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Detecting asymmetries in balance control with system identification: first experimental results from above knee amputees

A prosthetic leg can influence balance in various ways, but not all changes in postural performance can easily be identified with the naked clinical eye. Various studies have shown that dynamic posturography is able to detect more subtle changes in balance control. Here, we describe a modification of a new posturography technique we previously developed, which combines dynamic platform perturbations with (non) parametric system identification techniques to detect asymmetries in balance control of four subjects with an above knee prosthesis. The method also estimates the mechanical impedance of the prosthetic ankle joint. The time needed for the experiment and data analysis is less than 3 minutes. Results were compared to those of six healthy controls. Our pilot data show clear asymmetries in dynamic balance control. We also found asymmetries in weight bearing and centre of pressure movements, but the asymmetries in dynamic balance contribution were larger. Finally, asymmetries in weight bearing and dynamic balance in patients were not tightly coupled as in healthy controls. The relative contribution to dynamic balance control of the prosthetic leg was positively related with the stiffness of the prosthetic ankle joint. More transfemoral amputees have to be tested to more extensively evaluate the results of this pilot study.