Who's to say that athletes back then didn't juice?
Improbable-weights & drugs in sports were completely off the radar as recently as the 1970s & 1980s, depending on the sport. As mentioned, lifting and muscles were considered impediments-they would make you "musclebound' and "slow you down". A noticable change sometime around the early 70s in football and up to 10-15 years later for baseball. There would've been some evidence in the physiques, and you can see from pics they had nothing.