STUDY DESIGN: This study used rigid-body and finite-element models of forces in the cervical spine resulting from a rear-end motor vehicle impact based on data from 26 volunteer experiments. OBJECTIVES: To define the magnitudes and directions of internal forces acting on the cervical spine during rear-end impact, and to determine the effects of increasing the impact acceleration and the initial position of the occupant's head with respect to the head restraint. SUMMARY OF BACKGROUND DATA: In a number of studies using volunteers or cadavers, the kinematics of the occupant during a rear-end impact related to "whiplash" of the cervical spine have been reported. Few studies have described the mechanism by which internal spine forces are produced and how they may be affected by interaction of the occupant with the seat and head restraint during impact. METHODS: From a companion study on the response of 26 volunteers to rear-end impact, experimental data on head and torso accelerations were developed. Rigid-body mathematical dynamic modeling of a 50th-percentile male was implemented, along with a finite-element seat model, lap belt, and shoulder belt. The model was first subjected to a rear-impact pulse similar to that used in the volunteer study, first with a peak of 3.5 G, then with a peak up to 12 G. Initial head-to-head restraint distance in the model was varied from 1 to 12.5 cm. RESULTS: The major cervical spine forces were upper and lower neck shear causing intervertebral relative anterior displacements. Increasing the peak acceleration magnitude caused increased neck shear force magnitudes. With the head initially positioned closer to the head restraint, the time difference between the occurrences of the peak upper and lower neck shear forces was smaller; the C7-T1 intervertebral shear displacements were reduced; the head moved more in phase with the torso; extension of the head and neck was reduced; and late head flexion was increased. CONCLUSIONS: In this simulation, anterior shear was the major internal force acting in the cervical spine during rear-end impact. Increasing impact acceleration magnitude directly increased shear force. When the head was initially closer to the head restraint, the magnitude of the shear force was unaffected, but the time difference between its occurrences in the upper and lower neck was decreased and intervertebral translations were reduced. These results suggest how the seat could be improved to reduce peals forces and the time differences between them.