Abstract
Abstract
The purposes of this study are first to understand the kinematics of the human
cervical spine in rear impacts, and then to examine the effectiveness of a new seat concept for
reducing loading level to the cervical facet joints. A finite element based human model named
the Total Human Model for Safety (THUMS) was used to analyze the local and global
kinematics of the spine. Precise geometry of the cervical vertebral bodies and related soft
tissues were incorporated into the model. Their material properties were carefully defined
referring to anatomy textbooks. The model was first validated against human test data in
literatures by comparing vertebrae motion as well as head and neck responses. The model was
then used to simulate a rear collision at a delta-V of 25 km/h. The motion of the head and
torso was characterized by interaction events between the occupant and the seat. To
understand the mechanism of cervical joint motion during a rear impact, kinematics of the
cervical vertebrae was analyzed in a local coordinate system defined along the joint surface. It
was found that the trajectory of the vertebra was characterized by featuring points
corresponding to the interaction events. Based on the findings from the analysis, a new seat
concept was proposed to reduce the magnitude of joint deformation. For the elevated test
speed, the lower seat back frame was reinforced to withstand the severity of the impact. The
foam material in the seat back was softened to catch the occupant torso with less shock. The
location of the head restraint and the stays were modified to provide a firm support to the
occiput. To verify the effectiveness of the new seat concept, another simulation was
conducted under equivalent impact conditions. The magnitude of joint deformation calculated
using the THUMS suggested that the new concept seat was effective in reducing the loading
level to the cervical soft tissues especially stretch. Although shear deformation was increased
by the reinforced seat back frame, the magnitude of the increase was thought to be acceptable
considering the physiological range of joint motion. The study also revealed that the relative
motion between the head and torso was not a transient event but a continuous phenomenon
during impact. It was suggested that seat performance should be evaluated by conducting
dynamic tests rather than by static measurement.
Keywords - whiplash, rear impact, cervical spine, joint capsule, human model