College football is no pushover, but adjusting from the rigors of the college game to the NFL is one of the toughest parts of becoming a pro. At the running back position this could not be more true, and there are a handful of rushers in the 2013 class that won't withstand the beating.
This year's running back class has some huge potential, with guys like Marcus Lattimore, Eddie Lacy, Giovani Bernard and Kenjon Barner, among other players who could become regular contributors before too long.
But there's also no shortage of players who come into the league with too much baggage that don't fit the mold of a NFL running back.
Let's take a look at the rushers who won't be able to make a name for themselves in the NFL due to their physical detriments.
Auburn Tigers running back Onterio McCalebb entered the 2012 season with a chance to be one of the SEC's leading rushers, but it didn't pan out quite like he'd hoped.
Instead, McCalebb finished with just over 500 rushing yards on the season.
At 5'10", 174 pounds, McCalebb possesses a skill set that certain rushers have been able to utilize in the NFL. But excelling at that size and stature is awfully rare, as you have to do it in a distinct way. It was once thought that he could be in that group, but he proved otherwise last season.
A major part of Auburn's hapless 2012 season, McCalebb doesn't have what it takes to excel at the next level.
As an avid college football spectator, I became a huge fan of Theo Riddick's ability to use his speed to get to the outside and bust out big gains. But he doesn't show much that stands out as a player who can thrive in the NFL.
Riddick boasts a physical presence that stands out on paper (5'11", 200 pounds), but he doesn't use his body as much as his legs. With a 4.52 40-yard dash (per CBS Sports), he's fast but won't be flying past world-class defenders very much.
Sure, Notre Dame played some solid defenses and Riddick had a couple of big games. But he wasn't what made that team undefeated heading into the final game of the season.
Riddick surprised in 2012 after going from having a small chance to even get drafted to blossoming, but you don't want to look too much into the stock of one season alone.
Temple running back Montel Harris likely won't be drafted until late as an expendable back to do dirty work such as blocking, and that's about the extent that his effectiveness will be in the NFL.
Harris doesn't have the burst that it takes to get through the line in the big league. He's quick and can elude defenders, but he won't sprint past them and won't run them over. That's a recipe for disaster.
In the NFL, you either need blistering speed or overpowering strength (or both) to become an effective rusher. Harris obviously doesn't have either.
Florida Gators running back Mike Gillislee is another one of those players with a stout physical frame (5'11", 207 pounds) who doesn't quite use it like he should.
Instead of using those 207 pounds to move the pile and break wimpy tacklers, Gillislee opts to use his legs to shift around defenders and dance past the line. Although it helped him in his breakout senior season in racking up yards, it won't translate to effectiveness in the NFL.
Gillislee has the physical tools to succeed, if only he can become more of a bruiser. But if he continues to play in the style he embraced in 2012, his body and health won't last for long.
Raymond Graham is one of the shortest running back draft prospects you'll see this year, standing at 5'9" and 190 pounds.
With his short stature, Graham's best strength is obviously running past defenders and being quick on his feet. But he's struggled in the past with a torn ACL and hasn't been the same after coming back from the injury.
Anybody who follows the NFL knows how common ACL injuries can be, especially if you come into the league with injury baggage. Quite frankly, that should be enough to convince teams to stay away from Graham until late in the draft.