TAMPA, Fla. (AP) Up close, Ryan Clark doesn't seem all that dangerous. Just short of 6 feet and missing a couple of internal organs, he's a bubbly kind of guy who seems more likely to talk an opposing player into submission than leave him motionless on the field.

Put a helmet on him, though, and things change quickly. That's when the Pittsburgh defender becomes a 205-pound, ball-seeking projectile speeding across the field with little regard for his own safety or that of the offending player in his sights.

"I try to be the hammer," Clark says. "I try to be the one doing the hitting, because if you're the guy sitting back waiting for the other guy to get to you, it's going to be trouble."

The hammer has been doing some pounding lately.

He leveled New England's Wes Welker in a December game. Welker left the field and didn't come back. Against Baltimore in the AFC championship game, he knocked Willis McGahee senseless in a frightening helmet-to-helmet hit that put him in the hospital overnight.

Boxing, as Mike Tyson was always fond of saying after knocking someone silly, is a hurt business. But so, too, is football, where big hits bring big celebrations and every player must deal silently with the thought that he is always just one play away from being strapped on a gurney and carted off the field.

"This is just football," said Arizona receiver Larry Fitzgerald, who will be squarely in Clark's sights in the Super Bowl. "This is a man's game, and I know that every time I go up for a pass there is a possibility that I could be knocked out, and I'm willing to take that risk because I love what I do, and you play for the love of the game."

The machismo is shared by fellow players and celebrated by their many fans. It's why Ben Roethlisberger can come back from a concussion in the last game of the regular season to start in the playoffs, and why Anquan Boldin needed only two weeks off after a vicious hit that required seven plates and over 40 screws to fix multiple fractures of his face.

Got to show your tough. Got to earn the paycheck.

Got to keep somebody else from taking your place.

"For me it was just a part of football," Boldin said later. "It's an unfortunate incident that happened but, you know, you take your bumps and bruises."

Bumps and bruises are one thing. Early death is another.

Reality intruded this week in the celebration of all things Super Bowl with some sobering news that should be posted on locker room bulletin boards around the NFL. It won't be, because the NFL would rather invite Janet Jackson back to do the halftime show than deal with the sometimes terrible physical effects of playing in a violent league.

Forget for a moment that doctors from Boston University's School of Medicine felt the need to use the national media spotlight on the big game to publicize its latest research. Concentrate instead on what they had to say, which has to be troubling to anyone who has ever strapped on a helmet and pads.

Repeated hits to the head aren't just causing damage on the field. They may be killing former players.

Researchers say they have found evidence of a condition called chronic traumatic encephalopathy in the brains of six former players who died at relatively young ages. The condition, which can bring on dementia in people in their 40s and 50s, is more commonly found in boxers who have taken too many blows to the head.

No one has connected all the dots to conclusively blame the condition on concussions and head blows suffered while playing, and there is debate among researchers on how significant the study is. That's because players have to be dead before their brains can be examined. The NFL is doing its own study of retired players, but results are not expected for at least another two years.

Former New England linebacker Ted Johnson, who claims repeated concussions gave him brain damage, said players like Roethlisberger and Steelers safety Troy Polamalu, who has suffered at least seven concussions, should think hard about their future health.

"I kind of liken it to NASCAR racers, who don't like going to funerals or don't like going to the hospital because they don't like being reminded what could happen to them," Johnson said. "It's the same thing with football players. We don't want to know what could potentially happen to us down the road."

Count Roethlisberger among those players.

"I don't go out there and ever worry about getting hurt or being hurt in the past," the quarterback said. "I'm playing this game and living this life to the fullest."

So is Clark, who in addition to almost knocking himself out in the hit on McGahee has some medical issues of his own. Only 29, he has a trait of sickle cell anemia, and had to have his spleen and gall bladder removed in 2007 after his left side began hurting in a game against Denver.

Concussions don't concern him. If he's worried about anything, it's that the NFL will begin to clamp down on the kind of hits he's become known for.

"If they do anything else, we're not going to be able to tackle people," Clark said. "I'd like to see them stop talking about it on TV so much. It gets so much press when you see things like (his hit on McGahee). People are beginning to believe it's a barbaric sport."

People may be right.

Tim Dahlberg is a national sports columnist for The Associated Press. Write to him at tdahlberg@ap.org

Copyright 2009 The Associated Press.