Seven different NFL running backs have eclipsed 2000 rushing yards in a season including Minnesota's Adrian Peterson last year.

The previous six — including the Titans' Chris Johnson — never reached that milestone again leaving Peterson and Johnson as the only active players with a chance to make history by achieving the feat twice.

At the beginning of every season since his 2000-yard campaign of '09 Johnson receives the obligatory queries about whether he can do it again.

"Every year my goal is to rush for 2000 yards" the sixth-year back said matter-of-factly following Monday’s training camp session at Saint Thomas Sports Park."I already did it once" he added "so I really can’t go back and lower my standards to lower than that."

But has joining the likes of Peterson (2097 yards in 2012) Eric Dickerson (2105 in 1984) Jamal Lewis (2066 in 2003) Barry Sanders (2053 in 1997) Terrell Davis (2008 in 1998) and O.J. Simpson (2003 in 1973) become an albatross around Johnson's neck?

With his good but short of elite production from recent years some wonder if setting the 2K goal each season is unrealistic considering no back has ever done it twice.

"When he came into the league and he was running for five or six yards a carry everyone assumes that's how easy it is" Titans coach Mike Munchak said of Johnson. "But this league isn’t that way. He got his 2000. It just gets harder when you do that."

Certainly what Johnson has achieved in five seasons is remarkable including starting 77 of 79 games. As a 2008 first-rounder he has rushed for 6888 yards — second-best in the league for the period — while standing as only the sixth back in NFL history to gain that many yards in their initial five seasons.