An estimation of joint kinematics for a standing reach task using ground reaction data

The objective of this work was to derive a procedure able to estimate joint kinematics, relative to a simple, yet functionally relevant, motor task, starting from ground reaction data. The minimum number of input data has been used: force platform data, few and simple measurements relative to the subject, and protocol-specific parameters. Standing reach (SR) is the motor task analysed. The biomechanical model is a two degrees-of-freedom inverted pendulum moving on the vertical sagittal plane. Joint kinematics has been estimated solving the related direct dynamic problem stated in function of ground reaction data. The original nonlinear differential equation system of the model showed a high sensitivity to errors affecting initial conditions and experimental input data. Consequently, an approximate solution has been looked for in order to reduce the coupling between the model differential equations. This was possible taking into account the peculiar characteristics of the motor task. An optimization procedure has been deemed necessary in order to minimize the effects of the assumed approximation. The method has been tested both with simulated and with experimental data. In this latter case the validation of the angular kinematics estimated by the proposed method has been performed by means of data obtained by a stereophotogrammetric system. Results show a satisfactory behaviour of the whole optimization procedure. Very good results have been obtained in the case of slow reaching tasks.