“Nobody could accuse Kenneth MacMillan of lack of courage”, began the review of the first night of Isadora by Alexander Bland of The Observer. In a two act work MacMillan pushed the boundaries of ballet theatre well beyond any previously accepted limits.

Isadora Duncan embraced free love, free expression and disdained ballet dancers as little more than trained acrobats. Nonetheless the dancers at the Mariinsky theatre in St Petersburg who saw her in 1904 were highly struck by the force of her stage presence and by a technique unfettered by the conventions of court ballet. She danced barefoot and, unlike ballet, her dancing acknowledged gravity and the body’s weight.

When MacMillan made Isadora to open the Royal Ballet’s 1981 season, his choice of subject made strange sense. For Duncan dance was liberative, and of a piece with her radical politics, a view with which MacMillan could straightforwardly empathise. MacMillan aimed to reshape dance as a medium of social comment. And his work was increasingly attracting interest from a new audience which had not hitherto been much engaged by dance. So much so that ITV decided to show Isadora in prime time and the current affairs broadcaster David Dimbleby wrote an approving profile of MacMillan for The Times.

He was not concerned, MacMillan told Dimbleby, whether audiences thought Isadora was ballet or not. “I am not worried by the style. I just want people to feel that they have had a good night at the theatre. I am steeped in classical dancing, but this is 1980 and I cannot go on doing what they did in the last century. I’ll do anything to break away.”

MacMillan’s focus was less on Duncan the dancer than on her tumultuous personal life; her affairs with Ellen Terry’s illegitimate son Gordon Craig; with the Russian poet Sergei Essenin whom she married; with the sewing machine millionaire Paris Singer; and the devastation of losing her three children to early death, one in childbirth, the other two when the car in which they were passengers ran into the River Seine. MacMillan’s innovations in multi-media story telling went well beyond the techniques he had deployed in Anastasia and Seven Deadly Sins. The use of the spoken word in aid of the dance was both extensive and appropriate; Duncan herself was a polemicist, for whom the dance and the word were all of a piece. MacMillan borrowed from Duncan’s Victorian lecture-demonstration technique and this device is deployed throughout the ballet.

At the premiere Merle Park danced the role of Isadora, with the actress Mary Miller (who had attended the same dance school as MacMillan as a child) speaking lines from her autobiography. The ballet’s most profound episode of choreographic imagination came with Isadora and Paris Singer dancing their grief for the death of their children after they drown in the Seine. In a compelling pas de deux, each by turn falls and picks the other up with silent screams, as they fumble and circle about the stage. The ungainliness underscores their grief. Of this section Clement Crisp later wrote: “The rawness of their shared despair, and also the phenomenal outlines of the movement partake of the force of Picasso's brush-strokes in Guernica. I sensed that beyond this expressive point academic choreography could not go. That audiences were profoundly moved by this harsh, beautifully ugly exposition of grief, and responded to the drama of its imagery, cannot be doubted, even in a ballet as diffuse and as flawed in structure as this Isadora.”

Richard Rodney Bennett’s score provided pastiche music for the dancer's public appearances, and used Bennett's own music, his native voice, for Isadora’s private life and its relentless sexual turmoil. ''I must say that even though I've never worked as hard on any project as I have on this, I had the time of my life writing the music for Isadora”, he told The New York Times “There was a time, when I was 20 years old and a student of Pierre Boulez, when I couldn't think beyond the notes. But now, I'd much rather write about sex.''

Isadora’s two acts each lasted some 75 minutes. Only a minority of critics were persuaded of its cumulative effect. For Mary Clarke of The Guardian, Isadora needed “absolutely drastic cutting”, while Clement Crisp, of The Financial Times, its most fervent advocate, conceded that MacMillan’s treatment was “too generous in incident.” MacMillan himself was dissatisfied with the outcome and, in the months that followed, cut some twenty minutes from its length. In March 2009, the Royal Ballet restaged a single-act one hour version of Isadora

But in 1981 Michael Billington of The Guardian, the doyen of London’s theatre critics, thought it an important artistic breakthrough. “What worries me”, he wrote, “is that the kind of people who make up Broadway and Covent Garden audiences are no so stuck in a kind of artistic lockjaw that (unless the critics give their seal of approval) they seem unable to enjoy anything that breaks yesterday’s mould. What we need is not simply a new theatre: we also desperately need a new audience.”

• First performance: Covent Garden, London, 30 April 1981

• Company: The Royal Ballet

• Cast: Merle Park, Mary Miller, Derek Deane, Julian Hosking, Derek Rencher

• Music: Richard Rodney Bennett, Isadora

• Design: Barry Kay

• Scenario: Gillian Freeman

• Benesh notation score: Monica Parker/Adrian Grater (1981) Working score.

• Television: ITV, 23 February 1982