Ankle joint stiffness and damping pattern under different frequency of translation perturbation

Abstract

The change of e ffective stiffness and damping characteristic of ankle joint are able to indicate degeneration of balance ability due to ageing effect. This paper will discuss the ankle joint stiffness and damping pattern along repeat ed translation perturbation. Six young healthy subjects were exposed to five tr ials of five different frequencies of perturbation (quiet standing, 0.2 Hz, 0.4 Hz, 0.6 Hz and 0.8 Hz). The result showed that the me an of effective stif fness was reduced with the increase of frequency applied ; meanwhile the mean of damping value increased wit h increasing frequency . Additionally , a cubic polynomial curve (u - shape) was estimated to represent stiffness pattern when using curve fitting method with correlation R 2 >0.5 . These estimation s also suggested that ankle joint does not oscillate like sprin g - damper system which is based on inverted pendulum model ; however , it applied a different strategy to maintain balance, in particular during initiation, middle and termination of perturbation. These also indicate the influence of sensory processing and ad aptation to maintain balance under a long period of disturbance . On the other hand, damping pattern seems to be similar over different frequenc ies and under repeat ed perturbation . Besides, the change of stiffness patter n at higher frequency o f perturbati on (0.8 Hz) recommends the change in posture strategy from ankle to hip strategy . The se findings indicated that stiffness and damping are able to describe adaptation of human posture strategy to keep balance and motor learnin g under repeat ed perturbation.