The second order autoregressive model in the evaluation of postural stability

The properties of a new postural stability criterion based on the analysis of the difference between centre of pressure and centre of gravity are discussed. This difference may be regarded as the control variable representing active work of ankle muscles allowing to maintain equilibrium and can be effectively modelled as the second order autoregressive process. The model includes two parameters, which allow us to compute stability margin and respective peak frequency considered here as stability biomarkers characterising the human posture control system. The approach, originally designed for quiet stance, lets us effectively assess the stability changes during performance of postural tasks at different levels of difficulty. The latter feature may indicate that the method can find clinical application in the assessment of postural disorders as a supplementary tool reinforcing traditional measures of postural sway. This is particularly convincing in view of the fact that the correlations between old and new measures are low, what implies that the stability biomarkers contain different information concerning the stability of posture as compared with other indices.