In what year(s) in the NFL was the runner allowed to get up and run after being knocked off his feet, as long as they weren't tackled and held down? From memory, it only lasted 1 or 2 years because it caused lots of injuries. I think it was in the 50s but could have been in the early 60s also.
According to the NFL (emphasis is mine), in 1955,
The ball is dead immediately if the runner touches the ground with any part of his body except his hands or feet while in the grasp of an opponent.
The ball is dead immediately if a runner touches the ground with any part of his body except his hands or feet after being contacted by a defensive player.
So it was a rule in 1955 and maybe before (the site does not make this clear).