Cardio alone will not build muscle the way lifting does

Whether aerobic exercise can build muscle the way lifting does is a question the research literature has answered inconsistently, with some studies suggesting endurance work triggers meaningful growth and others finding little effect. That matters because many people choose one mode of training and hope it covers everything, including muscle size. This review set out to settle the disagreement by systematically gathering the evidence comparing hypertrophic adaptations to aerobic and resistance training, and to explore why the published findings have diverged so much in the first place.

The authors followed PRISMA guidelines for systematic reviews and assessed the methodological quality of each included study using the Downs and Black checklist. Twenty-one studies made the final cut, all rated good or moderate in quality. A random-effects meta-analysis was then run three separate ways: once for whole-muscle measurements of the knee extensors, once for the cross-sectional area of type I (slow-twitch) muscle fibres, and once for the cross-sectional area of type II (fast-twitch) fibres. This let them compare aerobic and resistance training at both the whole-muscle and the individual fibre level.

All three analyses pointed the same way. For whole-muscle growth, resistance training significantly outperformed aerobic training, with a pooled effect size of 0.66. At the fibre level the gap was larger still: 0.99 for type I fibres and 1.44 for type II fibres, meaning the fast-twitch fibres showed the biggest advantage for lifting. Notably, the studies agreed with each other to an unusual degree across all three analyses, which strengthens confidence that the difference is real rather than an artefact of a few outlier trials.

This review directly addresses the inconsistency that motivated it: taken together, the evidence says single-mode aerobic training does not promote the same muscle growth as resistance training, and the pattern holds whether you measure the whole muscle or individual fibres. The main open question is generalisability, since every included measurement came from the knee extensor muscles. The authors hypothesise that similar results would appear in other muscle groups, but that remains an inference rather than a demonstrated finding.

If building muscle is one of your goals, this is a reason to think aerobic work alone will not get you there, however good it is for other outcomes. The advantage for resistance training was consistent and largest in the fast-twitch fibres, which are the ones most associated with size and power. None of this argues against cardio; it simply clarifies that the two modes of training produce different adaptations, and that muscle growth sits firmly on the resistance side of the ledger.