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A review of the existing mathematical models of a car occupant in a rear-end crash reveals that existing models inadequately describe the kinematics of the occupant and cannot demonstrate the injury mechanisms involved. Most models concentrate on head and neck motion and have neglected to study the interaction of the occupant with the seat back, seat cushion, and restraint systems. Major deficiencies are the inability to simulate the torso sliding up the seat back and the absence of the thoracic and lumbar spine as deformable, load transmitting members. The paper shows the results of a 78 degree-of-freedom model of the spine, head, and pelvis which has already been validated in +Gz and -Gx acceleration directions. It considers automotive-type restraint systems, seat back, and seat cushions, and the torso is free to slide up the seat back.

Unembalmed cadavers were exposed to −Gx acceleration while restrained by applying a frontal load to the chest. A pre-deployed non-venting production air cushion mounted on a non-collapsible horizontal steering column provided the distributed load. The sled deceleration pulse was determined from a series of Part 572 dummy runs in which the HIC, chest acceleration and knee loads were at but not in excess of the limits specified in the current FMVSS 208. A total of six cadavers have been tested. In three of the runs, there were severe neck injuries of the type which have not been observed previously in belted tests. They include complete severance of the cord, complete avulsion of the odontoid process, atlanto-occipital separation with ring fracture. This study does not claim to establish the injury potential of air bags but uses the air bag to provide a uniform restraining load to the chest to investigate the mechanism of neck injuries.

A biodynamic model of the spine simulated the action of spinal musculature on the head, vertebral bodies and pelvis in the midsagittal plane. Muscle was treated as a force generator whose contractile force was dependant on muscle stretch, stretch rate and neural delay time. Eight model runs were conducted with and without muscle, simulating +Gz and -Gx impact acceleration. The model predicted that spinal musculature was incapable of affecting overall spinal column kinematics. However, as a result of muscle contraction, significantly higher local axial forces were predicted in the discs and facets than were predicted when muscle was absent.