A neck injury rate of up to 30% has been reported for older adult occupants of rear motor vehicle collisions (RMVCs) and older adults experience longer recovery times, as compared to younger RMVC occupants. Females experience higher neck injury risk and longer recovery times, as compared to males. Some epidemiological studies have found head restraints to be ineffective, while others have reported an effectiveness rate reaching only 20% for neck injury reduction. The specific hypotheses are: 1) for each spinal level, prevention of neck injuries in older adults (biomechanical instability, and ligamentous, neural and/or vascular injuries) depends upon the specific neck injury prevention system;and 2) for each neck injury prevention system, the potential neck injury severity depends upon the occupant gender and peak RMVC acceleration. There are three specific aims: 1) to advance our biomechanical understanding of traumatic neck injuries by developing a Human Model of the Neck (HUMON) and RMVC simulation apparatus in our experimental laboratory. HUMON will consist of a fresh-frozen human cadaveric specimen (entire head and neck through to T1 vertebra) stabilized with muscle force replication, mounted to the torso of an anthropometric test dummy. All specimens will be obtained from older adult donors, above 65 years of age, with an equal number of males and females. The RMVC simulation apparatus will include an automobile seat (with HUMON in it) rigidly attached to a sled on linear bearings, impacting mass and its power system, and a braking system;2) to determine the human neck injury tolerance due to RMVC. HUMON, without a head restraint, will be used to obtain baseline data (n=20;10 males and 10 females) during simulated RMVC, to determine the human neck injury tolerance, differences among gender, and to validate its dynamic biofidelity against in vivo data. Neck injuries (biomechanical instability) will be quantified using flexibility testing before and after each RMVC. During the RMVCs, ligament strains, spinal canal and intervertebral foramen narrowing, and vertebral artery elongation (potential injuries to neural and ligamentous tissues and vertebral artery) will be determined using custom transducers and a high speed digital camera. Intact and post-RMVC imaging (CT and MRI) will document clinical injuries. Neck injuries and their prevention will be documented by measuring dynamic intervertebral motions and spinal loads. All neck injury criteria (IV-NIC, NDC, NIC, Nij, Nkm) will be compared. Specific Aim 3 is to evaluate the effectiveness of the active head restraint and energy absorbing seat for neck injury prevention. The protocol of Aim 2 will be repeated to evaluate the role of two newly developed active injury prevention systems in reducing neck injury severity in older adults during RMVCs: active head restraint (n=20;10 males and 10 females), and energy absorbing seat (n=20;10 males and 10 females). Differences in neck injury severity among gender will be determined. The present data, obtained using older adult specimens, above 65 years of age, will: 1) advance our biomechanical understanding of traumatic neck injuries;2) quantify the human neck injury tolerance due to rear motor vehicle collisions;3) evaluate the effectiveness of the active head restraint and energy absorbing seat for neck injury prevention during the collisions;and 4) determine differences among gender. The proposed project will provide important research-based data that may form the basis for future safety standards and help to decrease the frequency and severity of neck injuries in older adults, thus benefiting society.