Rear impact simulations were conducted using a validated human body FE model representing an average-sized male occupant. Prototype seat models were also prepared to simulate actual rear impact conditions. The features of occupant responses including head and neck kinematics were investigated considering the interaction between the occupant and the seat (and the head restraint). NIC and joint capsule strain (JCS) were taken as injury indicators. NIC is a widely used indicator in laboratory tests, while the joint capsules have recently been focused on as a potential site of neck pain. Precise modeling of the neck soft tissues enabled the estimation of tissue level injury. The results suggested that NIC corresponds to the difference in motion between the head and the torso, while JCS indicates the difference in their position.

Two studies on seat design changes were conducted to examine the contribution from the seat design parameters and to understand the meaning of injury indicators. A parametric study was conducted on thirteen cases where major seat design factors were changed on a single seat configuration, while the second study focused on three different seat configurations with greater differences in dimensions, structure, and mechanical and material properties. The parametric study revealed that the stiffness of the reclining joint greatly affects the resultant NIC values, while JCS was more influenced by the thickness of the upper-end of the seat-back frame. The other finding showed strong correlations between NIC and the head restraint contact timing (HRCT), and JCS and the neck leaning angle (NLA). Introducing the results of the three different seat configurations, the second study suggests that NLA could be used as an injury indicator instead of JCS in dummy tests, while HRCT would not be a good indicator in terms of injury assessment.