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Biomechanical models are used with force-plate information to determine the center of mass (COM) trajectory during standing. They are usually based on simplifying assumptions and are often limited to a single-plane analysis. The objectives of this study were to present a three-dimensional (3-D) model to calculate the excursion of the COM of the human body, validate it and compare its performance to a video-based system during quiet standing and antero-posterior (AP) and medio-lateral (ML) self-imposed oscillations. In addition to the vertical displacements of the COM, the originality of the method lies in eliminating the accelerations terms in the model and their related assumptions. The model was able to estimate closely the COM displacements in quiet standing [a root mean square (RMS) of 0.9 mm or less]. For the self-imposed oscillations, the RMS differences were 6.6 mm in the AP and ML directions and 1.6 mm along the vertical axis. For all three testing conditions, the coefficients of correlation of the COM displacements between the model and the video methods were above 0.8 with the exception of the vertical direction, where the values were more variable.