'A chief attraction of the sport, said to be one of the fastest-growing in Britain, is that it requires strength, stamina and agility, but also gracefulness'

Like spiders they clamber up the walls - children, young adults, pensioners - and then descend on cords. They are multiplying fast.

It all started more than 20 years ago in a secluded house in downtown Brussels, according to the many Belgians who claim it to be the world's first indoor climbing centre.

Now, people from all walks of life are taking to the walls in a trend echoed across the globe.

Climbing walls, or rock gyms as they are known in the United States, have cropped up in sports centres, shopping malls, fairs and schools, from Europe to the US to Asia.

"It's definitely a boom time," said Trevor Cooper-Williams, who owns a climbing hall in Stockholm.

"It used to be an extreme sport, a sport for daredevils, but now it's more mainstream."

In the US alone, there are estimated to be more than nine million "urban" climbers - some three percent of the population - with the number growing annually by over 15 per cent.

In Britain, climbing centres have mushroomed from just 40 in the late 1980s to 254 in 2003. A chief attraction of the sport, said to be one of the fastest-growing in Britain, is that it requires strength, stamina and agility, but also gracefulness.

While climbers certainly work up a sweat as they heave themselves around overhangs, hanging by the teeniest of grips, they must also work out how to place their feet and shift their weight to reach the roof.

"Climbing isn't only about upper body exercise, it can also be an elegant puzzle waiting to be solved with footwork to make a ballerina envious," said Jason Meyers, a 29-year-old climber at the Brussels wall "Terres Neuves" (New Ground).

Indoor climbing walls have a series of graded routes, indicated by footholds and hand grips of the same colour. Climbers generally work in pairs, one at each end of a rope suspended from the ceiling. While one scrambles up, wearing tight rubber shoes reminiscent of ancient foot binding, the other secures or "belays" his or her partner.

Enthusiasts believe this partnership is a core attraction because climbers develop a relationship of trust as they put their lives in each others' hands.

"It's always social, as you always need two. A lot of our climbers become close friends and go out climbing together on the weekends too," said Stephanie Thankappan from New York climbing centre ExtraVertical.

After a hard day in the office, an activity that offers an adrenalin buzz is an effective stress release.

"I love the way you come down from that wall and all the thoughts about work and life have been washed away," said Tom Brookes, a 36-year-old Brussels-based consultant.

Thierry De Henau, 40, also in the Belgian capital added: "I don't want to just go to the gym and lift weights."

The sport is most popular among people in their 20s or 30s, and growing enormously among young women, but climbing walls boasts members well into their 70s.

"We get kids, adults who climb outdoors and want to train and people who just want to work out," said Thankappan.

"For kids, it's brilliant," said Brookes whose daughters, aged 7 and 10, are keen climbers. "They can climb the same wall as a grown-up, even if they use different grips and there is lots of cool equipment like funky rubber shoes and a harness."

Adrenalin junkies

So-called extreme sports have boomed in recent times as adrenalin junkies bungee jump, snowboard and paraglide.

But while scrambling up a 15-metre wall might not seem quite as sedate as jogging in the park, it is a lot safer than outdoors where climbers have to set up the ropes themselves.

Indeed, staff at Terres Neuves say the venue has seen only five serious accidents in the past 20 years.

To climb indoors is also a lot cheaper with clubs charging about $10-12 for a session and renting out all the necessary equipment.

Outdoor climbers usually provide their own ropes and safety equipment, which can run into hundreds of dollars to buy.

Nonetheless, for those who do venture outside, the sky is the limit. Their feats include deep water soloing, which involves the heart-thumping exercise of climbing cliff faces that overlook deep water without securing devices.

There is also a campaign among competitive climbers to have their sport included in the Olympic Games.

Already, indoor climbers were invited last year to join the world games in Munich.

"This is quite a big deal for us and a big step towards becoming an Olympic sport," said Anne-Worley Bauknight, executive director of USA Climbing.

Belgian former world champion Muriel Sarkany said she has taken up the cause, lobbying vigorously.

"Climbing is my passion. It's part of my life," she said. "Enthusiasts of all levels say climbing gives them a combination of exercise and companionship and a sense of achievement that cannot be beaten."

"It's such an unusual mixture of gymnastics, dance movements, and trusting the people you are with. When you reach the top, there's a feeling that you just can't describe," said Jo McCready, senior instructor at Bristol climbing centre.