An iterative model for estimation of the trajectory of center of gravity from bilateral reactive force measurements in standing sway

The trajectory of the human bodyʹs center of gravity (CG) during motion is usually determined from measurements of whole-body kinematics. Yet another method for estimating the CG trajectory is based on force-plate data of reactive forces and centers of pressure and on applying the equations of motion. However, using this latter method requires additional information on the rate of change of the angular momentum ( ) of the body about its CG. In this study we present a model for the estimation of the CG trajectory during standing sway, based on the reactive forces and centers of pressures as measured bilaterally, i.e. from two force platforms. An iterative algorithm was developed to evaluate and accordingly correct the estimated CG trajectory. A three-dimensional, four-joint, five-segment model of the human body was used to describe postural standing sway dynamics. In the first iteration, the kinematics of the segments was calculated from the CG trajectory obtained for = 0. From the second iteration, the estimated values of were used to update the CG trajectory. The above method is illustrated on a group of 11 able-bodied subjects and the results obtained indicate that the CG trajectory is negligibly affected by the contribution. The method is further implemented on two subjects with musculo-skeletal pathologies to illustrate the model behavior and resulting CG trajectory in cases where asymmetry during standing can be expected.