  Anticipating changes to vehicle interiors with future automated driving
systems, the automobile industry recently has focused attention on crash
response in relaxed postures with increased seatback recline. Prior research
found that this posture may result in greater risk of lumbar spine injury in
the event of a frontal crash. This study developed a lumbar spine injury risk
function that estimated injury risk as a function of simultaneously applied
compression force and flexion moment. Force and moment failure data from 40
compression-flexion tests were utilized in a Weibull survival model, including
appropriate data censoring. A mechanics-based injury metric was formulated,
where lumbar spine compression force and flexion moment were normalized to
specimen geometry. Subject age was incorporated as a covariate to further
improve model fit. A weighting factor was included to adjust the influence of
force and moment, and parameter optimization yielded a value of 0.11. Thus, the
normalized compression force component had a greater effect on injury risk than
the normalized flexion moment component. Additionally, as force was nominally
increased, less moment was required to produce injury for a given age and
specimen geometry. The resulting injury risk function can be utilized to
improve occupant safety in the field.
