Force-plate based computation of ankle and hip strategies from double-inverted pendulum model.

BACKGROUND Using statistical characteristics of the centre of pressure displacement or of the ground reaction forces, it is difficult to have quantitative evaluation of the equilibrium strategy employed (ankle or hip). The purpose of this study is to validate a new force-plate based approach that allows to evaluate postural strategies employed through a method based on a double-inverted pendulum model. METHODS This method allows to compute ankle and hip joint motion only from force plate data and relies on inverse kinematics with the centre of mass considered as the end effector. Furthermore, an index, depending on the covariance between hip and ankle angles, is proposed to quantify the strategy used. To validate the method and the strategy index, we compare the results of our computation to an optical stereophotogrammetry measurement of the angles considered as reference value. FINDINGS The experiments demonstrate that our method provides acceptable results. The root mean square error between computed and measured hip and ankle angles stands between 4.5 x 10(-3)degrees for ankle angle in static condition and 1.1 x 10(-1) degrees for hip angle in hip forced condition. INTERPRETATION The main interest of our method for clinicians is that it allows to retrieve the ankle and hip angles only using a simple and widespread device, the force-plate. Moreover, it proposes a new postural index that can be also be computed without videographic systems.