Occupant response to side-impact collisions is studied with a mathematical vehicle/occupant model, the MVMA Two-Dimensional Crash Victim Simulator. A primary goal is the investigation of head-torso relative motion and neck forces and moments. Computer simulations were performed in conjunction with a study on properties of the neck in lateral motion (not reported on here) involving 96 subjects. The subject pool consisted of six carefully defined population segments which represented young, middle-age, and elderly male and female adults of average stature. The side-impact simulations make use of much pertinent data resulting from the volunteer subject testing program, including head-neck lateral bend range of motion, sternomastoid muscle group strength, reflex time, and anthropometry. For the various population segments, the series of computer simulations investigates 10- and 30-mph car-to-car impacts, the effect of muscle reflex with muscle tension buildup, and the effect of varying degrees of pre-impact constant tension. It is determined that neck muscle contraction may significantly lessen the likelihood of hard-tissue injury resulting from excessive lateral flexion. The lesser muscular strength of female and elderly crash victims indicates greater susceptibility to neck injury for these groups. THE TERM “WHIPLASH” has commonly been applied to encompass the complex interactions which occur when the occupant of a vehicle is struck from the rear, resulting in cervical hyperextension-hyperflexion. It is also the case, however, that individuals incur “whiplash” injuries from forces resulting in lateral flexion of the neck such as would occur on side impact.A previous study conducted for the Insurance Institute for Highway Safety entitled “Bioengineering Study of Basic Physical Measurements Related to Susceptibility to Cervical Hyperextension-Hyperflexion Injury” (1)* was designed to study the influence of such basic factors as sex, age, and stature on neck properties in the sagittal plane. (See also References 2, 3, 4 and 5.) A second investigation, entitled “Basic Biomechanical Properties of the Human Neck Related to Lateral Hyperflexion Injury” (6) was conducted from October 1973 through December 1974. It was undertaken with the same representative population to investigate physical characteristics of the neck in lateral flexion and represents the culmination of nearly three years of intensive research into the basic mechanisms relating to susceptibility of individuals to “whiplash” injury. Only the mathematical-model simulation phase of this investigation is reported here. A companion paper (7) discusses in detail the other phases of the study. A summary of the research objectives is presented in the following section.