Nonlinear behavior of sitting humans under increasing gravity.

To determine the nonlinear properties of the upright sitting human body, driving-point impedance as well as the transmissibility from seat to head were determined under increased gravity on a centrifuge. Under this condition a shift of resonances to higher frequencies was observed. At the same time the transmissibility increased distinctly. To explain this behaviour with a model, a nonlinear multi-degree-of freedom model of the upright sitting body was calculated. It elements represent identifiable body parts. Protective measures for normal gravity conditions, as well as for sustained acceleration, were derived.