But for Julie Hill, this is a truly astonishing moment.

A car accident in April 1990 broke her back

Julie severed her spinal cord in a car accident eight years ago. She was told she would never walk again - let alone ride a cycle.

This is no illusion, however. Julie's legs are turning the wheels on her tricycle; she can stand up out of a chair and take albeit very faltering steps with the aide of a frame.

She can do all these things thanks to a revolutionary implant that directs a series of electrical impulses to the muscles in her legs.

Computer program

BBC Science Correspondent Sue Nelson: Years of being a human guinea pig

They stimulate her lifeless limbs into moving back and forth - her lower body now listens to the commands coming from a computer program instead of her brain.

But make no mistake, Julie is still paralysed. She spends most of her life in a wheelchair and the scientists, who are using her as a bionic guinea pig, have realistic goals.

The implant directs electrical impulses to the muscles

"Our target was to give sufficient function that Julie would want to use [the implant system] everyday - and would want to continue exercising with it every day," says Tim Perkins, from University College, London,

"But we are not in the business of miracles. The notion that she is just going to get up out of wheelchair and do a walking marathon is_no way. There is what is possible and what is clearly not possible."

Walking movements

Nevertheless, to the casual observer what Julie, Tim and his team have already achieved is quite remarkable.

Julie: Did not relalise how long it would all take

UK television viewers were first told about Julie's story in 1994. On the BBC's QED science programme on Tuesday, 1 December, they will get an update on her progress.

And they will see that it has been a hard, and at times, frustrating few years. Although the researchers managed to get Julie to stand fairly quickly, they have had difficulty getting her to adopt a posture that will allow her bionic system to mimic walking movements.

There are 47 muscles in each leg and they all require just the right amount of stimulation at just the right time. Getting the implant to match the sophistication of the human brain is no easy task.

Steady development

She can take one or two steps with the aide of a frame

"Research is what it is all about," says Julie. "I didn't realise in December 94, when I had the operation, that at this stage we'd still be talking about things that we were talking about a day after the op."

Nevertheless, progress is being made and the program that allows her to ride the trike is the most striking evidence of that steady development.

She loves her tricycle

If we could get even a modest capability in standing, steeping and cycling, then I'm pretty sure that quite a significant portion of the paraplegic population would be interested,"

Julie says, even with its currently limited capabilities, the implant has made a big difference to her life.

"I don't think I've ever, ever felt as fit as I do now. I'm getting such good aerobic exercise via the implant. It's really working well for me."