When the Bears went out and signed free-agent running back Michael Bush, it was believed they'd eventually cut backup running back Marion Barber. Instead, Barber retired on Friday, the Bears announced.

Barber, 28, rushed for 4,780 yards over seven seasons with the Cowboys and Bears, and scored 53 touchdowns in just 99 games played.

“I want to thank everyone who gave me the opportunity to play, and I’m very thankful to have had the chance to suit up for two of the NFL’s most storied organizations,” Barber said. "I want to thank everyone who helped me become a better player. I owe a lot to a lot of coaches, and am also very grateful to the owners and organizations I played for.

"Last but not least, I want to thank the fans for the support and inspiration they gave me."

Barber was always a big-time goal-line back who never really broke out the way people thought he might; he's ranked 53rd in rushing touchdowns in NFL history but 132nd in rushing yards in NFL history.

The lasting images of Barber, unfortunately, will be his run out of bounds in regulation and fumble in overtime in 2011 against the Broncos that set up a comeback win for Tim Tebow.

A more important reminder that Barber gives as he walks away from the NFL is the shelf life of running backs in professional football: at the age of 28, he had nothing left in the tank.