By Tom Brook In New York

An award-winning documentary with an unusual topic - charting the rise of modern vertical skateboarding - Dogtown and Z-Boys, has been released in the US.

It boasts a pulsing 1970s soundtrack and narration from Sean Penn.

Stacy Peralta, an original member of the Z-Boy skateboarding team who directed and co-wrote the film, found it was not difficult to get Penn's participation.

"Sean's a surfer, Sean was a skateboarder, so we just felt he would be the perfect guy to do it," says Peralta.

Sean Penn: Keen to lend support

"We never ever thought that he would do it. He agreed to narrate the film, didn't want any money for it.

"He said he was doing it because he thought it was a cool thing to do."

The film chronicles the emergence of vertical skateboarding from California surfing culture in the 1970s.

Disenfranchised

The sport began in Dogtown, a rundown seaside neighbourhood between Santa Monica and Venice.

Local street kids, many from broken homes, transposed their aggressive and daring surfing techniques into bold moves on planks of wood attached to roller skate wheels.

The pioneering Zephyr Skating Team practised their moves at local schools with asphalt-banked playgrounds that helped them perfect their distinct vertical styles.

The Zephyr team stole the limelight

Using old film footage, interviews and stills the documentary shows how this surfer-inspired sport was embraced by disenfranchised, mainly poor, teenagers from diverse cultural backgrounds.

These modern skateboarding pioneers were quite different from the bronzed, blue eyed, more middle-class surfers who epitomised California beach culture at the time.

Peralta recalls skateboarding gave the members of the Zephyr team a sense of identity.

"We weren't good at football, we weren't good at basketball, we weren't good at any of the traditional team sports," he says.

'Aggressive'

The 90-minute documentary captures pivotal moments in the evolution of the sport including America's first major skateboarding championships, the Del Mar contest in 1975.

This was where the Zephyr team stole the limelight and introduced their techniques to the rest of the world.

Peralta recalls the frosty reaction to the team from the traditional skateboarders at Del Mar.

Peralta was an original member of the team

"They thought what we were doing was trivial," he says. "The type of skateboarding they were doing was based on a decade prior, which was handstands, headstands and things like that.

"We came in with an approach based on surfing which was low, pivotal and very, very aggressive."

But the outside world took note and the Zephyr team's skateboarding style rapidly gained a cult following.

The documentary conveys how the sport, particularly in its early stages, had an illicit and subversive quality because many of the practice grounds were on private property.

'Virus'

A drought in California in the mid-1970s inadvertently helped the sport flourish because it left many large swimming pools in affluent neighbourhoods empty.

These pools with their undulating surfaces provided ideal terrain and became breeding grounds for young heroes of the new movement.

A drought emptied many pools

Peralta acknowledges that skateboarding developed rather like a virus.

"We started this sport in backyard pools, co-opting other people's swimming pool.

"We weren't allowed to be in their backyards, we weren't supposed to be in their backyards.

"But we saw something they had, just like a virus does. And that pool gave life to the sport."

Many of the big early heroes of the movement appear in the documentary.

They include Tony Alva, who can be seen doing dramatic vertical leaps on his skateboard into the air above the rim of empty pools.

Although Alva has been helping to promote the documentary, he is portrayed in less than flattering terms as a young skateboarder who could be selfish and a bit egocentric.

"I channel my energy in an aggressive manner and that maybe rubs people wrong occasionally," he admits.

Business

From talking to Alva it is clear that these pioneers of modern skateboarding view their skill as an art.

They take issue with people who might view the sport as the product of lazy California culture.

The film is intended to be seen as artistic project

"A lot of us are artists, photographers, musicians. We're just expressing ourselves on a different medium," Alva declares.

He also points out that skateboarders do not pollute the environment and that the sport has now become "totally part of modern society".

Indeed skateboarding has become a huge $1.4bn (�968m) a year business in America.

The documentary has drawn some criticism because it is financed by Vans, a shoe company whose footwear is very popular with skateboarders.

But most critics view the film as an artistic endeavour, and definitely more than just marketing oriented "infotainment".

The film won top honours at the Sundance Film Festival last year, and more recently at the Independent Spirit Awards in March.

Although the audience for documentary films is limited, Dogtown and Z-Boys should have strong appeal with its subject matter, Sean Penn's narration and a soundtrack that includes the likes of Jimi Hendrix, Led Zeppelin and Neil Young.