Also, some athletes, and Marner may be one of them, just will no be able to get faster, or have elite edge work, or whatever aspect of their game. Some things, like vision/hockey sense, or in McDavid's case - speed, are just something they are born with, and can't be taught no matter what. Be it body type, muscle structure, whatever. I am pretty sure McDavid/Barzal have not been taught to skate like they do, it's just natural. Tweaked.....sure, but not everyone is the same, and it's not as black/white as some people think.