Quantifying human balance : Analysis of force plate data

Sammanfattning: There is a lack of reliable methods for assessment of balance suited for routine use in clinical settings. The thesis focuses on analysis of force plate data for evaluation of balance during normal, unperturbed standing and sitting. Some commonly used force plate based methods during unperturbed standing are presented. Traditional measures, which have been used to quantify the postural sway during standing, were applied to seated children with and without spina bifida. Fundamental differences, that can not be observed visually, were found between the groups. However, the reliability on an individual level was limited since the method could not be used to distinguish between subjects from the two groups. It is not only of interest to measure how much the subject sways, but also what movement strategy the subject uses, that is, how he or she sways. A method for analysis of the movement strategy in the sagittal plane during standing is presented and tested. The subject's movements are compared to the movements of an inverted pendulum model. For movement analysis a kinematic measurement system is usually required, while the inverted pendulum method only needs force plate data for the analysis. To investigate the relationship between different force plate measures the measures were a) compared to each other and b) compared to a clinical balance test. Results on stroke patients showed that many of the measures quantify different aspects of standing. The standard deviation of the vertical ground reaction force was the only measure that was significantly correlated to the clinical balance test. Measurements on healthy adults and three groups of subjects with reduced or not fully developed balance were analysed to investigate the vertical force further. The healthy adults damped the vertical bodyoscillations caused by the heartbeats more efficiently than the other three groups. Finally, it was investigated under what conditions system identification can be used to model the balance system. In contrast to previous studies the model parameters were successfully estimated without any external perturbations and without a kinematic measurement system.