Gints Klimanis, a 37-year-old software engineer and martial arts instructor, started the invitation-only Gentlemen's Fight Club in Menlo Park in 2000 after his no-holds-barred sessions with a training partner grew to more than a dozen people. Most participants are men working in the high-tech industry.
"You get to be a superhero for a night," Klimanis said. "We have to go to work every day. We're constantly told to buy things we don't need, and just for a couple hours we have the freedom to do what we want to do."
The only protective equipment used is fencing and hockey masks. Several fighters have suffered broken noses, ribs and fingers.
Men involved in fight clubs often carry bottled-up violent impulses learned in childhood from video games, cartoons and movies, said Michael Messner, a University of Southern California sociology and gender studies professor.
"Boys have these warrior fantasies picked up from popular culture, and schools sort of force that out of them," he said. In these fantasies, "The good guys always resort to violence, and they always get the glory and the women."
There is also a sadomasochistic thread running through underground fight clubs, said Michael Kimmel, a sociology professor at Stony Brook University in New York.
"Real-life fight clubs are the male version of the girls who cut themselves," he said. "All day long these guys think they're the captains of the universe, technical wizards. They're brilliant but empty.
"They want to feel differently. They want to get hit, they want to feel something real."
Five-year fight club veteran Dinesh Prasad, 32, a heavily tattooed Santa Clara engineer, said he once broke a rib in a match but never complained to his fellow combatants. He also recently skipped his first wedding anniversary to attend a fight rather than drive to Los Angeles, where his wife is finishing law school.
"I came here to get over my fear of fighting, and it's working," he said. "I'm much tougher than I was five years ago. I'm not at the level of these other guys, but if things were to get tough, I can get tough, too."