Hundreds of teenage girls stand open-mouthed, screaming. The more bashful among them giggle and attempt to turn their faces from the traffic-stopping sight. Macy's department store in San Francisco has apparently never seen such a frenzy - but then it's not every day David Beckham unveils his wares on a 30m-high poster for fashion giant Giorgio Armani.

It's not the first time a sports person has turned a fashion moment into a genuine phenomenon. A few years ago Anna Kournikova's Wimbledon outfits made tongue-tied schoolboys out of the typically unflappable BBC commentators, while skateboarding legend Tony Hawk has inspired a generation of fans to emulate his relaxed dress sense.

After all, what do clunky white trainers have in common with exquisite silk gowns handmade in Paris - or, come to think of it, Alexander McQueen's latest, much talked-about collection and the contents of high-street sports shops?

It's a strange relationship, and one examined by Ligaya Salazar in her new book, Fashion v Sport. From the late nineteenth and early twentieth centuries, through the Lycra-clad days of Studio 54, up to that Beckham-worshipping moment last month, Salazar examines how the world of sport has affected fashion, and vice-versa.

"For quite a long time, sport has been a key influence on fashion," she explains. "I hadn't really thought about it much, though, until I started the research on this book. Sport's influence on fashion has never really been acknowledged until relatively recently with the coming together of designers and big brands. I just felt, with the Olympics this year, it was time to address this idea."

The effect of sport on fashion isn't so hard to grasp. After all, the honed and toned bodies of models - particularly male ones - bring to mind the superior physiques of sportsmen and women. You need only go back to the 1980s and recall how the craze for working out sparked an interest in body-conscious clothing as famously designed by Herve Leger.

There's also the constant experimentalism that goes with producing sports apparel, something such designer brands as Prada and Calvin Klein have taken advantage of. The reflective fabrics, thick diving-gear materials and ergonomic shapes so prized by and essential to the modern sports enthusiast have made it on to catwalks, creating some of the most forward-thinking contemporary trends along the way. Without the desire to go faster, run further and ensure body temperature remains stable, fashion would probably not have staples such as the humble polo shirt, swimming costume and, God forbid, Lycra.

"The Japanese brand Undercover is quite forward about using some hi-tech sports fabrics in their collections," says Salazar, whose book accompanies an exhibition of the same title at the V&A Museum in London. "It's quite obvious in their collections that they're using sports fabrics, but I think you'll start to see more designers using the improvements in technology in a more subtle way, so the consumer doesn't necessarily know the fabric they're wearing started off in an item of sports clothing."

As with any other relationship, the twinning of sport with fashion isn't a one-way street. The ability of the fashion world - on the catwalk or via street fashion - to drag sports brands into the future cannot be overstated. Nike, for one, hires a team of so-called "cool hunters" to scour the streets for breaking trends that will grip mass consumers in the seasons to come, while other brands such as Puma hire burgeoning designers to inject glamour into their clothing.

Stella McCartney provided fashion with its most famous sporting relationship when she joined forces with Adidas to create a trendy sports wardrobe, cementing the fashionable future of sportswear. Although McCartney's feminine designs weren't far from the shapes and styles already around, by lending her name to the brand she instantly made sports clothing desirable rather than simply functional.

She also added a much-needed elegance to sportswear. Although every sports brand already featured a collection for women, the over-use of bright pinks and a lack of subtle design meant women often viewed sports apparel as a matter of necessity rather than style. Not any more, considering you can now buy sports equipment from the likes of Chanel.

Nevertheless, while big designer brands have made moves into the sporting sphere, professional athletes are unlikely to be seen running around the Olympic track in China this year draped in Gaultier. The fashion world, though increasingly close to sportswear manufacturers, remains rooted in the arena of mass consumerism rather than professional sport.

"Part of the reason this book is so timely is because of the Olympics," says Salazar. "Everyone's thoughts will be on sport this year and I felt this relationship hadn't really been explored in the past, in spite of all the collaborations and history between the two worlds. We also found when researching this subject that sports companies have started to revisit their archives - something they never used to do in apparel.

"Historically, sports companies put most of their research and time into shoes, which is where the difference can be made. But now they're revisiting their clothing archive and producing some very popular retro styles."

The current relationship between fashion and sport seems tied between the uber-famous sports people who lend their faces to the brand, and designer collaborations that continue to secure the mass-market appeal of sporting apparel. As for those posters of a muscular David Beckham - turned modern-day Michelangelo sculpture - in his Armani underwear, well, that's just a nice bonus.

Fashion v Sport by Ligaya Salazar is published by V&A Publishing, priced £19.99. The exhibition of the same name runs from August 5 until January 4, 2009, at the V&A Museum, London.

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