Human-seat interactions are investigated through measurement and analysis of distribution of interface contact force and area under vertical vibration. The time histories of dynamic ischium pressure, effective contact area and contact force on a soft seat revealed significant asymmetry, under large magnitude vibration excitations occurring near the resonant frequency of the human-seat system. The asymmetric response characteristics of the cushion are mostly attributed to the nonlinear force-deflection properties of polyurethane foam materials, contour shape of human buttocks, body-hop motion and cushion bottoming tendencies. The results are utilized to propose a nonlinear and asymmetric seat cushion model incorporating body hop motion and cushion bottoming under vertical vibration. A combined human-seat model is derived upon integrating the proposed cushion model with a bio-dynamic model of the seated occupant. The proposed analytical model is validated under both low and high magnitude excitations using laboratory measured data. A parametric study is performed to study the influence of various design parameters on the comfort and body hop characteristics of the seat. The results show that cushions with high linear stiffness, damping coefficient and large static deflection may induce less vibration transmission under large magnitude excitations. The vibration transmission under such conditions can further be reduced by designing the seat with a flexible seat pan.