Steady-state cardio won't build muscle, pooled trials suggest

Cardio is often credited with all sorts of body-composition benefits, and some researchers have argued that steady, moderate-intensity continuous training (MICT) can build muscle as well as fitness. The evidence has remained genuinely controversial, partly because many earlier trials lacked a non-exercise control group, making it hard to tell whether any muscle change was down to the training at all. Protocols, measurement methods and participant characteristics have also varied enormously between studies. This review set out to settle whether MICT actually promotes muscle hypertrophy in healthy adults.

Following PRISMA guidelines, the authors searched MEDLINE, Embase, Web of Science, Scopus and the Cochrane Library for chronic human interventions comparing MICT with non-exercise controls. Eligible trials had to report whole-body or localised muscle mass outcomes — lower-limb lean mass, muscle cross-sectional area, fibre cross-sectional area or muscle thickness — in healthy adults free of chronic disease or musculoskeletal injury. Thirteen studies met the inclusion criteria. A random-effects model then compared MICT against control for total and regional muscle mass outcomes, both combined and separately.

The pooled standardised mean difference was 0.050 (95% CI: -0.147 to 0.248) for total and regional outcomes combined — an effect statistically indistinguishable from zero. For regional outcomes alone it was 0.080 (95% CI: -0.277 to 0.437), again crossing zero. In plain terms, adults doing moderate-intensity continuous training did not show consistently more muscle than people doing no exercise at all. The authors conclude that the current evidence does not consistently support a hypertrophic effect of MICT, while flagging that variability in study design, populations and measurement sensitivity means the conclusion should be read with caution.

This pooled analysis pushes back against the idea, floated in some individual trials, that steady cardio meaningfully grows muscle. By restricting the analysis to trials with genuine non-exercise control groups, it addresses a key weakness of the earlier literature. It does not close the question entirely: the authors note considerable heterogeneity in training protocols and assessment methods, and measurement sensitivity varied between studies. Whether particular intensities, volumes or populations respond differently remains an open question.

If you do steady-state cardio for heart health, endurance or general fitness, nothing here argues against it — muscle growth was simply never its strong suit. But this analysis is a reason not to count moderate continuous cardio towards your muscle-building efforts, since the pooled effect on muscle size was essentially nil. Anyone whose goal includes visible hypertrophy has grounds to look beyond MICT for that particular outcome. As ever, a cautious read is warranted given how much the underlying studies varied.