The interference effect: cardio and lifting together

The interference effect is real, but only for a few hours after your workout, inside a single muscle cell. Follow real people over months instead of hours, and it mostly disappears. That gap between the lab and your actual training week is the whole story.

Here's the worry: cardio and lifting send opposite instructions to your body, so doing both drags each one down. There's a genuine biological reason people believe this, which is why the idea won't die. There's also about forty years of ordinary people getting stronger and fitter side by side, which is why, for almost everyone reading this, the worry turns out to be smaller than it sounds.

Where the idea came from

The theory has a specific birthday: 1980, when exercise scientist R.C. Hickson put a group of trainees through ten weeks of heavy lifting stacked on top of hard endurance work, nearly every day, in both. It was about as much combined training as a body can absorb.

The group doing only strength work kept getting stronger for all ten weeks. The combined group tracked right alongside them for about seven weeks. Then their strength gains stalled, and by weeks nine and ten they'd actually lost ground. Their aerobic fitness climbed exactly as much as the group doing only endurance work, so it wasn't a fatigue problem across the board.¹ Something about mixing the two was costing the combined group strength they'd otherwise have kept.

That's the finding that gave the effect its name. But notice the dose. This wasn't someone squeezing in two easy jogs a week. It was near-daily hard training in both disciplines, at the extreme end of what anyone would realistically do. The interference showed up at the far edge of the volume scale, and that detail got lost as the idea spread.

The cellular tug-of-war

Here's why the fear has real teeth: lifting and endurance work flip on two different master switches inside a muscle cell, and those switches compete for some of the same resources.

Heavy lifting fires up a pathway built around a protein called mTOR. Think of it as a construction order: once it's active, the cell ramps up protein production and starts adding contractile machinery, which over time is muscle growth. Endurance work fires a different sensor, called AMPK, which works more like a low-fuel light on a dashboard. It notices the cell burning through energy fast and responds by making the muscle better at producing energy, mainly by building more mitochondria.

The friction is this: when AMPK lights up, it can dial mTOR back down. An energy-saving alarm and a construction order don't make much sense running at full volume at the same moment, so the cell tends to favor one over the other.

A lab experiment in 2005 caught this switch in the act. Researchers electrically stimulated rodent muscle with two different pulse patterns. Slow, endurance-like pulses switched on AMPK and a mitochondria-building protein called PGC-1 alpha. Fast, strength-like pulses instead drove the mTOR pathway and multiplied protein synthesis several times over. Same tissue, opposite programs, depending entirely on the signal it got.²

Hard cardio flips on an alarm that quietly tells the muscle this isn't the moment to build. Lifting flips on the order to build anyway. Stack a lot of endurance work right around your lifting, and that alarm can turn the volume down on the construction order.

What actually happens over months, not hours

A molecular signal that spikes for a few hours after one workout is a long way from a smaller muscle a year later. That gap between the lab and real life is where most of the fear falls apart.

A widely read look back at the human trials, by Murach and Bagley, put it bluntly in its own title: "Contrary Evidence for an Interference Effect." Their read was that the effect on muscle growth "is not as compelling as previously thought," and that under some conditions, adding cardio might even nudge muscle growth up rather than down.³

The two biggest attempts to settle it with hard numbers disagree, and that's worth being upfront about. The first, run by Wilson and colleagues in 2012, pooled 21 studies and 422 effect sizes and found real, if modest, damage. Lifting alone scored an effect size of 1.23 for muscle growth versus 0.85 when paired with cardio, 1.76 versus 1.44 for strength, and 0.91 versus 0.55 for explosive power. The more and the longer people did endurance work, the bigger those gaps got, and running dragged results down where cycling didn't.⁴

A decade later, with more than twice the data, the picture changed. Schumann and colleagues pooled 43 studies in a 2022 update, also in Sports Medicine, and found adding cardio barely moved the needle on muscle size (a standardized mean difference of -0.01) or maximum strength (-0.06), differences small enough to count as noise. The one real casualty was explosive power, down by -0.28, and even that was worst when cardio and lifting were crammed into the same session instead of spaced apart.⁵ This bigger, newer analysis also found no meaningful gap between running and cycling, and no penalty for training more often.

