TUESDAY, April 8, 2014 (HealthDay News) -- Four men paralyzed below the waist have regained some movement in their legs after a series of electrodes implanted along their spinal cord reawakened nerves long thought deadened, researchers are reporting.

Electrical stimulation of the spinal cord has given the men the ability to voluntarily flex their toes, ankles and knees, and the strength and precision of their movements has improved over time through intense physical rehabilitation, the researchers said.

"The really exciting news that has emerged from the study is that spinal cord injury may no longer mean a lifelong sentence of paralysis," said Dr. Roderic Pettigrew, director of the National Institute of Biomedical Imaging and Bioengineering at the U.S. National Institutes of Health. "This is a substantial milestone that has been reached."

All participants are now able to synchronize leg, ankle and toe movements in unison with the rise and fall of a wave displayed on a computer screen. Three of the four are able to change the force at which they flex their leg.

The findings were published online April 8 in the journal Brain.

The first man to receive the implants, Rob Summers, can stand unassisted for brief periods of time, following physical rehabilitation that involves standing and stepping while suspended in a harness over a treadmill.

"At this point I can stand, and I can move my ankles, toes, knees, hips," said Summers, 28, who first received the implants four years ago and lives in Portland, Ore. "I can do sit-ups, which is crazy. Today I am doing about an hour of sit-ups without any help."

Researchers said they were most surprised by the improvement shown in two men who had complete motor and sensory paralysis.

In those patients, the pathway that sends information about sensation from the legs to the brain has been disrupted, as has the pathway that sends information from the brain to the legs to control movement.

Researchers had thought that the sensory pathway needed to be at least partially intact for the men to be able to control the nerves reawakened through electrical stimulation.