NFL Running Backs: The Rapid Rise and Fall of Football's Most Physical Position

Three years. That is the average career of an NFL running back. To them, it might as well be three decades.

Every NFL team needs a good running back. Seeing as that is, the running back is heavily relied upon, and in return, they experience the biggest physical pounding.

Defensive lineman, sometimes a hundred pounds heavier, break through the line many-a-time to bury a running back, one of the smaller positions in football (save for the kickers and punters).

So it's no mystery why running backs, even the great one's, don't last in the league as long as other positions.

Consider the fact a few of the best quarterbacks of all-time, Dan Marino, Joe Montana, and John Elway, played a combine 48 years in the league, while great running backs such as Jim Brown, Barry Sanders, and Earl Campbell, only played a total of 27 seasons.

While it is one of the most important roles on the field, running back is also a hit-or-miss, hot-then-not, position on the gridiron.

Take Chris Johnson for example. His 130 yards this past weekend was what he was used to putting up his first three years in the league. But before the game, Johnson only had gained 366 yards in the first eight games.

While losing production out of nowhere is the name of the game for many running backs, the physical pounding leading to injury is the main cause.

Arian Foster has been outstanding, a year after running for over 1,600 yards. But injuries seem to already be adding up for Foster, and like every running back, there is uncertainty of even the near future.

But running backs can't worry about such things. If they do, they'll find an even quicker exit from the league. Some people have recently been criticizing Giants' running back Brandon Jacobs for his lack of bullish runs that he once demonstrated time after time.

Whether he has been holding back or not, the stats have shown that there is clearly something not the same as before with Jacobs. And even with having a lighter dose of carries throughout his career, there is the thought that Jacobs' career is already coming down the homestretch at 29.

It seems like the age 30 may be a number resembling a production wall to most running backs. Only one 30 year-old back could be found in the top 10 in rushing coming into last week (Willis McGahee), and he is the only 30-or-over any where near the top 25.

It's hard to see once-dominant backs crumble with age. LaDainian Tomlinson was likely the best one of the 2000's, and has lasted longer than expected.

But has eight straight seasons of over 1,000 yards came to an end in 2009. His age then? 30, the new 40 it seems when it comes to running backs.

Another fine example of how quickly a running back's career can take a turn for the worse is the situation in Kansas City in the past decade.

Priest Holmes was the main fixture of the Chiefs offense in the early 2000's, with three-straight All-Pro seasons. After three more years, Holmes was out of the league, due to the mounting injuries that he was plagued with.

The Chiefs simply moved on from Holmes, replacing him in the lineup with Larry Johnson. On cue, Johnson rushed for over 1,700 yards for two-straight seasons, including All-Pro honors in 2006.

After an astounding 37 touchdowns in those two seasons, Johnson has not found the end zone since 2008. What's to blame? you guessed it, injuries. With the 416 carries he had in '06, it's easy to see how he had so many.

So while Johnson has bounced around with five teams in the past four years, Jamaal Charles has established himself as the next solid Chiefs' back.

That is until a torn ACL left him out for the whole season, further showing the tough reality of the NFL, the "Not For Long" reality.

No one can be sure if Charles will make it back to full health. No one can be too sure about any running back for any period of time. So we ask how long Adrian Peterson will be the best runner in the game, or how long Maurice Jones-Drew will play far bigger than his size?

It's hard to tell what will happen, and how long they will last. But if one thing is for certain, there is nothing certain when it comes to an NFL running back.