"It's not as if Clinton Portis can extend his career with plastic surgery, a facelift like one of those actress-creatures who look like they sleep in crypts. There is no tricking up his sore legs with cosmetics. Time treats NFL running backs more cruelly than arguably any other professionals. How would you like to be "aging" at 29?

It's not over for Portis, who celebrated his 29th birthday Sept. 1, but it's getting there. A groin injury has forced him to cede his starting job to Ryan Torain, the 24-year-old with the hydraulic pumps for knees, and it may be difficult for him to get it back. Coach Mike Shanahan said Wednesday that Portis will be out four to six weeks.

But anyone who thinks Portis is easily replaced, who thinks this isn't a huge loss, is forgetting just how much he does for the Washington Redskins, not only as a runner but a self-sacrificing blocker. At the bottom of the conundrum that is Portis - the absurd charm, the apparent casualness, the uneven practice attendance - is one of their most physically fierce players.

NFL statistics suggest just how disposable backs are. Their average career span is just 2.5 years, and even the great ones seldom last a decade. In the 90-year history of the league, just 25 men over the age of 30 have run for 1,000 yards in a season. There is no mystery as to why. It's a simple matter of physics: mass times acceleration, and the damage those forces do to a body. Portis is in his ninth season. By my count that means he has taken somewhere around 6,500 hits. Scientific study suggests they're the most severe blows any player on the field suffers.

We often talk about the pounding that NFL running backs take, and recognize it viscerally, but we don't quantify it. A look at hard numbers explains why Portis has been banged up these last couple of years, and why he has broken fewer long runs than he used to. It also perhaps explains his famous reluctance to practice every day, which now seems more smart than lazy, given what researchers are learning about long-term health risks from repetitive blows.

A defensive back's speed, say 4.5 seconds in the 40-yard dash, combined with his mass can produce up to 1,600 pounds of tackling force. Recently scientists have begun attaching sensors to players' helmets and equipment, to measure the effects of such impacts on their bodies, especially concussions and repetitive brain injuries. "