Drift in Posturography Systems Equipped With a Piezoelectric Force Platform: Analysis and Numerical Compensation

Piezoelectric force platforms are electronic instruments widely used for posturography in movement analysis laboratories. As is well known, drift affects piezoelectric transducers; therefore, the accuracy of piezoelectric measurement systems should be periodically checked. The objectives of this paper were as follows: (1) to detect the occurrence of drift in the posturographic signals derived from a piezoelectric force plate (FP) using an easy-to-use standard procedure; (2) to determine the impact on drift itself of two clinically relevant factors: applied load [for values related to children/adolescent weights (range 49-588 N)] and test duration; and (3) to suggest numerical solutions, if any, to compensate for drift effects. Drift in piezoelectric FPs is essentially caused by leakage currents and/or poor insulation resistances. Moreover, it may be considered as a low-frequency event affecting the accuracy of standing posture descriptive parameters. The measurement system outputs available to the end user, namely ground reaction force (GRF) components and center of pressure (CoP) sway, were analyzed both in the time and frequency domains. According to the obtained results, it was possible to demonstrate that drift significantly affects the signals of the posturography system under study and that it can be assessed by the analysis of the GRF and CoP component values. As it exhibits a constant rate versus time, drift can be well approximated by a linear regression line. The main results are represented by the independence of drift rate from applied load and test duration and by finding that drift can relevantly affect CoP evaluation for test duration that is equal or longer than 1 min when the applied load is less than 245 N (which could be the case for children and young slim adolescents). High-pass numeric filtering with a cutoff frequency of 0.15 Hz and linear detrending can compensate drift in the presented case.