... this year, Titans running back Chris Johnson has proved that his prolific rookie campaign wasn't a fluke.

And chances are, he'll be a force for years to come, regardless of whether his team is winning or not (see: this year).

But just how many years is the question, given the historically volatile nature of his position. 

Over the years, all have shown the signs.

From Franco Harris to Eric Dickerson; from Edgerrin James to Shawn Alexander, the age of thirty is kryptonite of sorts for running backs.

Holes become smaller as the once-quick first step wanes.  

Plays that would have once been easy touchdowns get snuffed in the backfield.      

Life is hard for star running backs in...

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