Recent design characteristic changes in a small segment of production passenger car front seats have focused attention on the influence of seat back strength on occupant kinematics and potentially injurious loads placed on occupants during rear end collisions.The National Accident Sampling Study database from the years of 1980 to 1993 was interrogated to determine the relationship between vehicle change in velocity, and the nature and severity of injuries sustained by passengers occupying those seats in rear end collisions.The results of the NASS data analysis show that the yielding seats in most current automobiles perform well as a passive restraint system. When the yielding passenger car seats are compared to the stiffer seat/cab, the passenger car seats offered improved protection. Additionally, the data indicate that the three point restraint system provides protection and restraint for front seat occupants in rear impact.To determine what effect seat back stiffness and other seat characteristics have on occupant responses, several seats were evaluated in dynamic Hyge rear impact sled simulations. Two types of production seats, with yielding seat backs, were modified to obtain higher seat back stiffness characteristics. Other production seats were chosen for various design features (i.e. head restraint and seat geometry, recliner systems, integrated seat belts, etc.), as well as seat back stiffness. All the seats were tested in a rigid environment with a restrained Hybrid III dummy. Seats A, A-M, and A-R, were tested at ΔVs of 9, 16, and 24 kph; all the other seats were tested at these speeds and also at a 40 kph ΔV.The results of the sled tests indicated that stiffer seats do not have any consistent advantages over yielding seats for the complete range of speeds tested. When the production seats were modified to obtain higher seat back stiffness, higher responses were seen at the neck, thoracic spine, and lumbar spine. The other, stiffer production seats of different design also showed no consistent improvement in occupant protection across the speed range. Even a seat with a head restraint located closer to the center of gravity of the occupant's head did not perform better than the baseline seat. Throughout the testing, the most sensitive response to seat design and crash severity was the lower neck extension moment.