Does Cardio Kill Gains? The Real Science of the Interference Effect

Does cardio kill gains? For most lifters, no. A 2022 meta-analysis of 43 studies found that adding aerobic training to a lifting plan did not compromise muscle size or maximal strength compared with lifting alone (Schumann et al., 2022). What the data does show is a smaller, conditional “interference effect” that depends on how much cardio you do and what kind. For the full programming blueprint, see our concurrent training guide. This piece sits inside the wider joint-friendly lifting myth-busting guide, because the cardio fear stops a lot of lifters from doing the thing that protects their heart and joints.

Let us separate the myth from the real, much narrower effect.

Where the “cardio kills gains” idea came from

The idea has an actual scientific origin. Research on concurrent training (lifting and endurance work in the same programme) started with studies of endurance athletes and rodents, and some of the early human trials showed lifters who also trained for endurance gained less strength than those who only lifted. A 2014 review put it plainly: current evidence suggested that combining the two may attenuate gains in muscle mass, strength and power compared with resistance training alone (Fyfe et al., 2014).

Then gym culture rounded “may attenuate” up to “cardio eats muscle.” Not the same sentence.

What the meta-analyses actually found

Three big reviews are worth knowing, and they do not say the same thing.

Wilson et al. (2012) pooled 21 studies and 422 effect sizes. The mean hypertrophy effect size was 1.23 for strength training alone, 0.85 for concurrent training and 0.27 for endurance alone. Strength and concurrent groups were both significantly greater than endurance only, and the review concluded that interference depends on the modality, frequency and duration of the endurance work (Wilson et al., 2012).

Schumann et al. (2022) updated the picture with 43 studies. Comparing identical strength training with and without added aerobic work, the standardised mean differences were close to zero for maximal strength (-0.06) and muscle hypertrophy (-0.01). Explosive strength was the exception (-0.28) (Schumann et al., 2022).

Lundberg et al. (2022) zoomed in on single muscle fibres across 15 studies and found a small negative effect on overall fibre hypertrophy (SMD -0.23, borderline at p = 0.050) (Lundberg et al., 2022).

So whole-muscle growth looks fine, fibre-level growth may take a small hit, and power is the most vulnerable. If you want maximum bicep or quad size, the practical difference is likely tiny. If you play a sport that needs jump and sprint power, it deserves more thought.

Does running hurt more than cycling?

This is where the reviews genuinely conflict, so here is each side.

Wilson et al. reported that lifting while also running, but not cycling, led to significant decrements in both hypertrophy and strength. Lundberg et al. found a negative effect on type I fibre size when the aerobic work was running but not cycling (SMD -0.81), and they called this preliminary. Schumann et al., on the other hand, found no significant difference between cycling and running for muscle size or strength.

Why might they differ? Running has an eccentric component (muscle lengthening under load with every foot strike), so it may leave more soreness and muscle damage than cycling. That is a plausible mechanism, but the abstracts above do not measure it, so treat it as an explanation, not a proven cause. Study designs, running volumes and populations also differ.

The cautious takeaway: if you run a lot and your legs feel flat in the gym, swapping some running for cycling, rowing or incline walking is a low-cost experiment. Our deeper comparison is in cycling vs running for lifters.

What is the AMPK vs mTOR story, and does it hold up?

The molecular version goes like this. Endurance exercise activates AMPK, an energy sensor. Lifting activates mTORC1, the pathway that switches on muscle protein synthesis. In rodents and cell studies, activated AMPK can inhibit mTORC1. So the theory says cardio flips a switch that blocks the growth signal.

The theory is tidy. In humans it is messier. Fyfe et al. noted that human studies had not observed the molecular interference after acute concurrent exercise that would explain reduced hypertrophy (Fyfe et al., 2014).

A direct test used eight trained men who either lifted only, or did interval cycling before lifting. AMPK was raised in the cycling condition. Yet S6K1 activity (a marker of mTORC1 signalling) rose the same in both trials. The authors concluded that cycling-induced AMPK did not inhibit mTORC1 signalling, though a marker of protein breakdown (MuRF1) did rise after the combined session (Apró et al., 2015).

