Why Some People Put on More Muscle Than Others

January 6, 2026

I have a runner friend who claims he has to avoid doing too many push-ups, otherwise he starts to bulk up so much that it interferes with his running. It’s hard to understate how annoying I find this. I also have trouble believing it, to be honest, as someone who never seems to add any muscle at all no matter how many push-ups I do. Is it really possible that people can have such widely varying responses to the same exercise stimulus?

As it turns out, this is a hot question in exercise physiology these days. The traditional view is that yes, of course there’s individual variation in how people respond to training. That’s part of how we define “talent.” In fact, some scientists even argue that there are some unlucky “non-responders” among us who don’t get fitter at all when they train. But a more recent line of research has pushed back against these notions, using statistical analysis to suggest that the apparent variations in response are just the result of measurement error and day-to-day biological variability. I took a deep dive into this question last year. Outside