Wilson da Silva, Melbourne
TEXTBOOKS say that our sense of balance relies on fluid-filled canals in the inner ear. But researchers in Australia now suggest that when we are standing still, our calf muscles take over. “Most people are surprised, but the results speak for themselves,” says Ian McCloskey, director of the Prince of Wales Medical Research Institute in Sydney.
McCloskey and his colleague Richard Fitzpatrick wanted to know why one in three Australians over the age of 65 has at least one accidental fall every year. They wondered if there were other balance sensors in parts of the body away from the inner ear that were deteriorating in elderly people.
To test their hypothesis, they invented a contraption that disabled the inner ear’s vestibular apparatus and allowed them to examine other possible balance sensors. Volunteers stood strapped to a fixed vertical pole with their heads held completely still and their feet on pressure-sensitive foot pads.
As well as supporting the volunteer, the pads controlled the movement of a second, weighted, vertical pole. The weight was roughly equal to the person’s body weight. The weighted pole moved to and fro according to the pressure applied by the subject’s feet.
“What is swaying is the contraption,” says McCloskey. “If the subject doesn’t sway, he can detect nothing with the vestibular apparatus.” The volunteers were then asked to keep the weighted pole upright.
Blindfolding the subjects had little effect on their ability to keep the weighted pole from swaying. So, suspecting that the soles of the subjects’ feet might be relying on touch to maintain a sense of balance, the researchers anaesthetised their feet. This local anaesthesia resulted in only a minimal increase in the sway of the weighted pole.
Even when McCloskey and Fitzpatrick made things even harder, by introducing small sways to the weighted pole, they found that the movements were easily detected and corrected for by the blindfolded and anaesthetised subjects.
“Our conclusion was that the only sensors left doing the detections and corrections were those in the calf muscles themselves— sensors that detect the lengthening or shortening of the leg muscles as sway occurs,” says McCloskey.
The calf muscle hypothesis ties in with the finding that many elderly people suffer from a degenerative condition called peripheral neuropathy—a shrinking of the axons of nerves in the calves that would impair any balance-keeping function. “Certainly, people with peripheral neuropathy have quite a high incidence of falls,” says McCloskey.
McCloskey and Fitzpatrick, whose study will be published in the Journal of Neurophysiology, suspect that the calf muscles are the principal balance sensors when standing. They believe that vestibular apparatus takes over during rapid movement.
“We’re not saying the vestibular apparatus does nothing, we’re just saying it does nothing to hold you upright when you’re standing,” says McCloskey.