"Downhill"

When did the word “downhill” enter the sports’ vocabulary as a positive, as opposed to a term describing players no longer at the top of their game?  It seems every NFL game I listen to has an announcer talking about how somebody is great at “running downhill.”  There are football hills I don’t know about?

And now I just read this description of a pitcher in the Yankees’ organization—he “creates good downhill plane towards the plate.”  Huh?  No uphill plane?  Why any plane at all?  Why not say that somebody’s a good runner, linebacker or pitcher?

Next, they’ll start talking about an NBA forward or guard with great “downhill acceleration.”  Which leads to this thought: What happens when everyone reaches bottom?