"Mike Tomlin believes in the two-back system. He said as much shortly after the Steelers hired him in 2007. Then, he went out and used Willie Parker, as promised at Tomlin's first midseason, until the wheels came off.

Parker carried 321 times in Tomlin's first season and led the NFL in rushing right up until he broke his leg in the second to last game and never was the same. By 2010, he was out of football, his rushing yardage totals nearly cut in half from 1,316 to 791 to 389 in the following seasons.

Tomlin might still believe in the two-back system, but that it might only occur when two backs he trusts can give him what Parker and his successor, Rashard Mendenhall, have.

Coincidentally, like Parker in his final productive season, Mendenhall carried precisely 321 times last regular season. That represented 77.5 percent of all the running backs' carries. The rest were sprinkled among Isaac Redman, Mewelde Moore and Jonathan Dwyer.

That will not change, barring an injury to Mendenhall.

Mendenhall is the horse, and, while Redman will get some short-yardage calls, and he and Moore will see some third-down action, the Steelers will give the ball to Mendenhall and hope the wheels don't fall off.

"I was taught that a long time ago, I never worry about a running back getting tired," said running backs coach Kirby Wilson, echoing a sentiment Tomlin made last season. "Usually in a ballgame, they'll take themselves out if they get exhausted. Over a long term in their careers that's never been an issue with me."