Cat Power for Chanel, Santigold for Stella McCartney and Little Boots for Topshop...

Which came first, the music or the fashion? The two art forms have been feeding off one other for decades: designers are inspired by music; musicians use fashion to create an identity. Simple. But what of the music that is featured on the catwalk at fashion weeks around the world? More thought goes into it than you might realise. While it's there primarily to get the models moving, it takes a team of people to ensure that the soundtrack to a show expresses precisely what the designer wants to communicate.

The grand dame of British fashion, Vivienne Westwood, has said that the music "is more important than the clothes... if the music isn't right, then people aren't in the right mood". The designer Henry Holland agrees that "music creates the atmosphere of the show". Fashion week is a hectic time and the schedules of the influential editors and buyers involve dashing all over town to dozens of shows. A designer needs to delight and impress these influential guests, who are likely to be tired and irritable. Music is a great way to send them off excited.

So what are labels trying to convey with their music choices? Sometimes they pick a literal soundtrack, something that says less about them but more about that season's collection. For his Spring/Summer 09 collection, the London designer Christopher Kane used The Jungle Book's "I Wan'na Be Like You" to complement a show that featured animal prints and gorilla dresses. For his Autumn/Winter 09/10 show, the clothes were more mature and sombre. Zany colours and fun prints were replaced with a monochrome palette and a sexier, darker vibe. This time the soundtrack, fittingly, was the theme from the film Natural Born Killers.

More often, the music played during a show is not so much influenced by the collection but is rather an extension of the brand's values; it represents the girl they are designing for. London label PPQ is known for designing clothes for the type of girl who stays out too late and runs around town looking for the next gig or party. Theirs is a brand steeped in music. The founders, Amy Molyneaux and Percy Parker, also set up the 1-2-3-4 record label and the music for their shows is done in-house, often using the latest band they have signed, one with whom said girls would probably end up bar-hopping. Molyneaux also points out that "bands always turn up to the shows here. In New York it's much more about having actresses on the front row". Holland agrees that the music can "relate to the girl that you're designing for when you're doing the collection. Once we did 'When I Grow Up' by the Pussycat Dolls because that really reflects our type of girl: fun and a bit brash", he explains. "I nearly always use a Girls Aloud song, too".

Sometimes the music used is purposefully cutting edge, featuring little-known artists to showcase how ahead of the time and trendsetting a label is. No-one's going to think of a designer as edgy if they send their models down the catwalk to the sound of the Kaiser Chiefs. Topshop always ensures that they feature the next big thing in their shows. A spokesperson says, "Topshop Unique always shows against a backdrop of old and new but we certainly prioritise little-known artists that we feel reflect the tastes and interests of our consumer. We always aim to set trends rather than follow them, both in terms of music and fashion." Sure enough, predicting the swarm of new female pop stars in 2009, last September Topshop Unique used Little Boots and Ladyhawke to soundtrack their catwalk show.

So once the designer has an idea of an artist or vibe they want to use, who do they turn to for picking the remaining tracks and mixing them to be catwalk ready? There are a small number of prolific DJs who soundtrack most of the big shows. After receiving a brief from the designers, they will mix the songs and ensure that they are long enough to avoid models being left strolling uncomfortably down the runway after the music has finished.

Michel Gaubert is one of the best-known fashion DJs, having created soundtracks for Chanel, Balenciaga and Christian Lacroix. Frederic Sanchez (who, by the way, doesn't refer to himself as a DJ, but as a "sound illustrator") is another key player, soundtracking Marc Jacobs, Louis Vuitton, Prada and Versace shows. In London, ex-Boombox DJ Jerry Bouthier is a well-known figure and Steve Mackey, ex-Pulp bassist and husband of Katie Grand, collaborates with some of the biggest designers, including Giles Deacon.

For those wanting to create a more intimate ambience than mixed tracks allow, it is becoming increasingly common to feature a live performance for the models to strut their stuff to. A self-confessed music aficionado, Karl Lagerfeld often uses a performance from his latest music crush for Chanel's shows. "There is a warmth to live music," he explains. "It creates an atmosphere. A relationship formed between the audience, the models and the singer... There is a vibration in the room when it is live; it changes everything." By having the artist at the show, they also lend their qualities and personality to the label.

One effective live collaboration saw Cat Power playing at a Chanel haute couture show; her drama, beauty and elegance sitting comfortably with Chanel's own vision. Another is Tori Amos, who did an impressive job performing at a Viktor & Rolf show, her own eccentricities perfectly complementing those of the avant-garde Dutch duo. Two years ago Matthew Williamson pulled off a coup for his 10th anniversary at London Fashion Week when Prince leapt up on stage at the end of his show and sang "Chelsea Rodgers" with his two purple-clad dancers. And on Tuesday, rumour has it that Jeremy Scott's wacky prints will be showcased to live accompaniment from the colourful Lady GaGa, making his show one of the hottest tickets in town.

While using these more established artists can be very effective, record labels often hope that designers will look to one of their rising acts, as it can bring huge benefits to the artist. David Adcock, product manager at Columbia Records explains: "Finding fans that are trendsetters within their peer group is key to launching a new artist. Nowadays people are able to communicate and share their tastes – and thus influence – far more easily than ever before. The association with something cool has always helped and Fashion Week is a hotbed for these types".

Stella McCartney's label has shown great ingenuity in the past in picking rising talent. Santigold, MIA and Dizzee Rascal were all used during her catwalk shows right at the beginning of their careers. In a mutually beneficial deal, the designer gets kudos for being cutting edge while the artist receives influential exposure.

It is yet to be seen what music will be boomed out at Somerset House as London Fashion Week kicks off tomorrow. Whatever it is, be it rock or an unknown dubstep artist, you can be sure that it has been hand picked to underscore a designer's intentions and contribute to the spectacle of style that is the catwalk show. Sometimes, a catwalk show's influence can reach beyond your wardrobe and end up on your stereo, too.

The sound of London: Designers' favourite runway tunes

Topshop Unique "Walking on a Dream", Empire of the Sun

House of Holland "I Wanna Be a Supermodel", Letters to Cleo

Vivienne Westwood Red Label "I've Seen Enough", Cold War Kids; "Cities Burning Down (Naum Gabo remix)", Howling Bells

Luella "Happy When It Rains", The Jesus and Mary Chain; "Some Girls Are Bigger Than Others", The Smiths

Osman Yousefzada "Love Song", The Cure; "Standing On The Shore", Empire of the Sun

Christopher Kane Natural Born Killers original soundtrack

Jonathan Saunders "Small Town City", WhoMadeWho; "William's Blood (Aeroplane remix)", Grace Jones

Peter Jensen "M79", Vampire Weekend; "The Blue Wrath", I Monster

Roksanda Ilincic "Obsessions", Marina and the Diamonds; "Computer Love", Glass Candy