WALKING AWAY.

On a Tuesday afternoon a few weeks ago, the New York Giants running back Tiki Barber shuffled to the door of his apartment, on East Sixty-ninth Street, and apologized for the mess inside. His two-year-old son, Chason, had a playdate, and Legos, wooden horses, and used tissues were all over the floor. Tuesday is a professional football player's day off, and Barber had just woken up from a nap. He was wearing a hooded gray sweatshirt, gray sweatpants, and thick wool socks, and walked without seeming to lift his feet. He rubbed the top of his head a lot, like someone who'd just bumped into a street sign.

Pain tolerance is an essential requirement of his profession. "You get hit in ways that you can't even make up," he said. "It doesn't look bad. It just looks routine. But you'll get up and you'll say, 'God, I feel like my ribs are broken.' And that's how it was for me this week against Dallas." The Giants had just lost their fourth game in a row, to the Cowboys, and stood at 6–6, with their playoff chances in jeopardy. "We're in the red zone and I take a shot in my side: 'Ughh--that's going to kill me tomorrow.' And I knew it. I woke up yesterday morning and I couldn't even sit up." He was not exactly sitting up now. He had shut himself off in the den, to block out the noise made by Chason and his friend, who were attacking toy guitars. He was stretched out on a brown sofa, with one hand behind his head and the other arm folded across his midsection. He is, among his peers, small: about five feet nine and two hundred and five pounds. He has a shaved head, thick eyebrows, a boxer's jaw, not much of a neck, and a soft, nasal voice that makes him sound as though he had a perpetual cold, which enhanced, that day, the over-all effect of a kid staying home sick from school. "It's like someone's hitting you with a baseball bat," he continued. "That's really what it is, with the helmets and the pads. The way that our equipment has improved over the years, guys are getting less and less scared of contact." He paused, unhappy with the analogy. "It's hard to describe, because there's nothing else like it. Um, I'm trying to think. I'm sure there are things--like, if you fall off a ladder, that's probably what it would feel like. Or get in a car accident."

Barber is thirty-one, and several weeks earlier he had revealed that he planned to retire at the end of the season, his tenth. He was already, by a large measure, the most accomplished offensive player in the history of the Giants. He was also, at the moment, second in his conference in rushing, and had amassed more total yards in the preceding three seasons than any player in the league. He was quitting at the top of his game. When the news broke, David Letterman invited Barber on his show, and, speaking on behalf of puzzled Giants fans--football fans--everywhere, he said, "Before we get to this supposed retirement . . . are you really going to retire?"

Barber felt that he'd been fortunate to last as long as he had without suffering any injuries that would be considered major by football standards. (A fractured ulna in his left arm, late in the 2000 season, did not prevent him from playing in the playoffs.) An N.F.L. career usually lasts about three or four years. Running backs, who take more hard hits than players at any other position--as many as two dozen a game, and often from multiple bodies, and multiple angles, simultaneously--typically fare less well. On average, their bodies give out, or they're unceremoniously cut, after just two and a half years.

"You see this big-ass scar?" Barber said, pulling the leg of his sweats up and exposing a six-inch seam across his left kneecap. "I got that from riding my bike when I was twelve. And it's funny, because at the N.F.L. Combine they go over us with a fine-tooth comb, like they're inspecting a precious piece of equipment. They would see this scar and they would think I had surgery."

He almost had surgery once, during his rookie season, when he tore the posterior cruciate ligament, or P.C.L., in his right knee. ("That didn't hurt--it just felt weird.") One of the team doctors recommended an operation, but Barber consulted a number of veterans, and many of them advised against it: surgery, they said, only begets more surgeries; better not to start so soon. (Jim Otto, a center with the Oakland Raiders in the nineteen-sixties and seventies, has had forty-eight knee operations.) Barber built up the surrounding muscles to compensate for the damaged ligament, but he still can't lock his right knee, and standing up straight wears him out. His natural stance involves crossing his right leg in front of his left, and placing his hands on his hips for balance, like a catalogue model. Barry Word, a former Kansas City Chiefs running back, recently sent Barber an e-mail saying, "It gets worse." Word, who is forty-two, played only six seasons and retired in 1994. "My knees, my feet, my shoulders, my wrists--I ache," he told me. "Sometimes I have a difficult time walking." Like nearly forty per cent of all retired football players, he has degenerative arthritis.

"The short-term zeal and passion that comes from being an athlete, it's immeasurable," Barber said, with only halfhearted conviction. "Sure, you're mortgaging your future health to do it, but I think most guys will tell you it was worth it. And I'm not talking monetarily--because for a lot of those old guys it certainly wasn't worth it monetarily--but I think just the chance to be a hero. Football is America's game. Everybody says baseball is. But, nah, football is. Football defines a day of the week for six months of the year."