WHAT BJ Penn did to Joe Stevenson two months ago was bloody, brutal and - depending on how you view these things - either mercifully or disappointingly short. First, he dropped him to the canvas and pinned him to the floor. Next, he rained down blows with his fists on his grounded opponent's head. When Stevenson tried to resist, Penn responded with a withering look and a slashing elbow blow that produced an eruption of blood from the Californian's face.

Staggering on into the second round, Stevenson somehow survived another four minutes of Penn's relentless pounding. Strictly speaking, it was not the pulping of his features that brought the thing to a close, but the tightening of Penn's arm around his neck that forced the submission. Still, Penn did say afterwards that the blood had made it easier to slide his arm into position to complete the winning move.

Penn held his arms aloft as he celebrated his win before 8000 cheering spectators in Newcastle's Metro Radio Arena. Ringside, or rather cageside, he spotted Sean Sherk, his rival for the lightweight title. Sherk was commentating on the event while serving a ban for testing positive for steroids, but the two men are due to fight in May. "Sean Sherk," screamed Penn. "You're dead."

It was not what you would call a particularly helpful contribution to the argument that Mixed Martial Arts (MMA) should be considered a respectable sport. The US-based Ultimate Fighting Championship's (UFC) visit to the northeast of England may have thrilled a few thousand Geordies, but its bill-topping contest would have served only to reinforce the prejudices of the political, religious and medical figures who have dismissed cage fighting as the most grotesque and basest form of human barbarism.

And yet, it is one of the world's fastest-growing sports. In the age of the internet, YouTube and pay-per-view, and in the wake of the movie Fight Club, it has expanded rapidly across the globe over the past 15 years, finding particularly fertile ground in America, Japan and Brazil. A jostle of promoters and an alphabet soup of rival formats measure its massive commercial potential, but there is still something edgy and subversive about it all.

Which is hardly surprising when you consider that an MMA bout bears an obvious similarity to the kind of brawl that might spill down the steps of a city pub late on Saturday. The fight between Penn and Stevenson was unusually lurid in its own gruesome way - enthusiasts argue that drawing blood is rare - but its pattern was typical: tentative, jabbing preliminaries, a sudden fall to the floor, a period of grunting and grappling or a hail of fierce punches.

The origins of MMA lie in the confluence of traditional combat disciplines such as boxing and wrestling with such Far Eastern martial arts as judo, ju-jitsu and kick boxing. In Brazil, the all-in fight sport of vale tudo - from the Portuguese for anything goes - began to take shape in the 1920s, but modern MMA has only really evolved over the past 20 years. Enthusiasts call it an amalgamation of the best of many different fight forms; critics dismiss it as the worst form of blood-lust imaginable.

Certainly, it has far more respectable sporting credentials than the soap opera on steroids that is WWE wrestling, but its promoters are still not above drumming up interest with a little contrived menace. There is also a thick layer of theatricality around every event, with smoke machines, thumping music and apoplectic announcers hailing the gladiatorial entrances of the fighters.

It is, in short, an easy target for hand-wringers. When Cage Wars Productions, one of the UK's leading promoters, staged their first major event at Glasgow's Braehead Arena last year, there was fierce criticism from Sandra White, one of the city's SNP-list MSPs, and detective chief superintendent John Carnochan, head of Strathclyde Police's violence reduction unit. There was also a crowd of about 5000, a figure that is expected to be exceeded comfortably when Braehead hosts its second promotion next Saturday.

In the heart of Glasgow's Cowcaddens district, just off Garscube Road, the Griphouse Gym occupies the top floor of one of those solid Victorian four-storey buildings that the developers somehow forgot to tear down. Inside is surprisingly clean and airy, without the stale-sweat smells so characteristic of many fight gyms, but there is still a strong sense of purpose. Its main room is dominated by a boxing ring. Heavy bags hang from the high ceiling.

The bags are taking a pounding from the Griphouse regulars as Guy Ramsay, who set up the gym three years ago, looks on. Ramsay has a background in rugby and Thai boxing, but his premises have become the leading training ground for MMA fighters in Scotland. A graduate in sport science, he makes an eloquent case for considering MMA to be a valid form of sport. "The most important thing is that it tests you in every aspect of combat sport," he begins. "It's like the decathlon of all the fight sports. You need a bit of everything: wrestling, ju-jitsu, boxing, Thai boxing. You also need to be able to train and compete at a good level to be a complete fighter. The other big thing is that you simply cannot do this without discipline. The guys who come here are training two or three times a day, Monday to Sunday, doing long sessions. A day off is an absolute luxury.

"They need the skills, the conditioning, the sparring practice. They've got to keep hammering away at everything, moving between all the disciplines all the time. The guys here are absolutely the top athletes at the moment, and people from other sports who come in are always really impressed by what they see."

