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Dynamic properties of flexible polyurethane foam materials for car seats are highly complicated. In this paper, characterizations of dynamic stiffness of several foam specimens based on static stiffness obtained from IFD(Indentation Force Deflection) curve measurements are presented. It is observed that dynamic stiffness and its drift with static loading duration in logarithmic scale are proportional to the static stiffness at a given static loading regardless of types and dimensions of the foam. A three degree-of-freedom model of seated human body based on apparent mass measurements together with the characteristics of foam materials were incorporated for transmissibility prediction of the passenger-seat system.