Rollover accidents are one of the major types of crashes contributing to the serious or fatal injuries to occupants [1]. The roof crush in rollover accidents is associated with serious injuries to head and neck system [2]. The roof crush intrudes into the occupant survival space and imparts force to the head. The excessive force on the head subjects the cervical spine to injurious level. A commonly observed cervical spine injury in rollover accidents is locked facets with no major bony fractures that are often associated with the flexion-distraction type of loading [3]. Although numerous studies addressed the mechanism of locked facet injuries and the survival space issues [4-9], limited comprehensive efforts have been advanced so far. It is noted that humans tend to duck their head while startled due to sudden fear [10,11]. It is hypothesized that the occupants inside the vehicle tend to duck their heads as a protective mechanism to avoid impact on the head. Although range of motion of the cervical spine is well reported [12], the change in downward movement of the head-neck system (ducking) is not studied well. The present study quantifies the downward movement of the head-neck system of volunteers while seated erect.
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