The improvement of neck protection in rear impact requires a better understanding of the interactions between occupant, seatback and headrest. A mathematical approach was developed to analyse the interactions and to quantify the influences of different design parameters on neck responses. The first phase of this development consisted of neck modelling. To begin with, the RID-neck was compared to the Hybrid-III neck in terms of sensitivity with regards to some design parameter changes. A series of mini-sled tests showed the sensitivity of the RID-neck to be better than that of the Hybrid-III neck. Following this preliminary study, a numerical neck model was developed on the basis of the RID-neck and of two existing sources of biomechanical data on the human neck behaviour in rear impact. For the second phase of this development, the interactions of the thorax and the pelvis with the seatback were modelized, as was that of the head and the headrest. Component tests were conducted to characterise these interactions. With these data, two types of seatback model were constructed. One is a global seatback modelling and the other a more detailed approach which allows the consideration of more design parameters. To evaluate the models, a series of sled tests was performed and the validation level of the above models was assessed against these tests. Finally the influences on the neck responses of four design parameters - head to headrest distance, seatback joint stiffness, upper and lower seatback stiffnesses - were analysed with this model. Special attention was paid to the interactions between these parameters. The results indicate that softening of the upper seatback allows reduction of all neck injury risk indicators and enhances the headrest performance. Softening of the lower seatback increases the moment force at the C7/T1 joint and the head extension angle. Stiffening of the seatback joint aggravates, for a classical upper seatback structure, the moment loading of the neck and the head extension angle. Only the moment force at the C7/T1 joint is significantly affected by all parameter changes.