Denver » When the Broncos rattled the NFL's sensibilities with a rare blockbuster, star-for-star trade in March 2004, Clinton Portis was a 22-year-old franchise-back-in-waiting, a player with nothing but sunshine over the horizon and two 1,500-yard rushing seasons already on his résumé.

Six seasons later, the Broncos will play Portis' team today, and Portis will not be in uniform. He has played much of this season on sore ankles and is recovering from a concussion.

Portis is now a 28-year-old running back facing one of the cruelest truths of professional football: The clock is ticking. And perhaps at no position on the field is life more about the here and now than in the backfield, because the vocational end comes quickly, and often with little warning.

"I think the data shows when you start closing on 30, and you're talking about the end of the life span of a running back, you're not going to be the same player that you were when you were the energetic, vibrant, fresh-legged rookie you were at 22, 23 years old," said former Titans and Oilers running back Eddie George. "That's just a fact. It's not like other positions that way.

"It just isn't and guys can do a lot of things to make themselves great players, but not many guys can beat time. Not as running backs, anyway."

As George says those words, he is one of the most well-conditioned students at Northwestern University's Kellogg School of Management. A ferocious inside runner who never missed a start in eight consecutive 300-carry seasons, George hit his 30th birthday as a 1,000-yard rusher but was retired by 31.

He was done in by the hits of a job description that required taking the ball and running into harm's way play after play, game after game.

"It's a tough job," said Broncos running back Correll Buckhalter, now 31 with three major knee surgeries and no 300-carry seasons in his career. "Maybe I'm a little biased. But basically you go into the pile with one hand on the ball, maybe two, with no real way to protect yourself other than putting your shoulder into somebody until you get out into space a little.

"And then you do it over and over again. No question, that adds up."

"A punishing job"

Heading into Week 10, there is just one running back older than 27 among the NFL's top 15 rushers: the Jets' Thomas Jones. And on that list are three 24-year-olds, a 23-year-old and two 22-year-olds.

Nowhere to be found, at least not until the No. 39 spot, is the Chargers' LaDainian Tomlinson. For most of eight seasons he was the league's pre-eminent offensive player -- seven 1,200-yard rushing seasons to go with seven years of 50 receptions.

In 2008, at 29 years old, he dealt with knee, toe and groin injuries, finishing with a career-low 1,110 yards on 3.8 yards per carry. In the offseason, the Chargers asked him to renegotiate his contract, to reduce his salary on the way to a reduced role in the offense, and he now splits carries with Darren Sproles.

Shift to passing lane

For running backs, 30 is usually the end of the line. In league history, just 25 running backs age 30 or older have rushed for 1,000 yards in a season and only three -- Emmitt Smith, Walter Payton and John Riggins, all Hall of Famers -- did it three times each.

And so it goes. It seems the birthdays come and go, and so do the running backs having them.

"These guys [on defense] are big and fast," Broncos running backs coach Bobby Turner said. "Some [teams] are using three backs. It's hard to keep those guys healthy. This is my 15th year here, and at one point we were fortunate enough to be going with the one back, but now we were one of the first teams that were using the two-back rotation -- and now it's just a given."

Added George: "I look at carries. You see guys with a lot of years with 300 carries back to back, like I had, those careers just aren't going to be as long as the guys who are consistently in the 200s, at 150.

"But if you run well, they're going to give you the ball more and more, just not for that long, I guess. And there you are, studying for your law final."