Cardio and strength training: do you have to choose between them?

“If you do cardio, you’ll lose muscle.” This fear is persistent, and it even has a scientific name: the interference effect. It really does exist, it has been measured, and we have a fairly good idea of how much it costs. But the most recent research paints a much less frightening picture than the changing-room myth — provided you know which type of cardio to choose.

The interference effect, in figures

The leading study on this topic is a 2012 meta-analysis, which pooled 21 studies and 422 effect sizes. It directly compares the results achieved by people who do strength training alone with those who combine strength training and cardio.

So there is indeed an effect across all three measures. But take a close look at the bars: the ‘weight training + cardio’ groups are still making progress – and significant progress at that. They’re simply making slightly less progress than those who do weight training alone. It’s a slowdown, not a halt.

Not all cardio is the same

This is the most useful finding of this meta-analysis, and the one that is never cited. By breaking down the different exercise modalities, the authors found that combining weight training with running led to significant reductions in muscle hypertrophy and strength — but that this was not the case with cycling.

There are two other factors to consider, and they’re within your control: the frequency of your cardio sessions (a negative correlation of -0.26 to -0.35 with gains) and, above all, their duration (from -0.29 to -0.75). The longer and more frequent your cardio sessions are, the higher the cost.

How much does it actually cost?

A 2016 study involving 23 active men provides concrete figures on the matter. Over an 8-week period, the participants were divided into three groups: strength training alone; strength training plus high-intensity interval training; and strength training plus moderate-intensity continuous cardio.

Two key takeaways. Firstly, the order of magnitude: we’re talking about a 28 per cent increase in strength rather than 38 per cent – not a collapse. Secondly, the authors’ conclusion: the intensity of cardio training is not the critical factor in short-term interference — interval and continuous training both had a comparable negative impact on strength.

Recent research is far less alarmist

Since then, several reviews have revised the findings downwards, and one of them even goes so far as to reverse the result.

A contradiction worth noting, as it is a genuine one: the 2024 meta-analysis identifies high-intensity interval training as the method that best limits damage, whilst the 2023 meta-analysis finds that it is precisely when interval training is incorporated that weight training alone regains the advantage in terms of muscle fibres. On this point, the literature has not yet reached a consensus.

Should you do cardio before or after?

That’s the question everyone asks. The honest answer: it’s less important than people think. A 2008 study involving 48 students over 12 weeks found no significant difference between ‘cardio followed by strength training’ and ‘strength training followed by cardio’. However, both groups made less progress than the group that did strength training alone.

A 24-week study offers a more nuanced view: the order and time of day only influence the extent of adaptations after 12 weeks, and starting with cardio has been shown to be more beneficial for developing endurance. It is also worth noting that the interference works both ways: a 2017 review points out that a strength training session leaves residual fatigue which impairs the subsequent endurance session if recovery is insufficient.

What the professionals do: a survey of 43 fitness coaches in elite rugby union shows that 77 per cent take the interference effect into account in their training programmes, and that 63 per cent believe that performing strength training before aerobic exercise is more conducive to strength development. This is a view from the field, not an experimental finding — but it is consistent with the 2017 review.

What if you’re not a top-level athlete?

So the question probably doesn’t even arise for you. The most reassuring results actually come from those groups who aren’t striving for that final one per cent in performance:

Among rugby players monitored for 29 weeks during the peak season, no decline in strength or power was observed despite a combined training programme — the authors attribute this to the prioritisation and sequencing of the sessions. In moderately trained runners, 8 weeks combining speed endurance and heavy weight training improved 10 km times, 1500 m times, running economy and dynamic strength. And in older adults, combining the two improved quadriceps cross-sectional area by more than 7 per cent and strength by more than 35 per cent.

In summary

So the verdict is actually quite liberating. If your goal is maximum strength performance, every hour of cardio comes at a slight cost. If your aim is to be muscular, fit and healthy, the question ‘cardio or weight training’ is misguided: the answer is both, provided you keep your cardio sessions to a reasonable level.

And to schedule your weight training sessions properly throughout the week, have a look at how many sessions a week you need to build muscle.

Every figure in this article is taken from the studies listed below, summarised on the basis of their abstracts alone. Please bear in mind the study populations: several trials involved untrained individuals or elite athletes, for whom the circumstances are not the same, and the duration of the studies ranged from 8 to 29 weeks. This is for information purposes only and is not a substitute for professional advice.