Cardio and weight training: do you have to choose?

« If you do cardio, you lose your muscle. » The fear is persistent, and it even has a scholarly name: the interference effect. It really exists, it has been measured, and we know roughly how much it costs. But the most recent work paints a much less frightening picture than the locker room legend — provided you know which cardio to choose.

The interference effect, quantified

The reference on the subject is a 2012 meta-analysis, which gathered 21 studies and 422 effect sizes. It directly compares what is achieved by people who only do weight training and people who combine weight training and cardio.

The interference is therefore real, across all three qualities. But look closely at the bars: the « weight training + cardio » groups still make progress, and by a lot. They simply progress a little less than those who only lift iron. It is a slowdown, not a cancellation.

Not all cardio is equal

This is the most useful result of this meta-analysis, and the one that is never cited. By separating the modalities, the authors found that combining weight training with running led to significant drops in hypertrophy and strength — but that this was not the case with cycling.

Two other parameters weigh in, and they are under your control: the frequency of cardio sessions (negative relationship from -0,26 to -0,35 with gains) and above all their duration (from -0,29 to -0,75). The longer and more numerous your cardio sessions are, the heavier the bill.

How much it costs in concrete terms

A 2016 study on 23 active men puts tangible figures on the matter. For 8 weeks, three groups: weight training alone, weight training + high-intensity interval cardio, weight training + moderate continuous cardio.

Two takeaways. First, the order of magnitude: we are talking about 28% strength gained instead of 38%, not a collapse. Second, the authors' conclusion: cardio intensity is not the critical factor in short-term interference — interval and continuous training penalized strength comparably.

Recent work is much less alarmist

Since then, several reviews have downplayed the issue, and one of them goes so far as to reverse the result.

A contradiction worth pointing out, because it is real: the 2024 meta-analysis views high-intensity intervals as the modality that best limits the damage, whereas the 2023 one finds that it is precisely when intervals are incorporated that weight training alone regains the advantage at the muscle fiber level. On this point, the literature is not yet settled.

Should cardio be done before or after?

This is the question everyone asks. The honest answer: less important than people think. A 2008 study on 48 students, lasting 12 weeks, found no significant difference between "cardio then weight training" and "weight training then cardio". On the other hand, both orders progressed less than the weight training alone group.

A 24-week study qualifies this: order and time of day only influence the magnitude of adaptations beyond 12 weeks, and starting with cardio proved more favorable for endurance development. Also note that interference works both ways: a 2017 review points out that a weight training session leaves residual fatigue that degrades the following endurance session if recovery is insufficient.

What professionals do: a survey of 43 elite rugby union strength and conditioning coaches shows that 77% take the interference effect into account in their programming, and 63% believe that placing weight training before aerobic work better favors strength development. This is a field opinion, not an experimental result — but it is consistent with the 2017 review.

Two levers stand out to limit the damage. The first is nutrition: a 2022 review notes that increasing protein intake during combined training protects strength and hypertrophy. The second is workout organization — a review aimed at rowers and cyclists details how to schedule strength and endurance so that one does not eat into the other.

Interference also works both ways, which is often forgotten: a weight training session also impairs the endurance performance that follows. And it is not inevitable: a protocol combining speed-endurance work and weight training improved both performance and running economy, while in older individuals, adding blood flow restriction to combined training yielded good results with a light load.

What if you are not a high-level athlete?

Then the question probably does not apply to you. The most reassuring results precisely come from populations that are not chasing the last percent of performance:

In rugby players monitored for 29 weeks during the season, no drop in strength or power was observed despite combined training — the authors attribute this to prioritization and session order. In moderately trained runners, 8 weeks combining speed endurance and heavy resistance training improved both the 10k, the 1500m, running economy, and dynamic strength. And in elderly individuals, combining the two improved quadriceps cross-sectional area by over 7% and strength by over 35%.

In summary

The verdict is therefore rather liberating. If your goal is maximum strength performance, every hour of cardio comes at a slight cost. If your goal is to be muscular, fit, and healthy, the question "cardio or weight training" is poorly framed: the answer is both, keeping cardio sessions reasonable.

And to properly schedule your weight training sets during the week, check out how many sessions per week to build muscle.

Every figure in this article comes from a study you can open: the underlined mentions lead to the work from which the result is drawn. These summaries are written solely from the abstract. Pay attention to the populations: several trials involve untrained individuals or high-level athletes, for whom the stakes are not the same, and durations range from 8 to 29 weeks. Informative content, which does not replace professional advice.