An injury to the spinal cord, once considered a lost cause, may no longer mean an inevitable slide into chronic illness and physical decline. Studies are increasingly supporting the once-controversial idea that exercise can improve sensory and motor function long after the initial injury.

The latest evidence comes from scientists who used electrical impulses during exercise therapy to stimulate muscles. That approach has been around for more than 30 years, but the cost of the machines—about $15,000 each—and years of poor equipment design made it impractical.

If you stimulate the heck out of a muscle, you would expect it to get stronger. Diana Cardenas, University of Washington

Researchers at the Baltimore-based Kennedy Krieger Institute followed 48 people who were paralyzed an average of five years prior to treatment for nearly two years. Individuals randomly received either restorative therapy—riding a specially constructed bicycle three times each week while their leg muscles are stimulated through electrodes placed on the skin's surface—or stretching exercises. Of the restorative group, 40% regained some motor function such as the ability to walk without a walker or better bladder control as compared with only 4% in the stretching group. Exercise also cut muscle spasticity—uncontrollable jerky movements—by 47%.

The therapy also boosted muscle mass by an average of 30% while trimming fat by an average of 44%, factors that can cut the risk of diabetes and heart disease. “Those are benefits of exercise we know already,” says Lead researcher John McDonald. “But exercise is never offered to paralysis patients.”

More controversial is exactly how exercise influences recovery. Some scientists say physical activity merely strengthens long-atrophied neural connections. “I'm not surprised that people can regain some function,” says Diana Cardenas, chief of rehabilitation medicine at the University of Washington. “If you stimulate the heck out of a muscle, you would expect it to get stronger.”

But McDonald says his data suggest there's more going on. McDonald treated the late actor Christopher Reeve, who was paralyzed from the neck down after a horse-riding accident in 1995. Before he died in 2004, Reeve had regained sensation in 98% of his body and some motor function, McDonald says.

Reeve could, for example, stand unassisted in a swimming pool, breathe on his own for several hours without the aid of a respirator, and hold a glass. His recovery was especially remarkable because he began to improve after five years of exercise therapy, hinting that some nerves had regenerated. But critics say individual cases do not prove that recovery is possible in most people.

Mounting evidence from animal studies also supports the idea that physical activity promotes regeneration. For instance, Fred Gage at the Salk Institute in California has consistently shown that physical and mental activity substantially boost the birth and survival of nerve cells in rats.

McDonald's lab has found that when they electrically stimulated the hind limbs of injured rats, more than 32% of stem cells transplanted below the injury site had differentiated into neuronal cells as compared with only 9.1% in controls.

“McDonald's work has fostered a lot of interest in the field,” says Randal Betz, chief of staff at Shriners Hospital in Philadelphia, “but we need more evidence.”