Eight men is a small sample, and one session is not months of training. Still, it is the most direct human test of the idea, and the clean “AMPK blocks mTOR” story did not show up in it. For a fuller explanation, see our piece on the molecular interference effect.

What cardio can do for a lifter

Here is the upside the myth hides. A 2025 meta-regression of 353 studies for mitochondria and 131 for capillaries found that low or moderate intensity continuous training raised mitochondrial content by about 23% and capillaries per fibre by about 15%. High-intensity intervals gave similar gains. The changes were larger in people with a lower starting fitness, and capillary gains were seen mainly in untrained to moderately trained participants and in the early weeks (Mølmen et al., 2025).

Translation: easy cardio builds the aerobic machinery inside your muscle, and people starting from a lower fitness level get the biggest return. Some coaches argue this helps you recover between sets and between sessions. That sounds reasonable, but the studies above did not test it directly, so call it plausible rather than proven. Our guide to Zone 2 cardio for lifters covers the practical side.

There is also the health side. The US Physical Activity Guidelines recommend at least 150-300 minutes a week of moderate aerobic activity, or 75-150 minutes of vigorous activity, along with muscle-strengthening on 2 or more days a week (Piercy et al., 2018). Lifting alone does not cover the aerobic part.

Does this apply if you are over 50?

Mostly yes, with the usual caveat about checking in with your doctor. A 2025 meta-analysis of 53 randomised trials (2873 people aged 50 and over) concluded that concurrent training was as effective as resistance training for increasing muscle mass, and as effective as aerobic training for reducing body fat measures (Khalafi et al., 2025). In that age group, adding cardio did not cost muscle compared with lifting alone.

How to do cardio without losing muscle: the practical rules

Here is what the evidence supports, plus some coaching judgement where the evidence is thin.

• Lift first when you share a session. Cardio first leaves you tired for the heavy work. (This is a practical rule. The 2022 fibre-size review found no effect of same-session order on fibre growth, so it is about the quality of your heavy sets.)
• Separate sessions when you can. Schumann et al. found explosive strength was attenuated more with same-session training than with sessions at least 3 hours apart (Schumann et al., 2022). For many people, a morning run and an evening lift is a neat split.
• Pick the gentle modality. Cycling, rowing, incline walking and the cross trainer are easier on your legs than high mileage running. More on timing and sequencing in timing cardio and weights.
• Start with 2-3 easy sessions a week. There is no proven ceiling, but that range fits well around a 3-4 day lifting plan. Wilson et al. linked higher frequency and duration of endurance work with smaller gains, so if your lifts stall, trim the cardio first.
• Keep the heavy lifting heavy. Do not swap strength sessions for cardio. In a calorie deficit, keep protein around 1.6-2.2 g/kg and keep progressing your lifts.
• Eat for the total work. More training burns more fuel. Dal, paneer, curd, eggs, soya chunks and roti or rice around training all help, especially in hot weather when you sweat a lot and drink less than you think.

Common mistakes lifters make with cardio

The most common mistake is going from zero cardio to five hard sessions a week, then blaming “interference” when the squat stalls. That is just too much new fatigue.

Another is doing a long run the day before leg day. Your legs will not recover, and the interference you feel is soreness and tiredness, not a molecular switch.

A third is cutting calories, adding cardio and dropping lifting volume all at the same time. When muscle shrinks in that situation, the deficit and the missing training are the likelier culprits. We cover the threshold question in can cardio cause muscle loss.

When to see a doctor

Cardio is safe for most people, but speak to a doctor before increasing it if you have known heart or lung disease, diabetes, high blood pressure that is not controlled, or you are returning after a long break. Stop and get medical advice if you notice chest pain, unusual breathlessness, dizziness or fainting during exercise.

The bottom line

Cardio does not kill gains. High volumes of running may shave a little off muscle growth, and explosive power is the thing most worth protecting, but for most lifters moderate cardio is a fair trade for a stronger heart and better work capacity. Lift first, space sessions out, favour cycling or walking when your legs matter, and keep your lifting heavy. If you want the full picture of how endurance and strength fit into one plan, see the maximal strength guide.