When the wheels come off RBs

Many older readers probably recall a day years ago when their Little League manager gravely warned about throwing curveballs and sliders at age 12, suggesting in somber tones that an arm "only has so many pitches" in it.

Workhorses like Nolan Ryan debunked that cautionary warning, of course, rendering it nothing more than an old and unsubstantiated sports wag's tale.

In recent seasons, however, NFL running backs seem to be at least partially validating the hackneyed alarm, albeit it in applying the adage to their lower extremities. It only seems the likes of former Detroit Lions star Barry Sanders and maybe even current New York Jets starter Thomas Jones can run forever. But it also appears that many of today's most reliable ball carriers are losing steam.

The wondrous Adrian Peterson of the Minnesota Vikings, who led the NFL in rushing in 2008 and is ranked second behind Cincinnati Bengals resurrected back Cedric Benson through seven weeks this season, certainly isn't buying into the notion that overworked often leads to under-producing.

Following Sunday's loss to the Pittsburgh Steelers, Peterson said: "Most [running backs] I know, the more they get the football, the happier they are, really."

Still, the anecdotal evidence suggests that runners with considerable workloads have problems maintaining a high level of performance. And it seems that the running back spot, which according to every NFL Players Association survey of the past two decades has the shortest career expectancy of any category in the entire league, is evolving as a disposable position.

Arguably more than at any other position, runners get "used up" by franchises, and then discarded when there is too much tread rubbed off their tires. Longevity at the position isn't much of a long suit anymore.

The lack of continuity at running back in the NFL is hardly a new problem. But in recent seasons, perhaps because of the decline of perennial Pro Bowl-caliber backs LaDainian Tomlinson last year and Clinton Portis in 2009, the situation has become more pronounced.

Nearly one-third (10 of 32) of the NFL's franchises have starting tailbacks in 2009 who are different from those who opened 2008. Only five backs who finished the 2008 season in the top 10 in rushing yards currently rank among the league's 10 most prolific runners. At the outset of the 2009 campaign, just seven tailbacks had started at least eight games for each of the three previous seasons with their current clubs.

"It's really not a position with a long shelf life," said former Pittsburgh tailback Jerome Bettis, who started 10 or more games in each of his first 11 NFL seasons, and then six games combined in his final two years. "They don't hand out a lot of gold watches [at running back], you know?"

The primary culprits for the high attrition rate at running back typically are age and workload.

There are only five starters at the position who are currently in their 30s or will turn 30 by the end of this season. The 32 starters average 25.19 years of age and 4.53 seasons of NFL experience (counting this year). Eleven of the starters are 24 or younger and a dozen three seasons' tenure or fewer.

But it's not just the age of a runner that can affect his performance. A player's workload, especially if it is significantly increased over a short time, can be a factor, too.

After averaging 57.0 carries in his four seasons with San Diego, and never logging more than 80 in a year, Michael Turner carried the ball 376 times in 2008, his first season with the Atlanta Falcons. Last year, Turner posted eight games of 100 or more yards, averaged 4.5 yards per carry, and led the NFL in rushes of 10 or more yards. Yet this season, despite playing behind an offensive line that returned intact, a healthy Turner has rushed for only 403 yards (a 16-game pace of 1,075 yards), and averaged 3.4 yards per attempt. He has one 100-yard game and seven runs of 10-plus yards.

The sixth-year veteran is only 27 years old and has been a starter for just one season prior to this one.

"I feel exactly the same [as 2008]," Turner said recently.

Turner isn't the only 27-year-old tailback who is not playing to expectations. Massive Brandon Jacobs has 40 more rushes (120-80) than Ahmad Bradshaw but only 9 more yards (464-455) than his understudy. Bradshaw, 23, is averaging 5.7 yards per carry to 3.9 for Jacobs, and there are many who feel the backup has been the more explosive player of the two. Jacobs has been so frustrated by his output that he claimed he might retire at season's end if it didn't improve.

"You've got to understand, you could lose [a step] in a heartbeat, from one day to the next," said Hall of Fame tailback Eric Dickerson. "You feel the same physically … but on the plays where you'd normally gain 3 or 4 yards, you're getting 1, or even being hit in the backfield. It might be a [cumulative] thing, but it can also sneak up on you all of a sudden."

Before his 30th birthday, Dickerson averaged 373.7 carries per year and 24.9 attempts per game. In seven seasons (1983-89), he posted 300-plus rushes five times, and went over 1,000 yards in all but one season, when an injury limited him to three appearances in 1987. After 30, Dickerson played all or part of four seasons, averaging 136.5 carries per year and 13.3 per game, and never gained more than 727 yards.

Tomlinson, 30, is experiencing a similar fate. In his first seven seasons, he started all but one game, averaging 332.1 rushes and 395.5 touches from scrimmage. He has missed two games this season, and is on pace for just 213.5 carries and 238 touches -- and Chargers mighty mite Darren Sproles seems to have supplanted him, not as the starter, but as the team's go-to back in critical situations.

It certainly seems as if Tomlinson's legs have exceeded their quota of carries.

Len Pasquarelli, a recipient of the Pro Football Hall of Fame's McCann Award for distinguished reporting, is a senior writer for ESPN.com.