The major car and crash related risk factors for Whiplash Associated Disorders (WAD) 1-3 long-term neck injuries in rear-end crashes are the shape of the crash pulse, the seat-force characteristics and the head restraint position. However, the specific roles of these factors are not yet fully understood, which makes it difficult to find adequate countermeasures and to design protective car seats. In order to study these issues, a mathematical MADYMO model of the first version of the Biofidelic Rear Impact Dummy (BioRID I) has previously been developed. In addition, a neck injury criterion, NICmax, has been proposed and evaluated by means of dummy, human and rear-end impact simulations. In this paper the MADYMO BioRID I and four car seats ranked differently according to a disability ranking list are used to study the influence of crash pulse, seat-force characteristics, and head restraint position on the NICmax in rear-end crashes. A set of 64 crash pulses with four pulse shapes, a peak acceleration from 2.5 to 10g, and speed change (Δv) from 2 to 5 m/s were used. Also, 22 real-life rear-end crashes, where the duration of the occupants' symptoms were known, were used in the simulations.The results showed a correlation between the NICmax outcome and the disability ranking of the four seats. The critical parameters regarding the seat force character istics were found to be the recliner torque stiffness and yield limit. The head restraint position had a major influence on the NICmax for one of the four seats. Regarding the crash pulse, the speed change during the first 85 ms of the impact, Δv85 ms, equivalent to mean acceleration during the same time period, was shown to be the best NICmax predictor. For the real-life crash-pulses the levels of NICmax, and the Δv85 ms, correlated well to the duration of the occupants' symptoms.