Does Cardio Kill Gains? What Concurrent Training Does to Muscle

Does cardio kill gains, and what would it cost you if it did

You lift three or four times a week and you also ride, row, or run. The claim that this is a mistake usually arrives with the phrase “interference effect” attached, and the question underneath it matters: if the cardio is quietly taxing the muscle you are trying to build, that is worth knowing.

So: does cardio kill gains? A systematic review with meta-analysis in Sports Medicine is the closest thing to a direct test. It took every training study it could find that ran the same lifting program twice — once on its own, once with aerobic training bolted on — and looked at what happened inside the muscle.

Forest plot of 15 studies comparing muscle fiber growth after concurrent aerobic and strength training against strength training alone, with pooled effects of minus 0.34 for type I fibers, minus 0.13 for type II fibers and minus 0.23 overall

Figure 2 from Lundberg and colleagues (2022), reproduced under CC BY 4.0. Each row is one study. Dots to the left of the dashed line favour lifting alone; dots to the right favour lifting plus cardio. The diamonds at the bottom of each block are the pooled answers.

Does cardio kill gains, and what would it cost you if it did
What the study did: the same lifting program, with and without cardio
Does cardio kill gains at the muscle fiber level?
Running looked worse than cycling, on thin evidence
What did not matter: order, timing, frequency, training status
Where this sits in the wider evidence
What this means if you already lift and do cardio
The limits of this evidence
Common Questions About Cardio and Muscle Growth
How Pick It Up Helps
Further Reading & References

What the study did: the same lifting program, with and without cardio

Tommy Lundberg and colleagues searched six databases for supervised training studies lasting at least four weeks in which one group did aerobic and strength training together and another did an identical strength-training prescription with no aerobic work. That control condition is what makes the comparison readable: the lifting is held constant, so anything that shows up is attributable to the cardio.

Fifteen studies met the bar, covering 300 healthy adults — 153 concurrent, 147 lifting-only. Twelve used cycling as the aerobic training; three used running.

Most studies of this question use a tape measure, an ultrasound, or an MRI of the whole muscle. These took muscle biopsies and measured the cross-sectional area of individual fibers, split into the two families every muscle contains:

Type I fibers are the slow, fatigue-resistant ones, doing the work in endurance efforts and in the low-effort part of a set.
Type II fibers are the fast, forceful ones. Recruitment climbs as effort climbs, and in the thigh they make up a larger share of muscle area than type I.

An earlier review by the same group had reported no interference at the whole-muscle level, and almost every individual study claiming an interference effect had measured fibers rather than whole muscles. This paper pooled the fiber studies to see whether the disagreement was real.

Does cardio kill gains at the muscle fiber level?

A little, and less than the phrase “kills gains” implies.

Pooled across all 15 studies and both fiber types, the concurrent groups grew their fibers slightly less than the lifting-only groups. The gap was 0.23 on a standardised effect-size scale, where researchers conventionally treat 0.2 as the boundary of “small”. Its range ran from 0.46 down to exactly zero, at a p-value of exactly 0.050 — the analysis could not rule out no difference at all.

Split by fiber type, neither half reached statistical significance on its own:

Type I fibers came out at 0.34 in favour of lifting alone, with a range running from 0.72 worse to 0.04 better.
Type II fibers came out at 0.13, with a range from 0.39 worse to 0.12 better. That is a dead heat.

Where the effect landed is as informative as its size. The fibers that make up the bigger share of a thigh, and that heavy lifting leans on hardest, are the ones where adding cardio changed nothing measurable. What little interference showed up landed on the slow fibers — where endurance work would compete for the same adaptive machinery, which is the explanation the authors reach for.

Running looked worse than cycling, on thin evidence

The one subgroup that produced a clean-looking signal was the type of cardio.

Forest plot splitting the concurrent training studies by aerobic training type, showing cycling with pooled type I and type II fiber effects near zero and running with a type I effect of minus 0.81

Figure 4 from Lundberg and colleagues (2022), reproduced under CC BY 4.0. The left block is type I fibers, the right block type II. Cycling sits on the line; the three running studies sit to the left of it.

In the cycling studies, type I fibers came out at 0.22 and type II at 0.03 — too small to act on. In the running studies, type I came out at 0.81 against, with a range from 1.26 to 0.36 that stays clear of zero. That is the largest effect anywhere in the paper, and the authors’ suggested explanation is mechanical: running loads the legs eccentrically, over and over, with a stretch-shortening cycle on every stride, while cycling is almost purely concentric. More inflammatory and metabolic stress per hour, competing with the lifting for the same recovery.

Two things keep it from being the headline. It rests on three studies. The authors say so, and say caution is warranted. Three trials is a hypothesis, not a finding.

Running and cycling were not statistically distinguishable from each other. The test for whether the two subgroups differ came out at p = 0.204 for type I fibers and p = 0.094 for type II; neither clears the usual bar. What the paper established is that the running estimate was far enough from zero to be significant within its own subgroup. The authors still recommend that someone chasing muscle consider cycling over running — a reasonable low-cost bet rather than a proven one.

