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

The change of effective 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 repeated translation perturbation. Six young healthy subjects were exposed to five trials 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 mean of effective stiffness was reduced with the increase of frequency applied; meanwhile the mean of damping value increased with increasing frequency. Additionally, a cubic polynomial curve (u-shape) was estimated to represent stiffness pattern when using curve fitting method with correlation R²>0.5. These estimations also suggested that ankle joint does not oscillate like spring-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 adaptation to maintain balance under a long period of disturbance. On the other hand, damping pattern seems to be similar over different frequencies and under repeated perturbation. Besides, the change of stiffness pattern at higher frequency of perturbation (0.8 Hz) recommends the change in posture strategy from ankle to hip strategy. These findings indicated that stiffness and damping are able to describe adaptation of human posture strategy to keep balance and motor learning under repeated perturbation.