Abstract
Knee-thigh-Hip (KTH) injuries, although not necessarily fatal, pose a significant societal burden in frontal motor vehicle collisions, with differences in KTH injury risk varying with different occupant characteristics such as sex and anthropometry. KTH injuries generally arise when the flexed knees contact the lower instrument panel, knee airbag, or, in rear-seat configurations, the forward seatback or center console. The force is transferred from the impact at the knee, along the femur, and to the hip. Injuries can occur at any point along the load path; however, the force transferred to the hip comprises a fraction of the initially applied force at the knee: the force applied to the knee is expended by the acceleration of the increasingly recruited mass along the KTH load path. As the force-tolerance varies among the various anatomical locations along the KTH load path, the nature of the force transfer along the KTH can affect not only the location of injury but also the risk of injury as it is dependent on both the magnitude and duration of the applied load. Measuring this transfer in humans requires a combined experimental and modeling approach, since direct measurement of hip force would alter the body’s natural mass coupling. Prior work has focused on midsize males and has established the basis for predictive tools, yet the applicability of these findings to a broader population remains uncertain.
Current methods for evaluating KTH force transfer in females rely on the assumption that results from midsize male tests can be geometrically scaled and remain applicable for females. This includes occupant models used to assess vehicle safety, such as physical crash test dummies, which serve as surrogates for the small female KTH force transfer response in standardized tests. While scaling provides a rough estimation in the absence of female test data, analysis of injury patterns in the field data have consistently shown that anthropometry differences alone do not explain higher risk of KTH injuries females face in frontal collisions. Given the persistence of this trend despite overall advancements in automotive safety, the continued reliance on scaling may be concealing important sex-based differences in the KTH force transfer response, thereby limiting continued advancement of protection across diverse occupant groups. Specifically, differences in mass distribution and coupling, factors previously identified in male studies, may constrain the utility of current scaling techniques in representing the female KTH force transfer response to impacts at the knee.
To address this, KTH force transfer was quantified in female cadavers independent of scaling assumptions using an experimental and analytical framework adapted from midsize male studies. Bilateral knee impact tests were conducted in an upright, free-back configuration to isolate inertial behavior, using a hierarchical testing strategy that varied the mass coupled to the KTH region. Observed differences across impact responses at varying rates and mass coupling conditions informed the development of a one-dimensional analytical model, constructed in the reverse order of testing by sequentially reintroducing mass until reaching the WB condition. This process yielded a set of subject-specific models and a generalized small female model capable of estimating KTH force transfer behavior.
These experimental and analytical data were then used to develop more representative KTH injury assessments for small female crash test dummies. Three midsize male assessment methods were adapted for small female dummies and evaluated across impact conditions relevant to automotive safety testing. Biofidelity differences between dummies and female cadavers, primarily related to mass coupling, affected the ability of small female dummies to reliably capture the small female force transfer response. Specifically, scalar KTH force transfer approaches were less effective than the Injury Assessment Reference Boundary (IARB) method, which also incorporated impulse as an additional metric. Despite variations amongst each assessment strategy, their final implementation and conclusions largely aligned with prior midsize male evaluations, reinforcing confidence in the adaption of the methods and emphasizing dummy biofidelity differences as the primary limitation, which was consistent across both sexes.
Finally, the influence of mass differences across sex was examined to identify conditions in which scaling assumptions might fail when using a midsize male to infer KTH force transfer information about a small female. Comparisons between small female and midsize male KTH responses showed that scaling was generally effective. However, there were situations in which the assumption of scaling failed to capture the small female KTH force transfer response, and were attributable to underlying differences in total mass, mass distribution, and mass coupling. These findings reinforced the central hypothesis and provided important considerations for countermeasure design.
Ultimately, this dissertation presents new experimental data and analyses that quantify the biomechanical response of the small female KTH complex. It highlights how differences in total mass, mass distribution, and mass coupling across sex and between humans and ATDs influence the ability of current tools to capture female-specific responses. The injury assessment methods developed for small female crash test dummies provide an actionable alternative to existing methods that do not adequately consider female biomechanics, which improves representation in vehicle countermeasure design and safety evaluation. Deriving these assessments using established midsize male approaches helped address inconsistencies across current practices and supports the adoption of more consistent KTH injury criteria for vehicle safety testing, reducing the risk of over or under optimization in safety system design based on the method selected. Finally, the evaluation of sex-based differences in the KTH impact response emphasizes the importance of population variability in restraint system design and shows that solutions effective for midsize males may not provide equivalent protection for small females. These insights establish essential engineering constraints for future countermeasure development and highlight research avenues that can enhance vehicle safety for female occupants.
