The interference effect, where combining endurance and strength training negatively impacts gains, has been studied since at least 1980.

“The interference effect is what this is called. So this is all the way back to 1980, Bob Hickman's stuff, right, and he was actually working in a lab with John Holloszy, who's one of the fathers of exercise biochemistry.”

“But the general answer here is interference effect is sort of real, but it's probably greatly overblown.”

“So the concept of kind of the interference effect or issues with concurrent training come from the idea that you have both endurance training goals and resistance training goals and that if you perform those too close together that the endurance training might”

“But we really don't run into true uh interference effect type results unless we're doing tons of training and you know, you're doing your sprints before you do your leg workout.”

“strength first as cardio first can blunt muscle gains, but it's pretty small so I don't think it's a big deal either way”

“Yeah. So, you know, I think where the evidence is, and if you look at the latest systematic reviews and meta-analyses, what they're going to say is, maybe there's a slight interference effect, maybe it's there to a greater extent with high-intensity interval training. And certainly if you do it within the same session, maybe there's some blunting. But if you look at the work right now, there's some evidence to say cycling is okay, but running is not. There's some evidence to say actually running's okay, but cycling's not. So there's no clear answer. I think the bottom line is there might be a slight interference effect in some people, especially when they do it very, very close together or as part of the same session, but it's probably relatively small. It's probably relatively negligible in the big picture.”