Non-linear re-calibration of force platforms.

Force platforms (FPs) are used in human movement analysis to measure the ground reaction force and the center of pressure (COP), and calculate derived kinetic and energetic quantities. We propose a re-calibration method that compensates for the FP non-linearity induced by top plate bending under loading. The method develops a previous solution that was proposed for a linear re-calibration and proved suitable for both local and global error compensation (Cedraro et al., 2008). The new method was experimentally tested on 4 commercial FPs by estimating the non-linear re-calibration matrix in a first training trial and by using it to assess the three force components and the COP in a validation trial, comparing the new method to the previously proposed solution for global, linear re-calibration. The average COP accuracy (mm) in the training trial was (mean±std): 2.3±1.4, 2.6±1.5, 11.8±4.3, 14.0±2.5 for the 4 FPs before re-calibration, and 0.7±0.4, 0.6±0.2, 0.5±0.2, 2.3±1.3 after non-linear re-calibration. In the validation trial, for one of the 4 tested FPs, mean errors for the three force components (N) and COP (mm) were: 3.6±2.3 (F(X)), 3.0±0.7 (F(Y)), 5.0±2.5 (F(Z)), 1.2±0.68 (COP) after linear re-calibration, and 2.5±0.7 (F(X)), 2.6±0.5 (F(Y)), 3.9±1.2 (F(Z)), 0.6±0.3 (COP) after non-linear re-calibration. The proposed global, non-linear method performed equally well as the local, linear re-calibration method, proving well-suited to compensate for the mild non-linear behavior of FP with the advantage of estimating a single re-calibration matrix.