AND yet it is impossible to ignore the fact that MMA shows, particularly at the top level, are marketed in ways that emphasise raw, visceral aggression, not the skills or fitness of the participants. A cursory glance through MMA websites and magazines reveals a world of shaven-headed, grimacing combatants, camouflage gear and logos in Teutonic script.

Granted, one recent publication carried items extolling the value of vegetarianism and offered a recipe for turkey meat balls in tomato garlic sauce, but what is punted towards the audience is much more akin to red meat.

Ramsay shrugs. "I take the point on board," he says carefully. "But one of the things about MMA at the moment is that most of the people coming to watch still have an interest in the sport. I think you have to have that because, if you don't understand the principles of it all, especially when a fight hits the ground, then you'll be left wondering what's going on.

"The majority of the people who come to the events are fairly educated about the sport and what they're going to see. Yes, you will have a minority who are different, but I think it's up to the sport to try and educate its punters.

"Look at Scotland's obsession with football, and the problems there have been with football crowds for years. It's only now that the main clubs are trying to tackle that. It's unkind to a developing sport to expect it to be able to tackle the same issues without the resources that football has."

While many within the MMA world want to see it move into the mainstream, with clearer rules and regulations and an established competitive structure, Ramsay believes that it should be allowed more time to develop and evolve its own identity. The advantages of being within the tent of the sporting establishment might include access to lottery funding, but he fears it would also open the door to interference.

And it is easy to sense at least an element of mischievous enjoyment of the sport's pariah status. Paul McVeigh, the Irish bantamweight who is the Griphouse's most high-profile fighter - a status that may not be entirely unconnected to his surreally self-deprecating sense of humour - takes delight in debunking myths and demolishing preconceptions about the world of MMA.

"My girlfriend is a doctor," says the restless 25-year-old, who will travel to Japan soon to take part in a major tournament. "It's funny, because when you get a few doctors together all they ever talk about is medical stuff, so it's good to see their reaction when someone asks me what I do for a living. I say I'm a cage fighter, I get into a cage and fight people.

"I think we're all attention-seekers a little bit, me a huge bit, and it's cool to surprise people that way. For instance, there's no-one in this gym you would describe as a typical hard man. The last journalist who came here turned up when we were discussing the best place to get a cappuccino on Byres Road."

Like Ramsay, McVeigh has a degree in sport science and interests in psychology and nutrition. For both men, part of the appeal of MMA over traditional martial arts is that it offers the fundamental honesty of pure combat rather than a ritualised, sanitised and highly theoretical abstraction. It is what it is. If people like it, good. If not, tough.

"Those of us who are involved in it don't get hung up on the image," he shrugs. "I don't think we're barbaric. I'm too small to be scary."

Having gone through the experience of being thrown from a boxing ring, falling five feet and landing on his head, McVeigh can make the case, as most MMA participants do, that fighting in a cage is actually safer than the more traditional alternative. However, there is something suspiciously disingenuous about that argument, and he sees no reason why the sport should apologise for its presentation of itself.

"Cages are cool," he smiles. "That's the sport, that's its identity. I don't mind fighting in a ring, but cages are what we're about. Sandra White complains about strobe lighting and loud music, but Disney On Ice has strobe lighting, music and fighting and nobody talks about banning that."

If there is anything to the claim that MMA panders to the worst instincts and most bloodthirsty tastes of its audience, McVeigh counters with the argument that there is also an exemplary quality to the way he and other fighters live their lives: "People say that what we do would work really well on the street. I wouldn't know because I've never been in a streetfight.

"If you asked for my advice about self defence, I'd say don't be an arsehole, don't hang about with arseholes and don't stand outside kebab houses at three o'clock in the morning."

THERE IS still something quaintly homespun about MMA fighting today. Some estimates put the number of fighters in Britain at around 5000, but only the tiniest handful can make a living from the sport. If McVeigh's Japanese venture is successful he might be able to move into that elite, especially if it opens a door into the American market where professional careers are more accessible, but for the moment being a full-time fighter doesn't imply a full-time wage.

Critics of MMA might be a little less vehement if they appreciated that fighters are less concerned with breaking bones than breaking even. There is, to be frank, something slightly absurd about the overblown gladiatorial entrances made by blokes who would otherwise be swatting up for university exams and whose share of the purse is, as it will be for those on the Braehead undercard next weekend, critically dependent on how many tickets they have personally been able to sell.

But that's entertainment. "I suppose," says Ramsay carefully, "there is some tension between the marketing of the sport and what actually goes on."

As for the morality of it all, Ramsay remains adamant. "Ultimately, what makes it different is consent," he says emphatically. "Violence implies a lack of consent. What you have here is two mutually consenting adults sharing a challenge. They want it as hard as possible and if that means taking someone down and punching them in the face then that's what they accept."