What did not matter: order, timing, frequency, training status

The list of things the paper looked for and did not find covers most of the advice that circulates about cardio:

Lifting before cardio, or cardio before lifting, made no measurable difference within a session.
Same session, same day, or different days made no measurable difference.
Training frequency made no measurable difference. The comparison was roughly four or five total weekly sessions against six or seven — about two lifting sessions a week against three.
Training status made no measurable difference. Untrained participants and already-active ones showed the same pattern.

None of that makes these variables irrelevant to how a session goes. It means that across this body of evidence, none of them changed fiber size.

Where this sits in the wider evidence

This study does not overturn the field’s position on cardio and muscle. It refines it, and it comes from the same team that established the position it is refining.

The year before, Schumann and colleagues published the broader review this one is carved out of: 43 studies, same inclusion rules, looking at whole-muscle growth and at strength. Three findings come out of the two papers together.

Whole-muscle size and maximal strength are not compromised. Schumann’s pooled results were 0.01 for muscle growth and 0.06 for maximal strength — how much you can lift for one rep — neither statistically significant.
Fiber size may take a small hit, mostly in the slow fibers. That is this paper, on 15 studies, and the authors state plainly that it “does not necessarily translate into differences in whole muscle hypertrophy” — that being the word researchers use for muscle growth.
Explosive power is the outcome with a measured cost. Jumping and other rapid force production was attenuated by 0.28, which was significant, and worse when cardio and lifting happened in one session than when they were separated by at least three hours.

The first two are not in conflict. A finding at the level of the individual fiber and one at the level of the whole muscle are measurements of different things, taken with instruments of very different reliability — and fiber-level changes may simply show up first, with whole-muscle measurements catching up later.

The lever that moves muscle growth is not your cardio schedule. It is how many hard sets you do per week and how close to failure you take them, both of which rest on better evidence and are bigger dials than anything here.

What this means if you already lift and do cardio

Keep the cardio. Nothing here supports dropping it. The worst-case reading is a small effect on one fiber type, inside a body of evidence that finds no effect on the muscle you can measure, set against cardiovascular benefits not in dispute.
If you are choosing a machine and muscle is the goal, take the bike. The evidence is thin and the authors label it preliminary, but acting on it costs nothing. If you run because you like running or because you race, keep running — a possible 0.8 effect on type I fibers, from three studies, is not a reason to give up a sport.
Separate the sessions by a few hours if explosive power matters, per the same-session finding above.
Stop worrying about the order. Cardio first, lifting first, same day, different days — none of it changed fiber growth. If lifting first means you lift better, lift first.
Protect the lifting itself. This is the failure mode the research design excludes and a real week does not: in every one of these studies the lifting program was identical in both groups, whereas in practice the run happens, the session gets shortened, a set gets cut. That is not interference, it is lost volume, and it will cost you more than the run does.

The limits of this evidence

Fiber measurements are noisy. The authors flag this themselves as the biggest caveat on the paper. One reference study they cite found a 13% coefficient of variation for fiber-area measurements taken within the same person, and recommended averaging two or more biopsies from different sites — which the review’s own call for future work tells you the included studies mostly did not do.

The studies were small and the quality was middling. On the PEDro scale they averaged 4.3, which on the review’s own thresholds is medium — above 6 is high. Two were poor, one was good.

“Trained” is doing a lot of work in the training-status subgroup. Studies were classified as untrained only when participants were described as sedentary, previously untrained, or inactive; everything else went into one “active” bucket spanning recreationally active through well-trained. If you have lifted seriously for years, this paper cannot say whether you respond like its average participant.

Four weeks is the floor. Some of the pooled evidence therefore describes the first month of a training response rather than a year of one.

The overall result sits exactly on the significance line. A p-value of 0.050 and a confidence interval whose upper bound is zero is a coin landing on its edge. Small and uncertain is the whole of what it supports.

Common Questions About Cardio and Muscle Growth

How much cardio is too much when lifting? This review compared about four or five total weekly sessions against six or seven and found no difference in fiber growth between them, so within that range frequency was not the deciding variable. Where the ceiling sits it cannot say, because nothing in the pooled evidence went far beyond that.

Does cycling build muscle on its own? Some, and much less than lifting does. A separate meta-analysis of 21 studies comparing aerobic against resistance training found resistance training ahead by 0.66 for whole-muscle growth, 0.99 for type I fiber area and 1.44 for type II fiber area. Cardio is not a substitute for a lifting program.

How Pick It Up Helps

The risk to your lifting is less the interference effect than the Tuesday after a hard interval session, when a working weight that was fine last week may feel heavy. Grind out a set that costs more than it gives, or cut the session short, and enough of those add up to lost volume — the thing the evidence says drives growth.

That is the problem autoregulation solves. Pick It Up gives every set a target weight, a target rep count, and a target difficulty on a simplified RPE scale, then updates the next set’s targets from the weight, reps and effort you reported for the last. On a day when cardio has taken something out of you, the session adapts to what you have rather than to what the spreadsheet assumed. At the end of the block it re-evaluates your numbers across every session, so a training phase with heavy running in it shapes the next one instead of being ignored.

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