We live in the era of the disposable NFL running back. Tailbacks have never had less value in the NFL than they do now. Teams have never divvied fewer carries up between more backs. The few who do get a traditional workload never seem to consistently dominate. The few who dominate always hit a wall.

No position in any sport is exempt from Father Time. Age always takes its toll on speed, strength, flexibility and durability. Experience and technique can make up for these losses, but gradually all athletes' skills will erode, and their dominance will fade.

Even the NFL running backs fast, strong and skilled enough to win perennial starting roles for themselves always seem to lose traction earlier than most athletes.

Why?

The Big 2-8 Wall

When former San Diego Chargers and New York Jets superstar running back LaDainian Tomlinson retired, he was just admitting what fans had known for three seasons: He couldn't make an impact in the NFL anymore.

Tomlinson, at age 27, rushed for 1,815 yards on 348 carries; that's an astounding rate of 5.2 yards per carry. He scored a massive 28 rushing touchdowns, setting an all-time record. Incredibly, he added 508 receiving yards on 56 pass receptions and another three scores.

The next season, after Tomlinson turned 28, he still led the NFL in rushing yards and rushing touchdowns, but by far smaller margins. He rushed for 1,474 yards on 315 carries (a still-impressive 4.7 yards per carry) and 15 touchdowns. Nothing wrong with any of that, but he averaged half a yard less with every single carry and scored just over half as many touchdowns as the year before.

The season after Tomlinson turned 29, he rushed for 1,110 yards on 292 carries. That's only 3.8 yards per carry. He averaged nearly a full yard less on each carry than the year before, and 1.4 yards less than two years before. In two seasons, Tomlinson's performance level went from "maybe the best ever" to "probably the best in the NFL right now," to "pretty solid."

Tomlinson played two seasons as a starter after his 30th birthday. In those two seasons combined, he gained 1,644 yards (at 3.72 YPC) and scored 18 touchdowns. Before his 30th birthday, Tomlinson rushed for more yards than that in a single season three times and scored more rushing touchdowns twice.

So what happened?

Back in the year 2000, Doug Driner (then of Pro-Football-Reference.com) did a little data mining. For all running backs in the NFL from 1998 on, Diner looked at what percentage of running backs tended to improve statistically from one year to the next. He found that backs mostly improve up to age 27—as many improve as decline at age 27—and from age 28 on, they mostly decline—exactly what happened to Tomlinson.

The Curse of 370

In 2004, Aaron Schatz of Football Outsiders examined whether Ricky Williams was likely to bounce back from his disappointing 2003 season, surprise retirement notwithstanding. An analysis of conventional stats, as well as FO's own DVOA mark, led Schatz to formulate the following hypothesis:

A running back with 370 or more carries during the regular season will usually suffer either a major injury or loss of effectiveness the following year, unless he is named Eric Dickerson.

The anecdotal evidence is astonishing. Here's Schatz's compendium of all the running backs with 370-plus carry seasons, and the sticky end that befell all of them (save Dickerson and Tomlinson):

Though Tomlinson ultimately dodged the curse, he was only 21 when he carried the fateful load. Jamal Lewis' "Uh-oh" was appropriate indeed: Lewis not only got busted on federal drug charges and had to serve four offseason months in prison, he also missed four games, saw his average yards-per-carry rate drop a full yard (5.3 to 4.3), and saw both his yards (2,066 to 1,006) and touchdowns (14 to 7) cut in half.

Schatz's hypothesis was met with both interest and skepticism, especially by fantasy football players trying to avoid the dreaded "bust." Schatz penned a followup in 2006, which considered both receptions and postseason carries.

As it turns out, "Curse of 370" is even stronger if you rename it "The Curse of 390" and add in postseason workloads. Schatz said:

On average, running backs with 300 to 369 carries who do not play in the postseason will see total yards drop by 15 percent the following year, and yards per carry by just two percent. But the 14 players listed above [with 370 to 389 carries] averaged a 27 percent drop in total yards, and a 10 percent drop in yards per carry.

