Before the season, Michigan running backs coach Fred Jackson made it clear that he was confident in each of his five running backs — junior Mike Shaw, sophomore Vincent Smith, freshman Stephen Hopkins, sophomore Mike Cox and redshirt freshman Fitz Toussaint.

Jackson went as far to say that none of those five ballcarriers would be the No. 1 guy.

“I think you’re going to see the whole season and not really know who’s who,” Jackson said. “Because now I’ve got guys attuned, that can do everything."

But through half the season Michigan’s leading rusher has been its quarterback — sophomore Denard Robinson — and the majority of the Wolverines’ playing time at running back has gone to Shaw and Smith. They, along with Robinson, have accounted for 1,701 of Michigan’s 1,971 total rushing yards.

Hopkins has received an increased amount of work in the past few weeks and could be getting into the Shaw-Smith rotation.

But Toussaint and Cox have been relatively absent from the field, and in his weekly teleconference on Wednesday, Michigan coach Rich Rodriguez addressed their development.

The most prevalent concern with both players seems to be health. Toussaint especially has fought a nagging injury all season long, worrying many that he could receive the dreaded “injury-prone” tag.

"He's a little bit better, but he has not progressed to the point where we feel we can put him in a game yet," Rodriguez said. "It's kind of thing that's going to need a little more time."

As for Cox, Rodriguez said a lot of his progress will be judged on how quickly he learns the playbook, an area where he trails the other running backs right now.

“His growth in the offense has been limited because he’s had a knee issue,” Rodriguez said. “He works hard and he’s got to continue to learn the offense, so we can have enough confidence that we can leave him in for all the plays and not just certain plays.”

In a rushing attack dominated by Robinson — the nation’s leading rusher — there doesn’t seem to be enough room in the backfield for six backs, including Robinson.

But in the Michigan offense, especially under Jackson, lack of diversity in ballcarriers isn’t that uncommon.

With the exception of the last two years — when former Wolverines Brandon Minor and Carlos Brown nearly split 50-50 — Jackson’s running backs haven’t often shared carries.

For the four years before that, Mike Hart dominated Michigan’s carries. Then, it was Chris Perry. Then, B.J. Askew. And finally, it was Anthony Thomas.

In fact, outside of the previous two years, the Wolverines’ No. 2 rusher hasn’t accounted for more than 25 percent of the team’s rush yards since Clarence Williams did it in 1998.

Of course, Jackson compared Hopkins to Perry, and the pair of Smith and Toussaint to Hart, Michigan’s last four-year rusher. So there’s no telling whom the Wolverines’ will use against Penn State, which boasts a fairly depleted defensive line and linebacking corps.

But with the dynamic Robinson also in the fold, the combination on the field may not matter on Saturday.