While injury prevention strategies including legislation, behavior and awareness programs, and improved child restraint systems have reduced the risk of injury to children involved in crashes it is still widely accepted that motor vehicle crashes (MVCs) continue to be the leading cause of death and disability for children. Each year, more than 5,000 children and adolescents under the age of 21 die as the result of MVCs, 90,000 children are hospitalized, and over 2 million receive medical treatment for injuries sustained in car crashes. The most frequently severely injured body regions are the head and neck. Protection of the head and neck begins with the response of the thorax because the thorax is the main body structure that interacts with shoulder belt components of child restraint systems during a frontal impact MVC. The compressive response of the thorax helps dictate crash deceleration input to the thoracic spine, which in turn affects the kinematics of the cervical spine and ultimately the head.

The efficacy of child restraint systems is evaluated using anthropomorphic test device (ATDs) that are intended to mimic the geometrical size and biofidelic response of a child during an MVC event. Due to a (continuing) lack of pediatric post-mortem human surrogate data, child sized Hybrid III ATDs’ anthropometries and biofidelic responses are based largely on scaling of the midsize male Hybrid III ATD which was designed at a time when vehicle safety restraint use was low, airbags were almost nonexistent, and the role of muscle tetanus in crash dynamics was not well understood. As a result, the adult ATD thoracic response to anterior compression is now criticized as stiffer than the response of a human. Although scaling from the adult ATD facilitated child ATD design, a child is not merely a small adult. Changing body proportions, anatomical shapes, and material properties exist as children age and mature into an adult body form.

A novel method for determining the effective mass, damping, and stiffness of the thorax to compressive deflections utilizing nonparametric system identification techniques in combination with perturbation analysis has been developed. A series of three ATDs (6 year old, 10 year old, and midsize male Hybrid IIIs) and five adult postmortem human surrogates (PMHS) were tested in this study. ATDs and PMHS were tested in a custom made apparatus at defined levels of initial thoracic compression and perturbation speeds to encompass a range of crash relevant deflection. Impulse response functions calculated for each combination of compression and speed were fit by an underdamped second-order system for which effective mass, damping, and stiffness values were calculated.

Expected differences in effective mass, damping and stiffness values between the three ATDs were not substantiated in the study. The only significant difference (p = .001) was between the stiffness values of the 6 year old (mean 72.0 ± 30.8 N/mm) and the midsize male (mean 99.2 ± 24.3 N/mm). However, anticipated differences between the PMHS and adult ATD thoraces were confirmed in this study. PMHS effective mass (mean 5.71 ± 1.25 kg) was significantly larger (p <.001) than ATD effective mass (mean 2.66 ± 0.27 kg) and PMHS effective damping (mean 677.7 ± 274.1 Ns/m) was significantly larger (p = .002) than ATD effective damping (mean 375.6 ± 65.0 Ns/m) while PMHS effective stiffness (mean 38.4 ± 22.2 N/mm) was significantly less (p <.001) than ATD effective stiffness (mean 99.2 ± 24.3 N/mm). The findings of this study suggest that the midsize male Hybrid III thorax is overly stiff and underdamped with poor mass distribution when compared to the human thorax and that large differences between the midsize male, 6, and 10 year old Hybrid III thoraces do not exist. Testing the pediatric thorax is necessary to evaluate how well the child ATDs replicate the thoracic compressive response of children.