When Frederick Ashton choreographed Five Brahms Waltzes in the Manner of Isadora Duncan, he was drawing on his own 55-year-old memories of the great dancer. Back in 1921 he was a ballet-obsessed 17-year-old while Duncan, aged 44, was well past her prime. Her red hair was badly dyed and her semi-transparent drapes revealed a lot more wobbling flesh than Ashton wanted to see. Still, he thought he was in the presence of genius and returned to watch her perform night after night.
Duncan seemed to Ashton to distil essential qualities. Her movements were so responsively tuned to the music that she appeared to be carried by the notes. She had a startling expressive variety - quick and playful in her lightest moves, grand and resonating in her large gestures.
And she moved with a freedom he had never seen before. Within her seemingly restricted vocabulary of steps, runs and leaps she registered pure sensation, pure feeling. As Ashton later recalled: "She had a wonderful way of running forward in which she, what I call, left herself behind and you felt the breeze running through her hair."
It was these qualities he recalled in 1976 when he created Five Brahms Waltzes. The work started as a single waltz but Ashton extended it into a suite of five as a gift for Marie Rambert, whose company was celebrating its 50th anniversary. And the woman he chose to embody Duncan's memory was the 37-year-old ballerina Lynn Seymour.
Seymour, now 65, recalls that Ashton used a lot of visual imagery to help her re-create Duncan's movement style. "Fred brought along a huge book of photographs and a programme that he'd saved all those years. There was a little line drawing of Isadora that he'd painted in a peachy pink to remind him of the colour of her dress. I loved that."
But Seymour also had to use her own imagination to capture Duncan's unique spirit. "She was apparently unbelievably charming on stage. And the most astonishing thing was her stillness - she had this knack of holding a stillness and then moving at an exquisitely timed moment that made you crumble."
Duncan has gone down in history as a hard act to follow - a one-off original who, from mixed motives of exhibitionism and evangelicalism, believed she could change the world. She was born in California in 1877 and raised by her mother in a very West Coast bohemian style.
The whole Duncan family were romantic idealists and deeply susceptible to the prevailing Greek revivalist craze. While most enthusiasts limited their affiliation to wearing sandals and sticking to a simple diet, the Duncans attempted to live the life. In 1903 mother and four children decamped to the outskirts of Athens, where they tried to build their own temple and persuade the locals to revert to the customs of their ancient forebears.
In Isadora, however, wackiness proved to be the seedbed of greatness. As a child she had been taught conventional "fancy" dance steps and could easily have made a profitable career in music hall. But it became her mission to elevate dance into a language of liberation and transformation, to make it a vehicle for big emotions, big ideas and great art.
Duncan was a natural mover and she had reserves of obsessive energy. For inspiration she read the Greek poets, Nietzsche and Havelock Ellis. For music she chose Beethoven, Chopin, Wagner - great romantic composers who had never previously been used for the dance stage.
But Duncan was also a very canny operator. Her stages were decorated with flattering simplicity - grey voile curtains and soft pink lights. Even more radically, she danced without shoes or corsets. The fluid lines of her costumes not only liberated Duncan's body but also had the great advantage of showing her naked legs and the occasional glimpse of a breast.
By 1907 Duncan had become a worldwide phenomenon - and her real-life performances started to make headlines as much her stage shows. Notoriously, she took many lovers, including the stage designer Gordon Craig, the multi-millionaire Paris Singer and the Russian poet Sergei Esenin.
She danced before European royalty and before Lenin, and suffered a string of personal tragedies, including the drowning of her two children. By the time Ashton saw her in London she was drinking heavily and her stage performances were becoming erratic.
Her death, at 50, was as stagy as her life. As she was driving in a sports car, chauffeured by the young man she was lining up to be her next lover, the fringes of Duncan's shawl caught in the back wheel and broke her neck instantly. Jean Cocteau wrote: "Isadora's end is perfect."
Even though some of Duncan's dances have been reconstructed, it is hard for history to see her as she was. Some writers simply dismiss her as a phenomenon of her time, assuming that descriptions of her greatness were the gushings of an overwrought audience in thrall to a beautiful, half-naked woman. Yet, at the height of her fame, there were hundreds of Isadora wannabes - young women in Greek tunics who posed in drawing rooms and skipped on music hall stages.
Much more important, though, was the ground she opened up for other dance pioneers. Martha Graham's stark, serious modernism may have evolved far beyond Duncan's rhapsodies. But it was Duncan who proved that dance could be taken seriously outside the ballet academy and that a solo woman could take charge of her career.
Despite her opposition to what she called "the sterile gymnastics" of ballet, Duncan also made a profound impact on classical choreography. She danced several seasons in St Petersburg - the citadel of classicism - in the early 20th century, and among the reverential crowds she drew was Mikhail Fokine. At that time the choreographer was himself trying to envision a more naturally expressive style of ballet, and in Duncan's performances he found his inspiration and validation.
Her imprint is visible on several of his works, not least in the delicate arm movements in Spectre de la Rose and in the transparent romanticism of Les Sylphides, in which the poetic pulse of Chopin's music is evoked in ecstatic runs, haunted gestures and delicately nuanced rhythms.
Ashton's own debt to Duncan was various and large. It resonated in the held poses that form the still centre of Symphonic Variations, and led him to choreograph his duet Thais as a pure trance of unashamed, rapturous emotion. It influenced the peculiar plasticity and pliancy he always encouraged in his dancers' bodies, and it particularly inspired his own curtain calls. Even in his 80s, Ashton could generate an instant ovation with his dapper, if creaky, version of the famous Duncan run.
What he attempted in Brahms Waltzes was of course more direct. A few of the details are replications, such as the forward run in which Duncan strews rose petals from her cupped hands. But Ashton wasn't aiming for an exact reconstruction, rather a dialogue between his sensibility, Duncan's and Lynn Seymour's. Though the ballet's steps and structure were choreographed, Ashton left Seymour some latitude for spontaneity - essential in any homage to Duncan.
And for many viewers Seymour was an uncanny reincarnation. The rhythmic subtlety of her dancing, her rounded sculptural contours, her willingness to take risks all brought Duncan back to the stage. Five Brahms Waltzes has been danced by few other women since, but this year is Ashton's centenary and the piece is being revived by Rambert Dance Company and Birmingham Royal Ballet. Seymour is working with both companies and says that all the women to whom she has taught the ballet love it: "It's such a far cry from the unisex athleticism they get to dance today."
When Seymour selected the half-dozen dancers who would learn the role, she wasn't looking for a particular physical type but "for something a bit wild". She feels it is a huge gift for these women to get inside Isadora. "She was a pioneer - she had a huge, strong self-belief. You don't see a lot of that today."
Marie Rambert idolised Duncan and when she first saw Seymour dancing Ashton's homage she burst into tears, saying: "That's exactly what I remember." Today there can be very few people who remember Duncan in the flesh. But there are many who have seen Seymour and when they watch the women she has coached, some equally potent memories may be revived.
Dance passes through history by this peculiar laying-on of hands. Through Ashton, through Seymour, and now through this new generation, Duncan continues to perform.
· Rambert Dance Company perform Five Brahms Waltzes at Hall for Cornwall, Truro (01872 262466), on February 26, then tour. Birmingham Royal Ballet is touring at the end of April.