Matt Birk no doubt is polite when he encounters fans who wonder why he's on injured reserve for the 2005 season because of lingering repercussions from his sports hernias. After all, they were up and around just a week after their hernias -- note the absence of the word sports -- were repaired.

"I want to ask them, ‘What do you do for a living, sell insurance?’ ” Birk says, laughing.

Instead, the Vikings' Pro Bowl center smiles and heads back to rehab the torn left hip labrum on which he had surgery in the summer. He also is working on the right hip labrum, which was repaired in May. And on the sports hernias that plagued him last season and required three operations. That's five surgeries, all because of an injury that largely was absent from the sporting vernacular until it was revealed last month that Eagles quarterback Donovan McNabb has one.

Everybody knows about torn knee ligaments and rotator cuffs. But the sports hernia, or athletic pubalgia, as it is known clinically, is far less understood than it is widespread. And it's painful. "It's no joke," Birk says. "It's at your core, your power source. Everything comes from the center. If something's wrong there, you can't do anything."

Actually, "sports hernia" is a bit of a misnomer. With a typical hernia, there is a bulge resulting from the protrusion of an internal organ. But a sports hernia involves a partial or complete tear of muscle from the pubic bone.

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The two pubic bones are the hub of a network of muscles, tendons and tissues at the base of the pelvis. They have important roles in daily life, aiding in bending and twisting. The fusion of the two bones is a joint that "forms a fulcrum around which everything rotates," says Dr. William Meyers, chief of surgery at Drexel University and one of the nation's foremost experts on sports hernias. He's the physician treating McNabb.

When a tear occurs, there is considerable pain and a limitation of mobility and flexibility. As a result, the body begins to protect itself. Other muscles tighten, the better to assume the burden. That's why Birk's labrums tore. "You get pain from the injury itself and pain in other locations as the body tries to compensate," Meyers says. How much pain? That depends on the extent of the injury. Former NFL lineman Brian Baldinger suffered a full tear in the mid-1990s. "The pain is like a serrated knife going up through the pubic bone and into the lower stomach," he says. Ouch.

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The onset isn't nearly that painful. When Birk was first injured, during training camp in 2004, he had trouble exploding out of his stance after the snap. He was diagnosed with a sports hernia and underwent surgery. A mesh screen was attached to the muscle and bone to help bond the areas together. He returned to action at the start of the season but started noticing similar pain on the other side and tried to play through it, with mixed results.

"When you snap the ball, you have to roll your hips and push against the guy in front of you," he says. "I couldn't do that. I didn't have any pop and couldn't move laterally."

Eventually, Birk had surgery on the other side and missed four games. But that didn't solve the problem. When the season ended, he had another operation to repair both sides. Then came the labrum repairs. Former Vikings running back Robert Smith experienced similar misery in 1999 before undergoing surgery. "I couldn't lift my legs to full height or get into full stride," he says. "It would start to pull and be extremely painful."

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What does all this mean for McNabb? Meyers won't discuss McNabb's situation, but it's apparent his tear is not complete because he continues to function at a high level. But the injury could get worse, like a rope that frays until it breaks. Rest, however, is not an option. "Resting gives you a lack of tone in other muscles, which you need to compensate for the injury," he says.

Ultimately, McNabb's injury will require surgery. Meyers, who uses a series of sutures rather than the mesh, insists the outcome will be positive, no matter the degree of the injury. But surgery by Meyers probably would keep McNabb out at least six weeks, so he'll try to tough it out. He may take anti-inflammatory IVs or some pain blocks, as others have. But they help only with the pain, not the stability of the muscle.

This is not a doomsday prophecy, but McNabb's sports hernia probably will get worse. "It takes a toll, physically, mentally and emotionally," Birk says. "It beats you down."

That makes it hard to do anything -- even sell insurance.