Optimizing the determination of the body center of mass.

The position or trajectory of the body center of mass (COM) is often a parameter of interest when studying posture or movement. For instance, in balance control studies the body COM can be related to the ground reaction force or to the base of support. Since small displacements of the body COM are important in balance control studies, it is essential to obtain valid estimates of the body COM. The main source of error in the determination of the body COM is the estimation of the masses and centers of mass of the body segments. Especially the determination of the trunk COM is prone to error. In the current study five subjects maintained three postures, differing in trunk angle, during a few seconds. The relation between the center of pressure of the ground reaction force and the vertical projection of the body COM during the postures was used to optimize the trunk COM position. Additionally the subjects performed two lifting movements. The validity of the body COM trajectory estimation during the lifting movements, both with and without optimized trunk COM, was checked by relating the external moment of the ground reaction force with respect to the body COM to the rate of change of the angular momentum of the whole body. It was shown that the correspondence between the external moment and the rate of change of the angular momentum improved after optimization of the trunk COM. This suggests that the body COM trajectory estimation can be improved by the proposed optimization procedure.