Warrior is one of a number of well-equipped gyms teaching the centuries-old Siamese art of warfare that has sprung up in the city in the past few years. It has two boxing rings, three rows of punch bags and two large workout spaces.

In Causeway Bay, which hosts established gyms including Swish, Chok and Will Power, there's been a rumble in the concrete jungle this year. Offshoots of global boxing and mixed martial arts giants Everlast and Hayabusa have also entered the fray. Each now has two venues where members can slug it out in fight cages, scale climbing walls and tangle with grappling dummies. Although they offer training in mixed martial arts, Brazilian jiu-jitsu, krav maga and capoeira, Thai boxing is by far the most popular.

Warrior co-founder Billy Tam Po-kam attributes the sport's increasing following as a fitness choice to several factors: the growth of televised mixed martial arts, a long tradition of Thai boxing in Hong Kong and greater awareness about its culture thanks to the internet.

He reckons it's more popular than mixed martial arts because Hongkongers aren't so keen on close body contact. "People are still a bit scared of mixed martial arts, too, and associate it with hardcore cage fighting."

It also scares off many women, Tam adds. Although Muay Thai is no gentle pursuit, more than 60 per cent of Warrior's client base are women, he says. "Females tend to be harder working in terms of staying disciplined and training regularly."

Muay Thai offers a vigorous alternative to the tedium of the treadmill. It's a high-intensity cardiovascular workout that teaches the attack and defence aspects of martial arts. It also helps improve posture and coordination. Mental health benefits include stress relief, greater confidence and self-discipline.

Andy Lai Yin-hang, director of Hayabusa Martial Arts & Fitness Centre, describes the recent growth of Muay Thai in the city as "significant". He says it's popular because it is straightforward and has comprehensive skills.

"Many mixed martial arts lovers will go for Muay Thai training to enhance their performance," Lai says. "You can get fitness results quickly."

Hayabusa, which operates under licence from its US parent, opened its first gym in Central almost three years ago. In August the chain branched out into Causeway Bay because it sees further opportunities in the area of fitness, Lai says.

Thai boxing originated as a fighting technique for the battlefield. Its eight "weapons" are the fists, elbows, knees and shins. It became officially recognised as a sport in the early 20th century, when the ninth weapon - the head - was banned under the newly codified rules. Boxing gloves replaced rope binding around the fists for fights against foreigners.

The sport has not always had a positive image in Hong Kong; for a long time it was associated with the criminal underbelly. Triad bosses provided makeshift gyms for new members to hang out and learn to fight.

"All gyms, including Chinese martial arts and kung fu gyms, were triad hangouts in the past," says Mark Li Chun-wing, owner of Chok Muay Thai Gym and an instructor for more than 20 years. "They were called 'society gyms' in Cantonese. There were many rooftop and tong lau gyms … Many kids didn't even have to pay; they just showed up, trained and were pushed out to fight. Some of these still exist," Li says.

This image was reinforced in popular culture in the 1990s when triad movies were all the rage. By the early 2000s, however, the triad genre had lost its lustre and the fitness craze was gathering steam.

Muay Thai was starting to gain traction as a legitimate form of exercise. Becoming a fixture at some of the high-end conventional gyms, today it is offered by California Fitness and Pure Fitness, among others, and white-collar mixed martial arts specialist Impakt.

Li identifies three types of Muay Thai gyms catering to different enthusiasts. First, there's the old-school triad-run places. Then there's the more modern gyms such as Chok and Warrior, which focus on techniques, and have reasonable cardio and conditioning routines suitable for more types of followers.

"If there are students who are very passionate, we are also willing to nurture and invest in them to compete in tournaments," Li says.

The third type is those with a lot of financial backing that sell private training packages and offer a variety of disciplines beyond Muay Thai.

As for the enthusiasts, Warrior's Tam sees four categories. The largest is the general fitness crowd who work out once or twice a week with friends; then there are those who train for a few weeks then disappear for a while; hardcore training enthusiasts; and a minority looking for a fight.

Chalinene "Chali" Bassinah is in that minority. The finance worker has been Thai boxing for six years and entered her first tournament after just nine months. "It was one of the toughest things I'd ever done in my life. I never thought I would be doing this," she says.

Bassinah joined Warrior after moving to the city a year ago. Earlier this month, she took on world champion Valentina Shevchenko in Macau and lost on a close decision. At Kitec in August, she floored another rival in less than 30 seconds.

Bassinah says she enjoys Muay Thai for several reasons. "It's really cardio-intense, so you get a kind of endorphin rush similar to a runner's high. You can come every day and it's always going to be different.

"Also it's a social thing. You can never come to a Muay Thai class and not talk to anyone. I think fight training together builds a stronger bond. I've met people who I consider friends for life because we've been through hell together," she says.

The boom has also provided jobs for professional trainers, especially from Thailand. All the gyms boast champion fighters on their roster. At Warrior, seven of the eight instructors are Thai champions who earned their stripes at top Bangkok boxing stadiums such as Lumphini and Rachadamnoen.

All have 100 to 300 fights under their belt, and a success rate of up to 80 per cent, Tam says. They also have trainer certificates, which they need to secure a Hong Kong work visa.

Tanet "Fanta" Honchaipoom came here to work at Swish. Two years later, in 2010, he opened his own small gym in Causeway Bay, Fantasy Fitness & Muay Thai, where four of the trainers, including himself and a childhood friend, are from Thailand. Women also make up most of Fantasy's clients (70 per cent), Tanet says, adding that a similar trend is emerging in his home country.

"In Thailand, traditionally it's men who go to Muay Thai gyms, to learn to fight. But over the past three years it has also become popular for fitness. My friends tell me that even in Thailand it is very popular with women now," he says, adding that the sport's growing popularity is a worldwide trend.

Despite other gyms joining the fray, Tanet is not concerned about competition. "I have loyal customers. Some of them have been training with me since we opened," he says.

"I feel good that people like Muay Thai," he adds, noting that it's a worldwide trend.

Hayabusa's Lai believes there's still plenty of interest to sustain newcomers. "The trend is that people in Hong Kong are looking to do more exercise, and they are looking at Muay Thai for their training."

Li concedes new entrants are making it a more challenging market, especially factoring in rising rents. "But I never operated Chok as a business from the beginning. It's all about the passion."

Additional reporting by Ernest Kao

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