On the face of it, human gait seems a symmetrical affair. But int he last few years, varfious ways of measuring gait have shown otherwise. Video studies, sensory shoes and force-measuring mats have shown that every individual has a different gait, characterised by tiny asymmetries in stepping patterns, forces and accelerations.

These idiosyncracies are revealing but they dramatically complicate our understanding of gait, making it impossible for biomechanicists to agree on the fundamental properties of gait. Strange as it may seem, there is no agreement on the invariant parameters that all human walkers must share.

Without this definition, it is hard to model gait accurately and impossible to agree on how best to rehabilitate people with an impaired gait.

So the work of Yifang Fan and pals at Jinan University in China is a refreshing step forward (so to speak). Fan and co say they have discovered an independent measure of gait which they call the vertical gait reaction force. This is a measure of the amount by which a person’s centre of gravity raises during two complete steps (with the left and then the right foot).

In healthy adults, and Fan and co have tested 173, this force is symmetrical with regard to left and right foot, a finding they call the principle of least action (in other words, the body does no more than it has to move you around). That’s useful, they say, because no other measure gives this kind of symmetric signal for all healthy people.

In adults carrying an ankle injury or with arthritis, the pattern of force is not symmetrical. That allows a quick diagnosis simply by measuring this force and may in future point to the type of rehabilitation that may treat the condition.

Ref: arxiv.org/abs/0905.3499: Least Action Principle in Gait