Although much research has been devoted to constructing specific models or to measuring the response characteristics of seated subjects, investigations on a mathematical human model on a seat with a backrest to evaluate vehicular riding comfort have not yet attracted the same level of attention. For the responses of a seated body to vertical vibrations, mathematical models of the mechanisms must be at least two-dimensional in the sagittal plane. In describing the motions of a seated body, two multibody models representative of the automotive postures found in the literature were investigated, one with and the other without a backrest support. Both models were modified to suitably represent the different automotive postures with and without backrest supports, and validated by various experimental data from the published literature pertaining to the same postural conditions. On the basis of the analytical study and the experimental validation, the fourteen-degrees-of-freedom model proposed in this research was found to be best fitted to the test results; therefore, this model is recommended for studying the biodynamic responses of a seated human body exposed to vertical vibrations in various automotive postures.