Decomposition of three-dimensional ground-reaction forces under both feet during gait.

Three-dimensional ground reaction forces (3D-GRF) are essential for functional evaluation for rehabilitation. A platform path is required to obtain the 3D-GRF. The main shortcoming of these platform paths is that during double stance phases of gait, both feet can be placed on the same force platform causing the need for decomposing the 3D-GRF under each foot. Despite the high number of studies on force decomposition, there is still no method on the decomposition of 3D-GRF based on data from platforms. Objective: This study aims to present an automatic method using parametric curve fitting modeling to increase the accuracy of decomposition of 3D-GRF during double stances under each foot. Methods: The decomposition method was applied to the global 3D-GRF using 3rd order polynomial, sine, and sine-sigmoid functions. The computed 3D-GRF was compared to the 3D-GRF independently recorded by force platforms for each subject. Results: The relative average error between the computed 3D-GRF and the recorded 3D-GRF were equal to 3.3 +/- 1.6%. In details for the vertical, antero-posterior, and medio-lateral GRF, these errors were 2.9 +/- 1.6%, 6.3 +/- 4.3%, and, 9.5 +/- 3.6%, respectively, for 30 subjects. Conclusion: The global error on the GRF is the best one in the literature. This method can be validated on various populations with musculoskeletal disorders.