For the last nine years of his life, Christopher Reeve, the actor who played Superman, became a different kind of superhero. Paralysed from the neck down after a riding accident in 1995, he turned his wheelchair into a rolling campaign to promote scientific research into possible cures for spinal cord injuries.

A few months before his death in 2004, he sat down in front of a video camera and declared: “I don’t want to be melodramatic, but I’m 51 and the clock is ticking ... I don’t want to be a senior citizen before I’m cured.” The moving scene appears in Christopher Reeve: Hope in Motion, a pair of documentaries made by Reeve’s older son, Matthew, a 28-year-old film director.

By some kind of cosmic coincidence that would surely have enchanted his father, I met Matthew Reeve and his sister Alexandra in New York around the same time that scientists in Japan and Wisconsin were preparing to unveil what may prove the most significant achievement in stem cell research to date.

Two teams of scientists reported independently that they had succeeded in “reprogramming” ordinary skin cells to make them behave like embryonic stem cells, the magical building blocks of human development that can be transformed into brain, bone, heart or nerve tissue. Although the process is not yet perfect, it has shown initial success in treating sickle-cell anaemia in mice and has the potential to transform the lives of hundreds of thousands of people suffering from paralysis.

“My dad was always extremely optimistic that science could get him out of his wheelchair,” said Matthew, whose films about his father were recently released on DVD in America. “For him it wasn’t a question of if, but when.”

Alexandra, 24, added: “Our dad might not be with us any more, but the issues he cared most about still are.”

The image of Superman in a wheelchair remains one of the most haunting reminders of human fragility to emerge from recent American culture. Matthew watched at close quarters as the former Hollywood heart-throb grappled with the cruelties of life on a ventilator and the challenge of taking on a conservative Republican political establishment, vehemently opposed to experimentation on human embryos and seemingly more concerned with God than science.

His films offer an intimate view of Reeve’s furious struggle to escape his wheelchair. They capture all too painfully the claustrophobic life of a former action man who could now do almost nothing for himself. They record both his regrets at what he has lost - “Not to have made love for six years is almost unbearable to me,” Reeve says in 2001 - and his joy at his moments of triumph, notably when he discovers that he can wiggle his index finger.

“He used to call that his party trick,” said Matthew, who was born in London. He and Alexandra are the offspring of Reeve’s 10-year relationship with Gae Exton, a British former model. When the couple split up in 1987 they agreed on joint custody, and the two children spent much of their school holidays with their father in America.

The first film opens conventionally enough with thundering horses’ hooves. Reeve, a keen equestrian, was participating in a three-day event when he was thrown from his horse and landed on his neck, breaking his two uppermost vertebrae and rendering him quadriplegic.

Picking up the story on the sixth anniversary of his accident, Hope in Motion shows Reeve in buoyant spirits, joking to his nursing staff that “I appreciate all of you but can't wait for the day when I have to fire all of you”.

Some of the most striking footage shows Reeve addressing the camera directly, speaking with daunting matter-of-factness as he discusses his condition. “I was an incredibly independent person who loved to spend time alone,” he says. “And for six years I have never once been able to be alone ... I used to sail alone, I flew my airplane across the Atlantic alone, now I can’t go out in the driveway by myself.”

I asked Matthew how difficult it had been to watch his father’s struggles up close. “I definitely experienced both the highs and the lows of his progress,” he replied. “And of course there were setbacks, but that was what was so amazing about Dad. There were moments of darkness, but they stayed just moments. Then you go forward.”

Perhaps the most devastating moment occurs when Reeve goes for a critical test of his diaphragm function in the hope of finding that his lungs may be strong enough to breathe without an artificial ventilator. But he fails the test and the doctor can hold out little hope.

Reeve calls it “a crushing blow”, and admits to getting jealous “just seeing someone get out of a chair and walk across the room ... I want my life back”. But then he gets angry with himself “for being self-pitying”.

That first film ends with his family celebrating his 50th birthday. Matthew is reading something about a contest between Batman and Superman. Reeve, who mustered enough breath to blow out one candle on his cake, is in no doubt who would win. “[Superman] flies, Batman only has a cool car. It’s no contest,” he says.

