The “cardio kills gains” thing is basically a gym myth. A major systematic review and meta-analysis found that combining aerobic and strength training does not compromise muscle size or maximal strength development. So if you’re doing both, you’re not sabotaging yourself.
Moderate cardio can actually support your fitness overall — and some research suggests it may contribute to strength and muscle growth, especially in middle age and older adults. The idea that a few runs a week will melt your muscles just isn’t backed up by the science.
That said, there is one real nuance worth knowing. The same research found that explosive strength gains — think fast, powerful movements like jumping or sprinting — can be slightly reduced when cardio and lifting happen in the same session. And a 2022 meta-analysis (summarized by ScienceInsights) found only a small negative effect on muscle fiber growth when the two are combined, compared to lifting alone.
Small. Not catastrophic. But real.
So if you’re training for serious power or speed, it’s worth thinking about how you structure your sessions. Doing heavy lifting and intense cardio back-to-back in the same workout is where things can get a little less efficient. Splitting them into separate sessions — or separate days — is the smarter move if that stuff matters to you.
One more thing: if you’re ramping up both cardio and lifting at the same time, don’t overdo it. Piling on too much volume too fast is a real recipe for burnout or injury, no matter how you order your workouts.
For the average person just trying to stay fit, build some muscle, and keep their heart healthy? Moderate cardio is your friend, not your enemy.