Spinal injuries can be very difficult to treat
Hopes that people with spinal injuries could one day regain leg movement have been raised by research in rats.
US, Russian and Swiss scientists used drugs and electrical stimulation to train the animals, whose spinal cords had been cut, to run on a treadmill.
Their movement was "almost indistinguishable" from normal steps, they said.
But the report, published in Nature Neuroscience, stressed the movement was not directly controlled by the mind.
Spinal injury - damaging or completely severing the nerves connecting the body's muscles to the brain - is one of the most intractable to surgery or drug treatment.
Scientists have known for some time that if a tiny electrical current is applied to the nerve just below the injury, it produces a muscle contraction.
However, the act of walking relies on a complex sequence of such contractions delivered at the right moment, so that the legs can carry the weight of the body forwards.
The earlier discovery that these patterns of motor signals may be partly governed in the spinal cord itself is the key to the latest research.
It is suggested that nerve circuits called "central pattern generators" may exist even at the base of the cord, underneath the point of the injury.
The teams from the University of Zurich and the University of California at Los Angeles believe they have found a way to harness this.
They used drugs which stimulate unused motor nerve circuitry, coupled with electrical stimulation, with the aim of activating these central pattern generators and producing a stepping action in the legs below.
The rats, despite having no connection between their brains and their legs, were able to carry their own weight at walking, and even running pace, on a treadmill, with virtually no differences between their gait and the running style of a healthy rat.
The researchers suggested that in humans, this strategy could deliver "substantial levels" of motor control in the legs, although they conceded that it still offered no way for the patient to control the movement with his or her mind.
They suggested that the technique could be used in conjunction with "neuroprosthetic" devices, which attempt to bridge the gap in the spinal cord caused by the injury.
Professor John Priestley, a neuroscientist from Queen Mary's University of London, said that while there were major structural differences in the spines of rats and humans, and any human application would be "a long way off", it was a promising development.
He said: "They have established that you can take a rat with a completely severed spinal cord, and with the right sort of stimulation, produce almost normal movement.
"It shows that we might be able to get in there and manipulate that."