you burn. If food intake doesn't roughly keep pace, chronic energy deficiency can make lean-mass gain harder and recovery worse — which can look like 'cardio killing your gains' when the actual issue is under-fuelling the extra volume. That doesn't mean eating back every calorie burned, and there's no single universal number to aim for — but it's worth checking before blaming the cardio itself. See the Nutrition Guide for more on eating to support training.
For most recreational lifters, dropping cardio entirely to protect muscle isn't supported by the concurrent-training evidence. Whole-muscle hypertrophy and maximal strength hold up well alongside a sensible amount of concurrent cardio in the current research, and aerobic fitness has its own real health and performance value on top of that.
If you're chasing elite-level explosive power specifically, there's a small, real trade-off worth managing — mainly around same-session scheduling. For general size, strength and health, that trade-off is a minor programming detail, not a reason to choose one over the other.
Your heart and your muscles are not enemies.
BOTH.
A major 2022 meta-analysis found no detectable overall compromise in whole-muscle hypertrophy with concurrent aerobic and strength training, although a separate fibre-level analysis found a small negative effect. Context and outcome matter.
An influential 2012 meta-analysis found more interference with running than cycling, and a 2022 fibre-level analysis found preliminary evidence of a larger running effect for type-I fibres. However, the updated 2022 whole-muscle/maximal-strength meta-analysis didn't find a significant running-vs-cycling moderator effect. Running isn't automatically 'bad for gains.'
Do the priority first. If strength is the priority, a resistance-before-endurance order is sensible in a combined session; if endurance quality is the priority, reverse it.
There's no universal weekly threshold. The relevant dose depends on intensity, duration, modality, lifting volume, training status, nutrition and recovery — together, not any one number in isolation.
Not always necessary. Separating hard sessions is most useful when fatigue overlap matters, especially for power or demanding lower-body work on the same days.
Explosive strength and power show the clearest interference signal in the major 2022 analysis. Whole-muscle hypertrophy and maximal strength weren't significantly compromised overall.
Concurrent-training research uses different populations, modalities, doses and outcome measures, so results shouldn't be collapsed into one universal rule. Educational content, not individual medical advice.