991014-1 10.1038/nsu991014-1 8 October 1999 medicine Disease drags its feet SaraAbdulla Cheating friction is a major preoccupation of engineers, who will use all manner of lubricants to lower the energy lost when one moving surface rubs on another. Now US researchers claim to have found a new way to undermine one of the principal laws of friction and thereby to reduce frictional drag forces.

Subtle mathematical differences in the way people walk could soon allow doctors to assess disease, monitor treatment, and predict – and maybe even prevent – falls. As researchers at the Beth Israel Deaconess Medical Centre, Harvard, Massachusetts announced at this week's 'Bench to Bedside and Back' conference at the Harvard Medical School, there's more to gait than meets the eye.

Patients in the 'gait lab' wear a small recording device on their ankle. This keeps track of information gleaned from force-sensitive pads that can be slipped inside their shoes – one in each heel and toe. Thus researchers can monitor step-to-step variability over time.

This technology has revealed that the stride time of a healthy person follows a fractal pattern, as study leader Jeffrey Hausdorff explained. In other words, over the course of about an hour, minute differences in the inter-step time intervals fall into a pattern that repeats itself in a 'self-similar' manner. This recurring motif is best described by the mathematics of fractals – the same equations that are followed by the branching of a tree, which looks approximately the same at the millimetre scale of tiny, branching twigs as at the metre scale of branching trunks and boles.

Conversely, in a frail, elderly person who is at high risk of falling, or in someone with advanced Parkinson's or Huntington's, the stride profile is far more random. Indeed, "loss of 'fractalness' is approximately in line with severity of disease," he says. Toddlers, too, start off with low 'fractalness' as they learn to walk, but this improves as they reach maturity in their late teens.

Given that the technology behind this technique is of negligible cost, it could have potential as a 'fall-imminent' alarm, the researchers think. Which, given that falls cost billions of dollars a year in knock-on health costs and loss of independence, could be very useful. Gait analysis can also give much-needed insights into treatment efficacy, which is at present often monitored purely subjectively, by eye. Monitoring minute losses in 'fracticality' could alert physicians to people whose non-fractal gait pattern might be a harbinger of some deeper malaise.