Does cardio kill gains?

For most clients, no. Cardio does not erase muscle growth - the so-called interference effect is real but modest and manageable, and it mostly shows up with high volumes of hard, fatiguing cardio done close to the muscles you are trying to grow. Done sensibly - mostly easy intensity, a low-impact modality, and kept separate from heavy leg training - cardio sits alongside lifting without meaningfully blunting hypertrophy, while adding real health and recovery benefits.

The interference effect describes how doing a lot of endurance training alongside strength training can slightly reduce strength and muscle gains compared with lifting alone. The research suggests it is real but generally small, and it grows with the frequency, intensity, and volume of the cardio - especially when that cardio loads the same muscles you are training for size. It is a reason to program cardio thoughtfully, not a reason to avoid it.

When it is high in volume, high in intensity, uses a modality that fatigues the same muscles you lift (running and leg growth is the classic clash), and sits right before or in the same session as that muscle group. Stack several of those at once - daily hard running on top of heavy squats - and the odds of interference rise. Remove one or two of those factors and the conflict largely fades.

Make lifting the priority, then fit cardio around it. Favour a low-impact modality like cycling or the elliptical over heavy running, keep most sessions easy, separate hard cardio from heavy leg days, and start the weekly dose low. Add cardio only when health, conditioning, or a fat-loss deficit needs it, and keep an eye on lifting performance - if it slides for a couple of weeks, the cardio dose is the first thing to trim.

For protecting muscle growth, easy steady-state cardio is usually the safer default because it adds less systemic fatigue per session and recovers faster. Hard intervals are not banned - they are efficient for conditioning - but they carry more interference and recovery cost, so use them deliberately rather than by default. Keeping most cardio in an easy, conversational zone, which you can check against heart rate zones, is the simplest way to get the health benefits without taxing lifting recovery.

Yes. Even when hypertrophy is the goal, cardio builds cardiovascular health, supports recovery and work capacity, and helps manage body fat so the muscle a client builds actually shows. The point is not to cut cardio out but to dose it well. A couple of easy sessions a week plus daily movement gives most muscle-building clients the health upside with little to no cost to their gains.