We all know that people move a little slower as they age. But scientists are only now uncovering the reasons why elderly people lose muscle control, and tend to fall more often. They're finding that as we age, communication between nerves and muscles starts to change.

Christopher Knight, a physiology professor at the University of Delaware, studies how muscles and nerves function in older adults. Noting that every muscle has hundreds or thousands of muscle fibers in it and sets of those fibers are controlled by a single neuron, he focused in on one small muscle in the left hand. "And what I'm trying to record are the electrical impulses, or the action potentials, that are delivered from one neuron to those muscle fibers," he explains.

Knight worked with several dozen subjects, half college students and half over the age of 65. He had his volunteers 'practice' a repetitive movement with their left hands. The force of the movement was recorded on a computerized graph that they could see, so subjects knew how hard they were working.

Once the volunteers learned the different experimental conditions and graphs they were asked to follow, they came back for a second day of testing, which involved a more invasive procedure. "We insert the needle electrode into the muscle and record those action potentials while they're performing these different tasks," Knight says, quickly adding that the needle inserted into the subjects' hands was very, very small and the insertion, painless.

With this set-up, Knight was able record the electrical activity the nervous system needed to generate to get the muscle to move. "For the slower less forceful, aged muscle, the nervous system has to give larger, more forceful commands, to get it to perform as needed." With a laugh, he compares it to speaking louder to the muscle, but notes that once the older subjects' nervous systems created enough impulse, their muscles performed as well as the young subjects'.

Knight says this indicates that it's a good idea for older people to keep doing exercises that push muscles to their limit. "When you produce a fast movement, you send a large amount of neural activity to muscle," he says, pointing out that this sort of exercise seems to strengthens communication between the muscles and the nervous system, and could help older people maintain their mobility a little longer. Knight's research is published in the Journal of Applied Physiology.