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Interval training (HIIT) interferes more with muscle growth than steady-state cardio,
new meta-analysis finds

High-intensity interval training was heralded as cardio 2.0, a better and more time-efficient way to burn fat and develop endurance. "Train like a sprinter, look like a sprinter!"

Except you're not training like a sprinter. You're training like a soccer player. Olympic 100 m sprinters train with sprints of around 10 seconds and then often rest several minutes. HIIT is metabolically much closer to low-intensity, steady-state cardio (LISS) than strength training.

All forms of cardio can interfere with muscle growth, as endurance and strength training adaptations are partially acutely mutually exclusive. It's like trying to develop your body into 2 different directions along the strength-endurance continuum at the same time. This new meta-analysis confirmed that endurance training on average in the literature significantly reduced both type I and type II muscle growth from strength training ( the interference effect AKA concurrent training effect), although the effect on the whole-muscle level was not statistically significant.

Some people argue that the interference effect is not a real thing, despite numerous studies over the past decades demonstrating doing cardio and strength work in the same program can reduce muscular gains. The counter-arguments mostly rely on studies in untrained individuals. As I recently posted, some research finds that untrained individuals can gain just as much muscle from endurance as from strength training. They can get newbie gains from anything the first few weeks. So of course there's no interference yet at that point.

The novel finding of this new meta-analysis of the literature was that HIIT reduced muscle growth significantly more than LISS cardio. The reason for this may be that HIIT is always intensive, so it will always evoke significant endurance training adaptations and those can interfere with strength training adaptations. With steady-state cardio it's easier to stay under the threshold of evoking strong endurance adaptations.

For body recomposition and strength development, I've always preferred LISS cardio over HIIT. The vast majority of my clients never have to do any cardio at all though.

For health benefits or endurance training, HIIT can still be great. To minimize the interference effect, schedule HIIT sessions as far away from your strength work as possible, on different days.

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