It’s not exactly news anymore to say that everything that possibly could go wrong has done just that for the Jets. Many of Gang Green’s problems were unexpected, like injuries to four different quarterbacks and a sudden loss of explosiveness for prodigal son Laveranues Coles. But one problem should have surprised nobody: the decline of 32-year-old running back Curtis Martin.

Last year, Martin won the NFL rushing crown with 1,697 yards on 371 carries, establishing a career high with 4.6 yards per carry. A year later, he’s been one of the league’s worst backs,with just 706 yards and a miserable 3.4 yards per carry. When Martin turned in a 91-yard performance against New Orleans on Sunday, it was just the second time all year that he gained more than 80 yards on the ground in a game.

The loss of center Kevin Mawae and general problems on the offensive line certainly plays a part in Martin’s collapse. So does the parade of backup quarterbacks that allows defenses to focus on stopping the running game. But Martin struggled just as much when Chad Pennington was still under center early in the season, averaging just 65 yards in those three games with less than three yards per carry.

Still, as any student of the game will tell you, Martin’s collapse is mostly the result of his age. At 31, Martin was the oldest player in the modern era to lead the league in rushing, and with the endof-season accolades came the warnings from the punditry that the Jets had better not rest their hopes in 2005 on another league-leading performance from their star running back.They knew a top season from an older running back is a rare occurrence in the NFL. Only 19 times since 1978 has a running back in his 30s appeared among the NFL’s top 10 rushers.

NFL running backs generally begin to decline around age 28, as years of pounding from defenders begin to take their toll. Speed deteriorates, loss of agility makes it harder to avoid tackles, injuries become more common, and recovery becomes more difficult.

Last year, it seemed as if the normal effects of age had taken a holiday. In the past 20 years, only six players have set a career high with at least 1,250 rushing yards at age 29 or older, and half of these seasons came last year: Martin, the Giants’ Tiki Barber, and New England’s Corey Dillon.

But NFL aging patterns have come calling, and Martin is not the only player affected. Last year, Dillon ran for 1,635 yards in the regular season, but carried the ball a whopping 410 times if the postseason is included.This year, at 31, Dillon has missed four games due to injury.When he has played,he’s been ineffective, posting a career-low 3.5 yards per carry and just one game of more than 75 yards.

At age 31, Carolina’s Stephen Davis, who missed most of last season with knee injuries, returned as Carolina’s starter this season and has averaged a career-low 3.0 yards per carry. 29-year-old Fred Taylor of Jacksonville has battled knee problems all year, missing three games, and is averaging a careerlow 4.2 yards in 2005. 32-year-old Priest Holmes was averaging a career-low 3.8 yards per carry before a midseason injury knocked him out of action for a second straight season. This was no simple injury, either; a concussion led to the discovery of a spinal condition that is likely career-ending.

The one older back who hasn’t struggled this year is Big Blue’s own Barber. It’s likely that Barber’s body has less wear and tear than the average running back his age, mostly because he spent his early years as a part-time player. Barber carried the ball just 250 times in his first three seasons, and did not register a 300-carry year until his sixth NFL season. Another 30-year-old back having a big year, Atlanta’s Warrick Dunn, has followed a similar career path.

Nonetheless, the Giants do have to consider the possibility that Barber is going to be hit by the same aging trends that have felled Martin and Dillon, especially since Barber is called on to do so much for the Giants’ offense. The possibility that this will be Barber’s last year as an elite running back brings new urgency to the Giants’ surprising Super Bowl run, not to mention added pressure on Eli Manning to make defenses respect his abilities as a passer.

The magical barrier of 30 also explains why the league’s top two rushers, Seattle’s Shaun Alexander of Seattle and Indianapolis’s Edgerrin James, could become free agents after the season has ended. Each player was a free agent at the end of 2004, but neither team wanted to sign its star to a big-money contract. Instead, both teams used the “franchise” tag, retaining the players for one season at a salary that matches the average of the league’s top five running back salaries. The Seahawks, to prevent a holdout by Alexander, later agreed to a contract that equaled the franchise tag salary but blocked the team from giving Alexander the franchise tag again in 2006.

The Colts and Seahawks have the two best records in the league, and Alexander and James are major reasons why. Alexander is 28, James 27, so both players are squarely in the midst of their primes. But given the workload each has faced over the last few seasons – both are on pace to go over 310 carries for the third straight season – it isn’t hard to imagine that an expensive contract will result in one, maybe two years of strong performance, followed by a sudden loss of effectiveness to match what Jets fans have seen in 2005.

Mr. Schatz is the editor in chief of FootballOutsiders.com.