There is another hero lurking quietly at the margins of Matthew’s films. Or to be more precise, a heroine. Reeve’s wife Dana is probably best known for her constant presence at her husband’s side during his wheelchair years and for her crucial role in establishing their joint foundation to promote medical research.

When I met Matthew and Alexandra, I at first took their glowing descriptions of Dana with a pinch of salt. Children whose parents break up rarely wax lyrical about stepmothers who come along later.

Yet here was Matthew describing Dana as “an astounding person, a ray of sunshine ... just incredible in her devotion”. And here was Alexandra insisting that “we stayed a very close-knit family”. Matthew’s film helps you understand why.

Reeve met Dana when she was performing in a late-night cabaret show in Massachusetts in 1987, when he was 34 and she was 26. They were married five years later, and had a son of their own named William. He was two when Reeve fell from his horse.

Reeve once talked about the pressures on his wife as she adapted to life with a quadriplegic. “I had really pushed the marriage vows to the limit, the ‘in sickness and in health’ part,” he said. “This wasn’t what we had in mind.”

Yet Reeve soon found that the love of his wife and children was the difference between life and death. “Dana doesn’t let me go into the realm of how different things could have been,” he said. “And there’s no use in punishing myself ... they’d rather have me around than not. That helped me to put away thoughts of suicide.” Ten months after Reeve died in October 2004, Dana announced that she had been diagnosed with lung cancer, even though she was a non-smoker. She died in March 2006. Their son William, now 15, is being raised by friends in Connecticut. “He is in excellent hands with a very loving family,” said Matthew.

As time went on, Reeve underwent a successful operation to implant electrodes on his diaphragm, enabling him to get off his ventilator for short periods. But the breakthrough came at a price - he ended up with a hole in his chest and could no longer do the therapy he enjoyed in a swimming pool. The surgery also left him more prone to infection.

His last winter, 2003-4, was difficult as he developed bed sores after long spells in hospital. Yet the one thing that appears to have kept him from despair was the prospect of a medical breakthrough. His main goal, he said, was “to be prepared for whatever medical intervention comes my way”.

Matthew remembers his father as being “just so determined and driven ... to get out of his wheelchair. He spoke to as many doctors around the world as he could to find out what they were up to”.

Reeve was convinced medical science could deliver his cure if given the political and financial space to succeed. He was passionate about stem cell research and scathing about its opponents, whom he challenged to “spend one hour in a wheelchair without moving and see how you feel”.

The US government’s refusal to allow federal funds to be spent on stem cell research, because it involved the destruction of human embryos, was “very, very painful to me”, he said bitterly, six years ago. “If we’d had full government funding, we might by now be in human trials ... instead the whole thing is just a political football being kicked around.”

Now at last it looks as though the possibility of creating stem cells from skin may point the way to an escape from religion and politics and a return to science and medicine. Yet among the scientists who are hesitant to embrace the new techniques as a “miracle cure” are those at Reeve’s own foundation, which in addition to campaigning for stem cell research raises millions of dollars annually to assist victims of spinal cord injuries.

“This is an important step forward, but it’s still early days,” said Susan Howley, the foundation’s director of research. “We don’t yet know if the reprogrammed cells are capable of acting like embryonic stem cells. This will certainly energise the field, but it doesn’t necessarily mean the beginning of the end of the embryo debate.”

Other doctors agree that it would be foolish to stop the work on embryos, which has yet to yield any major cures or practical therapeutic discoveries.

There is one other point that emerges strongly from Matthew’s films about his father. Apart from a picture of Superman on Reeve’s library wall and that brief conversation about Batman, the Man of Steel is largely absent from the documentaries.

Matthew shies away from discussing his childhood experiences as the “son of Superman” - he must have been ribbed mercilessly at his English public school - and that may be because his father never took the image too seriously himself.

“They say he played Superman, now he is Superman,” Reeve at one point muses about himself. “Well, I don’t know about that.”

As for his supposed superpowers: “What I do is based on powers we all have inside us,” he says. “The ability to endure, the ability to love, to make the best of it. You don’t have to be a Superman to do that.”

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