Are Portis' best days behind him?

Lying pressed against the artificial turf inside the Georgia Dome, briefly unconscious and later unable to remember what happened, Clinton Portis looked like he smacked into a wall.

Maybe he already had.

It's a dirty job, NFL running backs. They get beat up and worn out, and that's just the physical part. Doubt and hesitation tug at the psyche. Diminishing returns arrive faster than at other positions.

Portis' career workload raises the question of whether that is now happening to him. He is, after all, in his sixth season with the Washington Redskins and his eighth as a professional.

Portis, who suffered a concussion against Atlanta two weeks ago and missed Sunday's win over Denver, has been working behind a patchwork offensive line.

Still, his typical quickness and burst seem to be missing. After rushing for 1,487 yards last season (fourth highest in the league) on 342 carries (third highest), he looks like a different runner.

Portis is averaging a modest 4.0 yards a carry (the league average is 4.3), and that number is skewed by a 78-yard run against Philadelphia. More than 47 percent of his runs have netted 2 yards or fewer. Meanwhile, his replacement, Ladell Betts, gained 184 yards for a 4.5 average in the past two games. Betts appears to be the more decisive, confident runner.

Redskins coach Jim Zorn attributes Portis' flat performance to a lack of consistent practice time because of nagging injuries. He made a point Monday of saying Portis will regain his starting job once he's cleared to play and said Portis remains an effective runner.

But a decline would not be unprecedented or unexpected because Portis' durability and ruggedness might be catching up to him. Going into the season, only four active runners had more career carries. All of them - Edgerrin James, LaDainian Tomlinson, Fred Taylor and Jamal Lewis - have seen better days.

At 28, Portis is the youngest of the group. But the odometer says he is older. He has been a workhorse throughout his career, especially after the Redskins traded All-Pro cornerback Champ Bailey to Denver in 2004 to get the fast, tough, durable running back required by new coach Joe Gibbs.

The No. 1 rusher in franchise history, John Riggins, played nine seasons for the Redskins. Portis has played fewer than six, yet he is second by only 880 yards. Portis averaged 293 carries during his first seven seasons and has four years of 300 or more carries in the last five. All running backs are different, but that many rushing attempts over a period of time - in addition to blocking and receiving - often are followed by a sharp, immediate decline, regardless of age.

James averaged 317 carries from 1999 through 2007. Now he is looking for work, cut by Seattle a few weeks ago. Cincinnati's Rudi Johnson was essentially finished after three straight 300-carry seasons. For Stephen Davis, averaging 301 carries over four seasons apparently was too much. Terry Allen, Jerome Bettis, Ahman Green, Corey Dillon and Shaun Alexander among others also dropped off considerably after several seasons of concentrated work.

"[Portis] is probably trending downward, but it's impossible to evaluate him based on his numbers," ESPN analyst and former quarterback Trent Dilfer said. "That is probably the second-worst offensive line in the league. Maybe the worst. Yes, he's trending downward, but he's not [finished] yet. ... Some players are just flat done because of the cumulative effect of carries and overuse. Other players, that time is near, but it is not now [for Portis]."

If not now, then when? Competing in the most violent of sports, NFL running backs probably absorb the most punishment and traditionally have had the shortest career expectancy. Some runners have been compared to "bowling balls," but according to former Redskins great Larry Brown, it's really the other way around.

"It's like you roll the ball down the alley and the pins essentially are the running backs," said Brown, whose running career was cut short by injuries and general wear and tear. "They're the ones that get knocked down every time."

Brown led the league in rushing in 1970. Two years later, he gained even more yards and carried the Redskins to their first Super Bowl. He retired at 29 in 1976 after eight seasons, a shadow of the 5-foot-11, 195-pound player whom coach George Allen used again and again and still again to implement his ball-control philosophy.

"I noticed a vast difference in what I couldn't do and what I used to be able to do," Brown recalled. "I got to the point where, when the ball was handed off to me, mentally I was in and through the hole and downfield, but my body wasn't there."

Portis, who has not spoken to reporters in nearly two weeks, predicted before the season he had "five great years" remaining. But Dilfer said no player would acknowledge any sort of decline while still competing. "I know I didn't," he said.

In addition to James, the players ahead of Portis in carries have not fared well lately. Despite a big game Sunday, Tomlinson's 3.3 yards a carry is a career low. Taylor, after a career-low 42.3 yards a game with Jacksonville, is nursing an injured ankle with New England. Playing for the miserable Cleveland Browns, Lewis is on pace for a career low in yards a game and plans to retire after the season.

Assorted leg injuries this year have bothered Portis, listed at 5-11 and 221-pounds. But pain is a way of life in the NFL, especially for running backs.

"It's like you got run over by a Mack truck because every part of your body is sore, from the crown of your head to the bottom of your feet," former Houston/Tennessee running back Eddie George said.

Remarkably durable over his nine-year career, George started to slip after year 5, when he had 403 carries, his fifth straight 300-plus season.

"Unless you put on the pads and experience 30-carry games, not just one game but numerous times, you wouldn't know what your body goes through and what you have to do each and every week," he said.

In 2003, 13 players had at least 300 carries. It was 10 in 2006 and five last year. Dividing the work among running backs is a growing NFL trend, and Dilfer said the Redskins should do the same with Portis and Betts.

"[Portis is] a split-carry back, and that's exactly what Clinton needs to be to maximize his career the next two or three years," he said.

Zorn last week played down the idea. He said Betts is not a "change of pace" from Portis like some others in a two- or three-back system.

On his weekly ESPN 980 radio appearance Tuesday, Portis said he deserves to remain the starter but does oppose the idea of sharing the position.

"[It] would be even better," he said. "I'm not a selfish guy. 'C'mon, give me the ball, give me the ball, give me the ball.' If Ladell is gonna help us win, it don't matter to me. I'm all for it."

In his 2005 MVP season, Shaun Alexander led the league with 1,880 yards and 370 carries as Seattle went to Super Bowl XL. Like Portis, it was his fourth 300-plus-carry season in five. The next year, Alexander ran for nearly 1,000 fewer yards, and his yards a carry dipped from 5.1 to 3.6. His production declined even more in 2007. He played briefly for the Redskins last year and is now out of football.

Zorn, then Seattle's quarterbacks coach, said Alexander suffered from a weakened offensive line. Dilfer, who played with Alexander from 2001 through 2004, saw something else.

"What happened was he stopped trusting his instincts because he started doubting his abilities," he said. "He was so instinctive as a runner. He could turn nothing into something and know when to turn it on and turn it off. Then, all of a sudden, it looks like he's questioning his instincts and thinking too much."