Tampa, Fla ? 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 more than 40 screws to fix multiple fractures of his face.

Got to show you’re tough. Got to earn the paycheck.

Got to keep somebody else from taking your place.

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.