...short stubby running back.
These guys are just too quick for average size defenders to deal with. Quick, jabby little steps and cuts and then...zoom.
The first of this breed was tiny 5-4 170 pound NCAA/AAU sprint champ Buddy Young out Illinois. He was a blurr and would get lost behind the line only to pop out and...zoom. He was the first of the great Afro American running backs having played in the 40's.
Then here came guy like...
Lionel "Little Train" James
Charley Tolar an anvil like fullback
Pro football has always (since Buddy Young anyway) had those kinds of players and there doesn't appear to be and end in sight since we see a lot of them in the NCAA.
Today it's Ray Rice and MJD both not having the kind of seasons they've had, but at their best they were dynamic.
Sproles is the best all around back of the short stubby backs, he can do it all.