It’s no joke that the NFL is a young man’s game. Age catches up with players quicker in this sport than any other. Couple that with the influx of young talent coming into the league every season and it’s no surprise that veteran players are constantly on the bubble.
One position especially susceptible to being on the bubble would be the running back spot. The life span of NFL running backs is lower than any other position, and these guys hardly ever seem to get big second contracts, unless they happen to be a franchise player.
Teams seem to operate under the assumption that after four-or-five seasons a runner is too beat up, and can be easily replaced with younger (and less expensive) guys who also have much more tread left on their tires.