Muscle-ups are fun, but I do not remember them being part of gym class in my schooling, probably because the chin-up bars were too close to the ceilings to do them. I wonder when the "muscle-up" term came into being as it was something that kids would just do on their own, y'know, jump up to a tree branch and just do it, but nowadays it too is seen as a physical feat. There's the Boomer meme of an 8th grade math quiz from the early 1900s going around, with pretty much the majority of modern Americans expected to fail it. I think most Americans today would fail an 8th grade gym test from the 1950s.