 All players with 390 or more carries, no matter how these carries were split between the regular season and the postseason, averaged a 33 percent drop in total yards, and an 11 percent drop in yards per carry.

Tristan H. Cockcroft of ESPN.com took it a step further: He looked at the dreaded curse-engaging season as not just the catalyst for an immediate implosion, but as "the wall" that running backs hit.

Look at the average values for missed games, rushing yards per game, rushing yards per carry, scrimmage yards per carry, touchdown rate (extrapolated to 16 games) and fantasy points per game. The trend is crystal clear: a huge drop-off after the 370-plus carry season, then a long steady decline for the next five seasons:

Or, There is No Wall or Curse

Schatz himself noted that the "Curse of 370" is, of course, not powered by any metaphysical force. In his 2006 revisit, he wrote:

Just so people understand, there's nothing magical about carry number 370 that makes a running back blow out his ACL, any more than there is something special about pitch 100 that makes a pitcher's arm fall off. It's simply a useful shorthand to represent the fact that overworking your running back with too many carries is a bad thing. The punishment gets worse and worse with more carries, and 370 is a close approximation of the tipping point.

Even with that caveat, and the extensive work Schatz and others have done, there's been some criticism of the "Curse of 370," none of which was more pointed, eloquent or exhaustively researched than by Brian Burke of Advanced NFL Stats for this article.

Burke pointed out that statistically speaking, backs with 370-plus carries don't suffer any higher injury rate than any other running back; they just dodged the injury bullet for 16 games. Backs take a ridiculous amount of punishment, so getting through an entire season unscathed means it's unlikely they'll do it again the next year.

Then Burke pointed out that 370-plus carry seasons are likely to be career years with everything clicking and working in favor of the running backs. Their performance, their health, the offensive line and the schedule all contribute to that magical season; it makes perfect sense that everything is unlikely to fall in place like that again.

Statistically, Burke found that running backs who hit their career peak in per-carry effectiveness very often suffer a steep decline the next season, regardless of number of carries.

Finally, Burke plotted out the data and realized Football Outsiders probably didn't just pluck the 370 number at random. Several players who had just under 370 carries in a season had significantly stronger follow-up seasons, and the group of players with 374 or more carries in a season had a pretty even distribution between good follow-ups and bad follow-ups.

There's a tight cluster of 370-to-373 carry seasons with significant drop-offs the next year, so setting the cutoff at 370 looks very strong. But there's no football reason to set the cutoff there, just the data itself. The reality is, there's no real connection between 370 carries (or any form of "overuse") and steep decline in performance.

The decline will happen no matter what.

A Position of Fine Margins

The great thing about the running game is that there's very little variance. It's why risk-averse football coaches have leaned on the running game for years. Late, legendary Ohio State head coach Woody Hayes originated the classic maxim: "There's only three things that can happen on a pass, and two of them are bad."

Running plays are far less likely to gain zero or negative yards than pass plays, so even though they're less likely to gain huge yards than pass plays, football coaches love them.

The problem is when 32 NFL teams are trying to run it down each other's throat, most of them aren't going to be significantly better or worse than the rest. As Burke described, over the years NFL teams have consistently passed more to take advantage of the pass' higher offensive potential.

Here's a distribution of per-team, per-play rushing and passing averages:

You can clearly see most NFL teams are very tightly clustered around the NFL average of 4.3 yards per carry. The best rushing team in the NFL only averaged 1.1 more yards per carry, and the worst only 0.8 fewer yards per carry.

Meanwhile, the spread of teams' passing effectiveness is twice as large. The average is 6.3, but there aren't many teams close to that mark. The worst passing team in the NFL averaged just 4.2 net yards per pass attempt, but the best hauled in twice as much, with 8.4 NY/A.

If a player is not exceptional at running the ball, he's mediocre. That's why it seems like players "hit a wall." If age robs a step or two, the always-rolling injury dice come up snake eyes, or the natural roster or coach turnover of the NFL weakens the back's supporting cast or systems, even history's greatest runners suddenly become pedestrian.