Say goodbye to the stuffy string quartet. Now they're the preserve of trendy twentysomethings.
The two men in the front glower menacingly into the camera dressed in flapping lumberjack shirts and trainers. Behind them stand a grim-faced blonde and a bearded man in black. They could be muggers awaiting their prey, or new recruits for Madonna's bodyguard team. In fact, they are members of a string quartet about to launch an anniversary concert programme at the Royal Festival Hall.

String ensembles? They're those clusters of constipated, grey-haired men in tail coats. Or are they? A curious phenomenon has occurred in the past few years: string quartets are trendy. Switch on to Jools Holland's show and you'll see a resident ensemble sawing away with the singers. The Spice Girls have used one, as has Icelandic pop star Bjork. Leafing through the programmes for Europe's summer festivals you'll notice a gallery of twentysomething foursomes in jeans and leopard-spotted bodies.

This revolution has been led by the American-based Kronos Quartet, which was founded 25 years ago. Its founder, David Harrington, says: "I don't subscribe to the idea that quartets play in some cloister on a Sunday afternoon. I have spent my career trying to bring the world we know into this medium called music." The piece of music that inspired him to found the group was a protest against the Vietnam war, George Crumb's Black Angels. Since then he has pursued musical politics with Steve Reich's Different Trains about the Nazi death transports, a piece featuring gays rioting against homosexual discrimination and a composition performed as a background to the voices of FBI director Edgar Hoover and the American anti-war polemicist IF Stone.

The Kronos mission to transform what they see as a staid genre into a mouthpiece for modern issues is shared by the British Brodsky Quartet, which also achieves its silver jubilee this year.

Jackie Thomas, the Brodsky cellist says: "We found the trend used to be to teach individual instruments, which was a shame because playing in a quartet is the best way to learn to listen to others."

Their youthful verve won them some unlikely fans. They formed the musical background to Paul McCartney's "Eleanor Rigby" and Elvis Costello learnt musical notation so that they could work together on The Juliet Letters.

Like the Kronos, the Brodskys have eschewed traditional evening dress and they prefer to perform standing up. "There is so much more energy," says Thomas. "The usual image of a quartet is of an elite, compact insular unit with the audience like voyeurs. Standing up lets the audience in more."

Most of the new quartets prefer to stick to more traditional repertoire than the Brodsky and the Kronos but inject it with such youthful vigour that chamber music is enjoying its biggest boom since the 1930s. One of the attractions is the size and the scope of the repertoire available.

The Polish violist Krzysztof Chorzelski formed the four-year-old Belcea Quartet with students at the Royal College of Music because he disapproved of what he calls the increasingly industrial approach of orchestras. With the youngest player only 21, the Belcea is already beginning to attract critical acclaim. "Players are becoming like an employee in a factory," he says. "Chamber music offers less financial stability but more creativity."

A more unconventional explanation for the trend is offered by Georges Zeisel who in 1987 launched Pro-Quartet, a foundation to raise the standard of chamber music in France. He believes musical tastes mirror the political climate and points out that the revival began in earnest in the 1980s when the Communist bloc was crumbling.

Jonas Krejci, 28, a cellist with the Prague-based Skampa Quartet agrees that independence, political or personal, is the lure for the modern musician. "I felt an orchestra was too impersonal and I didn't feel I could get my ideas across as one player among 60," he says. The group formed just after the Velvet Revolution which meant that they, unlike their predecessors, could travel freely abroad to establish their reputation. They became the first ensemble ever to be appointed a residency at the Wigmore Hall.

While there may be more room for individuality in a quartet, the players must do without the protection of an orchestra and the freedom of a soloist. Each must work in perfect accord with the others. The enforced intimacy is an added stress, particularly on tour. "It's like a marriage, so there's always some friction," says Krejci. "You are four people trying to be as one so there is no room for ego."

The Sorrel Quartet is classical music's equivalent to The Spice Girls. The all-woman band in their colourful silk shirts is another sign of the changing face of chamber music. "We try to be gutsy and not feminine, but we do sometimes have to make more of an effort because we are all women and people notice us." They have recorded works by Britten, Tavener and Doreen Carwithen, widow of the composer William Alwyn and are about to start the complete cycle of Shostakovich quartets. In June they give the world premiere of John Pickard's fourth quartet at the Wigmore Hall.

Whether all these ensembles will survive is another matter. The Brodsky's experience is encouraging for young quartets like the Belcea who are just starting out. "When we began we were conditioned to think that only when we were old and grey would people accept us," says Belton. "But we abandoned our tails to avoid the dusty quartet image and experimented with new kinds of music."