BALLETT FRANKFURT’S third visit to Dance Umbrella since 1998 will also be its last under the artistic directorship of William Forsythe. The revered American is moving on, having spent the past two decades shaping his company of 38 dancers into the world’s leading avant-garde troupe.

No choreographer since George Balanchine has pushed classical dance as far as Forsythe. Taut and extreme, his deconstructions of the academic vocabulary have helped to alter the way we perceive the human body in motion. Companies from San Francisco to St Petersburg have hungrily taken his pieces into their repertoires, while the Royal Ballet has used them to show off dancers from Sylvie Guillem to Darcey Bussell.

Although his works may contain spoken text, they have nothing to do with 19th-century narrative traditions. Instead they are mixed-media sensations featuring bold, sculptural lighting design, densely textured soundtracks and, as is the case with his new Kammer/Kammer, extensive and sophisticated use of film and video.

Forsythe divides audiences. To some he is a visionary genius doing his damnedest to keep ballet fresh, exciting and alive. To others his work is just damnable. His innovations are practically guaranteed to drive dance purists batty, but Forsythe’s artistic credit rating is so high that even those who don’t much like the work have to respect his integrity.

Which explains why recent developments at Ballett Frankfurt came as such a shock. In spring 2002 the news that Frankfurt politicians were planning to close down Forsythe’s government-subsidised troupe shocked the world’s arts community and, in particular, Forsythe himself. “I’m so dumbfounded I don’t know what to think or feel,” he said at the time.

Apparently the reasons for the move were partly monetary. Forsythe says Ballett Frankfurt’s productions enjoy a 96 per cent attendance rate, but city finances were in such dire straits that something had to give — and conservative local leaders, it seemed, preferred to see classical story ballets on the stage of their city’s premier dance company. Anyone familiar with Forsythe’s startling and challenging productions knows how alien staging, say, a straight Swan Lake would be to him.

Letters of protest flooded in, signed by everyone from Mikhail Baryshnikov to Anish Kapoor. The high-profile public outcry eventually prompted civic authorities to have a change of heart, but by then it was too late. Forsythe wasn’t about to stay where he was neither wanted nor understood.

For Umbrella audiences, the upshot of all the backstage drama merely increases the urgency to see Ballett Frankfurt in what amounts to its last great Forsythian gasp. True to form, Kammer/Kammer is not an easy piece to grasp. Premiered in 2000, its musical sources include Bach, Telemann, Busoni and the long-term Forsythe collaborator Thom Willems. It also draws upon two texts, Anne Carson’s Irony is Not Enough: Essay on My Life as Catherine Deneuve and Douglas A. Martin’s Outline of My Lover. Each is about love, desire and, jealousy.

The American dancer-choreographer Richard Siegal is one of Kammer/Kammer’s original cast members. “It would appear that we’re engaged in the telling of two separate stories on a collision course with one another,” he says. “Two principal characters, played by Dana a Forsythe muse and Tony (Rizzi), spiral into a vortex of jealousy. The other performers are more ambiguous. At times we complete the points of jealous triangles, at others we’re changing the physical context of the drama to elucidate a scene.

“There’s quite a bit of scenery,” Siegal says. “Primary are two mattresses on which much of the dancing is done. There are also many plasma screens onstage and in the audience. A camera woman shares the stage with us, while a video editor sits in the audience mixing the live images and stock footage. Of course being inside the performance means I actually see very little of this, but I’ve heard it’s beautifully rendered.”

In some quarters Kammer/Kammer has been accused of not containing enough movement. “Criticisms I’ve heard along these lines are most often based on certain expectations,” Siegal says. “For my part, I can say it is a work for which I need to be in top form. I dance hard.”

What, then, is the attraction of working with Forsythe? “Physically,” Caspersen has said, “Bill looks for the ability to co-ordinate in highly complex ways, using isolation and extreme articulation of head, neck, hips, torso and limbs.”

A strong ballet technique is a given although, as she explains, “if someone is extraordinary in other ways that isn’t necessarily a deciding factor. Often the dancers are involved in several sides of the creative process. So he also looks for artists and colleagues who are interested in his work, but who have their own hearts and minds and don’t wait for orders. These are people who have what I would term dance intelligence: curiosity, fearlessness and the desire to continuously reapproach dancing.”

Just like Forsythe himself, in fact.

• Kammer/Kammer is at Sadler’s Wells, London EC1 (020-7863 8000), from October 22 to 25