This paper presents a three-dimensional dynamic modeling of the upper part of a passenger's body, carried out using a three-mass system for the head, chest and hip. The equations of motions for the system are formulated using the Lagrange method. A dynamic excitation apparatus for the human body was manufactured, and experiments on the human body have been conducted for three exciting directions, the frontal, the transverse, and the vertical. Joint parameters, such as damping and spring constants of the neck, abdomen, and lower-hip portion are identified using the multistage method of the partial optimization technique with a least squares approach, for minimizing the discrepancy between the theoretical and excitation test results. It is shown through the comparison of calcurated results with those of experimental results that the motion of a human upper body under sinusoidal excitations can be easily analysed using this approach.