A coupled human-seat-suspension model is developed upon integrating asymmetric and nonlinear models of the cushion, suspension and elastic end-stops with a three degrees-of-freedom biodynamic model of the occupant. The validity of the model is examined under harmonic and stochastic vibration excitations of different classes of vehicles, using the laboratory measured data. The suspension performance under continuous and shock excitations, assessed in terms of Seat Effective Amplitude Transmissibility (SEAT) and Vibration Dose Value (VDV) ratio, revealed that attenuation of continuous and shock-type excitations pose conflicting design requirements. It is thus proposed to develop suspension design for optimal attenuation of continuous vibration, while the severity of end-stop impacts caused by shock-type excitations be minimized through design of optimal buffers. Two different optimization problems are formulated to minimize the SEAT and VDV ratios. The first optimization problem is solved to achieve optimal suspension stiffness and asymmetric damping properties, while the peak dynamic deflection is constrained to account for occasional end-stop impacts. The second optimization problem is solved to derive optimal properties of the end-stop buffers under large amplitude excitations. The results suggest that soft and lightly damped suspension with low degree of damping asymmetry coupled with low friction and large suspension mass can enhance vibration isolation performance. Thick and soft elastic buffers with nearly linear stiffness characteristics over a large deflection range reduce the severity of end-stop impacts. The proposed optimal design yields considerable reduction in both SEAT and VDV ratio response under selected classes of excitations.