RUNNING ultra marathons that turn feet into blistered, bloody stumps. Nordic skiing in winter's brutal cold. Riding mountain bikes on rump-numbing trails across the grasslands of the Inner Mongolia Autonomous Region.

For many Chinese, these have long been things that only crazy foreigners do for fun. But that's not so true anymore. The number of Chinese competing in such adventure sporting events has been growing rapidly in recent years as the country's leisure-loving middle class continues to expand.

Many companies are lining up to get a bigger piece of the market, while others see the trend as a new way to promote their products.

When the organizers of the North Face 100 began accepting applications for China's first large-scale ultra-marathon last year, they thought that most runners would opt for the 9.98-kilometer fun-run option, says Julia Cui, director of sports events at Octagon, the marketing company that promoted the event.

Only 100 slots were available for runners who wanted to do the grueling 100-kilometer race, which started at the Great Wall and went through the Ming Tombs outside Beijing last April, Cui says.

The organizers were stunned when 300 people tried to sign up and had to be turned away from the race sponsored by American outdoor gear company The North Face. "We didn't realize that this would be so popular," Cui says.

Octagon has seen a big spike in interest in other endurance events it organizes in China, like the marathon in the eastern city of Hangzhou, Zhejiang Province.

"From 2006, there were only 5,000 people attending the race, but in 2009, there were 14,000," Cui says. "So the increase was huge."

For decades, those who did sports in China were mostly an elite few who were plucked out of school at young ages because they had the right physique or coordination for gymnastics, table tennis, diving or some other event. Most other Chinese spent their lives in the fields, factories or pushing paper. They were busy just struggling to make a living. Extra income was spent on food, clothing and other necessities - not US$90 trail running shoes that would get you through an ultra-marathon.

But that's changing as the ranks of China's new middle class continue to swell. More people have the time and money for recreation, and leisure sports serve as a good "filtering system" for companies who are trying to reach consumers with money to spend, says Chris Renner, president for China of sports marketing agency Helios Partners.

"If you do your consumer-based research, you'll find that amazingly, unlike the United States, when you see people participating, doing walking, running, badminton, swimming, they're all higher-level income," Renner says.

"They're higher level because they're the only ones who have leisure time. The 800 million people working in the fields don't have time for that and certainly the migrant workers don't have time for that."

Renner says the growth potential is enormous.

"We're almost starting from scratch, honestly," he says. "And I think there is an appetite as the virtuous cycle of more income, more education, more leisure time starts spinning itself."

The Economist Intelligence Unit said in a report last October that China's sports sponsorship market is worth about US$1.5 billion to US$2 billion - just a fraction of the global market of US$60 billion. Although the state still dominates sports, the report said, commercialization is taking root. But much more development at the grass-roots level is needed, it added.

Daimon Ling is among the new class of white-collar fitness fanatics. The 31-year-old deputy general manager at a records management company in the southern city of Guangzhou, Guangdong Province, says that traditionally his peers liked to spend their leisure time playing mahjong. But more of them are getting into mountain biking and cycling on the roads.

"Ten years ago, there were no Websites in China about cycling that we could go to for information," says Ling, who rides an expensive carbon-fiber model produced by famed Italian bike maker Tommasini. "But now, there are about 10 of them that I check."

Ling is the ideal customer for many outdoor gear companies that are aggressively moving into the market. In Guangzhou, the Columbia Sportswear Co runs infomercials on small flat-screen televisions in taxi cabs. One ad features a young man who works in an airline company but has a passion for hiking in the Tibet Autonomous Region. He's shown wearing a floppy hat and fancy hiking boots, trekking among snow-capped mountains and bright blue lakes.

Trek Bicycle Corp is another company that's focusing on China's growing masses of weekend warriors. The American company - which already has 250 dealers across China - recently moved its Asia director, Philip McGlade, from Japan to Beijing.

"The potential here is massive," McGlade says.

He adds that another factor that makes the Chinese market so attractive is that it's relatively easy to reach great places to ride because suburban sprawl is rare in China.

"Even in cities like Shanghai and Beijing, the two largest cities, the ability to access the outdoors is actually really good," McGlade says.

One of the pioneers in introducing mass athletic events to the Chinese is the Swedish company Nordic Ways Group, which for 11 years has been organizing Scandanavian-style sporting events in China. The firm known for organizing the Vassalopet cross-country ski festival started out in China in 1998 by promoting orienteering (a race using a map and compass).

Now, Nordic Ways has branched out into mountain biking in the Inner Mongolia Autonomous Region, grassland marathons and Nordic skiing. It organizes between 20 to 25 events each year, says Niclas Hellqvist, the group's Beijing-based managing director.

The company strives to replicate famous Swedish athletic events in China. Last September, Nordic Ways organized a 177-kilometer road cycling race around Fuxian Lake in Yuxi in southwestern China. The inaugural event, which attracted 300 cyclists, was modeled after the annual Vatternrundan in Sweden, billed as the world's largest recreational cycling event.

The race's sponsor list included the makers of PowerBar energy snacks, Pearl Izumi sportswear and Look bicycle components. Most of the participants were local Chinese.

Hellqvist says that the Beijing Olympics in 2008 helped get more people interested in sports. It also got more local officials interested in hosting events, he adds. More of them are viewing sports as a good way to highlight their cities and attract investment.

Hellqvist declines to discuss how well Nordic Ways has been doing financially.

"Put it this way, we have been in China since 1998 and we're still here," he says. "It's a tough job but we're still here."