CARTHAGE, Mo. – Ultimate fighting was once the sole domain of burly men who beat each other bloody in anything-goes brawls on pay-per-view TV.

But the sport often derided as “human cockfighting” is branching out.

The bare-knuckle fights are now attracting competitors as young as 6 whose parents treat the sport as casually as wrestling or Little League.

The changes were evident on a recent evening in southwest Missouri, where a team of several young boys and one girl grappled on gym mats in a converted garage.

Two members of the group called the “Garage Boys Fight Crew” touched their thin martial-arts gloves in a flash of sportsmanship before beginning a relentless exchange of sucker punches, body blows and swift kicks.

No blood was shed. And both competitors wore protective gear. But the bout reflected the decidedly younger face of ultimate fighting. The trend alarms medical experts and sports officials who worry that young bodies can’t withstand the pounding.

Tommy Bloomer, father of two of the “Garage Boys,” doesn’t understand the fuss.

“We’re not training them for dog fighting,” said Bloomer, a 34-year-old construction contractor. “As a parent, I’d much rather have my kids here learning how to defend themselves and getting positive reinforcement than out on the streets.”

Bloomer said the sport has evolved since the no-holds-barred days by adding weight classes to better match opponents and banning moves such as strikes to the back of the neck and head, groin kicking and head butting.

Missouri appears to be the only state in the nation that explicitly allows the youth fights. In many states, it is a misdemeanor for children to participate. A few states have no regulations.

Supporters of the sport acknowledge that allowing fights between kids sounds brutal at first. But they insist the competitions have plenty of safety rules.

“It looks violent until you realize this teaches discipline. One of the first rules they learn is that this is not for aggressive behavior outside (the ring),” said Larry Swinehart, a Joplin police officer and father of two boys and the lone girl in the garage group.

The sport, which is also known as mixed martial arts or cage fighting, has already spread far beyond cable television. Last month, CBS became the first of the Big Four television networks to announce a deal to broadcast primetime fights. The fights have attracted such a wide audience, they are threatening to surpass boxing as the nation’s most popular pugilistic sport.

Hand-to-hand combat is also popping up on the big screen. The film “Never Back Down,” described as “The Karate Kid” for the YouTube generation, has taken in almost $17 million in two weeks at the box office. Another current mixed martial arts movie, “Flash Point,” an import from Hong Kong, is in limited release.

Bloomer said the fights are no more dangerous or violent than youth wrestling. He watched as his sons, 11-year-old Skyler and 8-year-old Gage, locked arms and legs and wrestled to the ground with other kids in the garage in Carthage, about 135 miles south of Kansas City.

The 11 boys and one girl on the team range from 6 to 14 years old and are trained by Rudy Lindsey, a youth wrestling coach and a professional mixed martial arts heavyweight.

“The kids learn respect and how to defend themselves. It’s no more dangerous than any other sport and probably less so than some,” Lindsey said.

Lindsey said the children wear protective headgear, shin guards, groin protection and martial-arts gloves. They fight quick, two-minute bouts. Rules also prohibit any elbow blows and blows to the head when an opponent is on the ground.

“If they get in trouble or get bad grades, I’ll hear about it and they can’t come to training,” he added.

In most states, mixed martial arts is overseen by boxing commissions. In Missouri, the Office of Athletics regulates the professional fights but not the amateur events, which include the youth bouts. For amateurs, the regulation is done by sanctioning bodies that have to register with the athletics office.

The rules are different in Oklahoma, where unauthorized fights are generally a misdemeanor offense. The penalty is a maximum 30 days in jail and a fine up to $1,000.

Joe Miller, administrator of the Oklahoma Professional Boxing Commission, said youth fights are banned in his state, and he wants it to stay that way.

“There’s too much potential for damage to growing joints,” he said.

Miller said mixed martial arts uses a lot of arm and leg twisting to force opponents into submission. Those moves, he said, pressure joints in a way not found in sanctioned sports like youth boxing or wrestling.

But Nathan Orand, a martial arts trainer from Tulsa, Okla., said kids are capable of avoiding injuries, especially with watchful referees in the rings.

“I can see their point because when you say ‘cage fighting,’ that right there just sounds like kids shouldn’t be doing it,” Orand said.