Does cardio really kill your gains? The interference effect, in full

Ask whether cardio kills your gains and you will get a confident yes from one corner of the internet and a confident no from another. A 2012 meta-analysis in the Journal of Strength and Conditioning Research pooled 21 studies and 422 effect sizes specifically to find out which parts of "cardio" actually interfere with strength training — and the honest answer is: it depends heavily on what kind of cardio, how much, and how often.

The headline numbers

For hypertrophy, the effect size for resistance training alone was 1.23, for endurance training alone 0.27, and for concurrent (combined) training 0.85 — meaningfully closer to strength-only than to endurance-only, and significantly ahead of cardio alone. For strength, the numbers were 1.76 (resistance only), 0.78 (endurance only) and 1.44 (concurrent) — again, combined training loses some ground but stays much closer to strength-only. Power output showed the biggest hit: 0.91 for resistance only versus 0.55 for concurrent training, a proportionally larger interference effect than seen for size or strength.

Interference effects of endurance training are a factor of the modality, frequency, and duration of the endurance training selected.

The modality that matters most: running versus cycling

The most actionable finding in the whole analysis: resistance training combined with running produced significant decrements in both hypertrophy and strength, but combining resistance training with cycling did not. The authors also found that more frequent and longer endurance sessions were consistently correlated with smaller hypertrophy, strength and power gains — the interference effect scales with dose, not with the mere presence of any cardio at all.

What this actually means for your programme

Put together, the data describes a dial, not a switch. A couple of moderate cycling or Zone 2 sessions alongside a serious lifting programme costs you very little, if anything, on size or strength. Frequent, long, high-impact running volume stacked on top of heavy lifting is the combination most likely to blunt your progress — particularly your power output, the quality most sensitive to interference. Separating hard endurance sessions from your heaviest lifting days, favouring lower-impact cardio modalities when strength is the priority, and keeping total endurance volume proportionate to your lifting goals are the levers that actually matter, far more than avoiding cardio altogether.

This is why Misi schedules cardio and lifting around each other rather than treating them as competing priorities — spacing hard interval sessions away from your heaviest lower-body days and defaulting your steady-state cardio toward lower-impact, Zone 2 work when a lifting goal is active.

Misi is a coaching and wellness platform, not a medical service, and this article is educational and not a substitute for personalised training advice.