Does doing cardio kill your muscle and strength gains?
No. Cardio doesn't kill your gains — the interference effect is real but modest, and it mostly dents power and explosive strength, not raw size. How you do it (running vs cycling, how hard, how close to lifting) decides how much it costs.
The fear traces back to a real finding. In 1980, Robert Hickson had people train hard for strength and endurance at the same time and watched their strength gains stall out and then slide in the final weeks — while their endurance climbed just fine. That study coined the interference effect, and it’s spooked lifters ever since. The catch: Hickson’s subjects ran and cycled hard six days a week on top of heavy lifting. Almost nobody trains like that.
Pool the modern evidence and the picture calms down. The largest recent meta-analysis — Schumann and colleagues in 2022, 43 studies — found basically no penalty to muscle size or maximal strength from adding cardio; the hit to hypertrophy was a rounding error. Where interference reliably turns up is power and explosive strength: jumps, sprints, rate of force. If your sport lives on a max vertical or a fast barbell, that’s the tax you pay.
But ‘it depends’ is doing real work here, and the variables are worth knowing.
What actually causes it
Modality matters most. Running interferes more than cycling — Wilson’s 2012 meta-analysis found running dented both size and strength while cycling didn’t, probably because its eccentric pounding piles muscle damage onto the same legs you’re trying to grow. A 2022 fiber-level review found the same split: running blunted slow-twitch fiber growth; cycling left it alone.
Dose and timing do the rest. Interference scales with how much cardio you do and how often, and stacking it right before — or in the same session as — your lifting is the worst case for power. Much of the effect is just fatigue: showing up to squats already tired. Put a few hours or a day between the two and most of it evaporates.
The practical version
Cardio won’t sabotage a size or general-strength program; staying fit helps you recover between sets and handle more work. Keep most of it low-impact — bike, row, incline walk — rather than hard running on leg days, separate it from lifting when you can, and don’t bury the session you care about under fresh endurance fatigue. Save strict separation for when peak power is the whole point.