More data and tighter methods usually beat less of both, so the newer read is probably closer to the truth. At normal, recreational training volumes, your squat and your muscle mass are safe. What's most likely to take a hit is fast, explosive power, mostly when you're already tired and doing everything back to back.

Where the caution still applies

The molecular story and the real-world data don't connect as neatly as anyone would like. We can watch AMPK dial down mTOR within a few hours in a muscle biopsy. We can watch people build strength and endurance together over months in the real world. The bridge between those two facts is still partly a guess.

Most of the human trials behind these numbers also ran for a few weeks to a few months, in fairly young, mostly healthy, and mostly male groups. That matters most if you're chasing the last few percent. Competitive lifters and athletes fighting for a maximal vertical jump or a personal best are exactly the people for whom a small interference effect is a real decision. If your goal is a body that works well at 70, that tradeoff barely registers.

Make it work in your training week

• Leave a few hours between hard cardio and heavy lifting when your schedule allows it. The evidence points to roughly three hours to protect explosive power.
• If they have to share a session, lift first when strength is the day's priority, so your heaviest work happens on fresh legs.
• Lean on cycling, rowing, or an incline walk over running if your legs already feel beat up. It's not that running signals differently inside the muscle, it's that it's higher impact and leaves the legs you squat with more sore.
• Watch total fatigue, not just the pairing. Most stalled progress on a combined plan comes from too much hard work and not enough recovery, which is a volume problem, and it's fixable by backing off.
• If your only goal is health, stop weighing the order at all. Moderate cardio plus a couple of lifting sessions a week is close to the ideal combination, and what it does for your heart and metabolism outweighs any small gym-specific cost.

Common questions

Will cardio kill my gains? Almost certainly not, at normal training amounts. The biggest pooled analysis on this found essentially no effect of adding cardio on muscle size or max strength. Interference is real, but it shows up mainly at very high endurance volumes, and even then it mostly costs you explosive power, not the everyday strength and muscle most people are training for.

Should I do cardio or lifting first? Whichever one matters most that day, while you're still fresh. Lift first if the session is really about getting stronger. Order barely matters on an easy aerobic day. Better still, separate them by a few hours instead of picking an order at all, if your schedule allows it.

Is running actually worse than cycling for lifters? The 2012 analysis suggested so. A bigger one a decade later found no meaningful signaling difference between them. The real case for cycling over running isn't molecular, it's mechanical: running is higher impact and leaves your legs more sore, which can quietly drag down your next lower-body session.

How much cardio is too much? There's no hard line, but interference climbs with endurance volume, so it's only a real risk once you're doing a lot of it. For most people training for health, the aerobic dose that actually helps your heart, roughly what we lay out in our longevity week, sits nowhere near the level that threatens muscle.

Does getting older change any of this? Not based on the evidence. The reassurance held up across age groups in the newer analysis. If anything, the case for doing both gets stronger as you age, since at that point you're defending muscle as an organ, not just chasing a number on a bar, and muscle quietly does a lot for the rest of your body.

Adjusting the mix as your week changes

The rules above aren't complicated, but living by them is a moving target. Your legs are still fried from a long ride, or your explosive power has quietly dropped and you haven't noticed, or a busy week keeps landing cardio and lifting in the same afternoon anyway. At Bespoke Fit, your coach programs the strength and the cardio together from the start, watches your lifts live over video, and adjusts the mix session by session based on what they're actually seeing, not a plan written three months ago. Your first month is $49, then $299 a month locked for life if you're one of the next 25 founding members, with unlimited live 1-on-1 sessions and a 30-day money-back guarantee. See how it works.

Sources

1. Hickson, 1980. Interference of strength development by simultaneously training for strength and endurance. European Journal of Applied Physiology and Occupational Physiology. ↩

2. Atherton et al., 2005. Selective activation of AMPK-PGC-1alpha or PKB-TSC2-mTOR signaling can explain specific adaptive responses to endurance or resistance training-like electrical muscle stimulation. FASEB Journal. ↩

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

4. Wilson et al., 2012. Concurrent Training: A Meta-Analysis Examining Interference of Aerobic and Resistance Exercises. Journal of Strength and Conditioning Research. ↩

5. Schumann 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. ↩