Does cardio kill your gains?

The fear

If you combine strength and endurance, you know the line: cardio eats muscle. For Hyrox athletes, runners who lift or combat athletes this is not a theoretical question. Their sport demands both. They cannot just pick a side.

What happens in your body

Strength training switches on a signalling pathway in the muscle cell that runs through mTOR. mTOR controls how much new muscle protein gets built. Endurance training, on the other hand, mainly activates AMPK, the cell's energy sensor. AMPK says: energy is low, save before you build.

And AMPK can put the brakes on mTOR. On paper that means cardio turns down exactly the switch you need for building muscle. Research calls this the interference effect.

What the research shows

A large meta-analysis from 2022 pooled the results of 43 studies. It compared people who only trained strength with people who combined strength and endurance.

For muscle growth and maximal strength there was, on average, no measurable difference. For explosive strength there was: those who combined both became less explosive. Most clearly when the two were combined badly.

So the switch in the lab is real. What actually reaches your muscle over months is a different question.

Why the problem is still real

"On average" does not mean "for you". A meta-analysis shows the average of many studies, with many different people and programmes. Your training week is not the average.

And exactly where the research does find a downside is where it hurts many athletes most. In a lot of sports explosive strength is not a side issue: the sled push, the final kick, the punch in the ring. When strength and endurance get in each other's way, you notice it first where you need performance when it counts.

Then there is fatigue. If you go into strength work tired, you move less load and often move it worse. At that point the interference effect is not just molecular biology but a planning question.

The real question

Not "cardio, yes or no". But: how much, what kind, when and in what order. For your sport, your life and your goal. There is no one-size answer, which is why I am not writing one here.