Pediatric and Young Adult Biomechanical Response to Far-Side Oblique and Lateral Low-Speed Impacts

The importance of head injuries to restrained far-side occupants, both children and adults, has been previously documented, but the literature lacks pediatric biomechanical data in response to far-side loading. Such data is essential for the design of safety countermeasures for far-side occupants. Therefore the objective of this dissertation was to investigate pediatric and young adult far-side dynamics and muscle activity in low-speed lateral and oblique impacts. Thirty male human volunteers, ages 9-14 years (n=20) and 18-30 years (n=10), were tested on a low-speed, sub-injurious crash sled at either 60° or 90°. The safety envelope of the crash pulse was defined by an amusement park bumper car impact. The acceleration pulse was provided by a custom-designed hydro-pneumatically-driven sled system composed of a cart on a set of low friction rails (max pulse: 1.91 g; rise time: 53.8 ms; pulse duration: 146.5 ms). Each subject was restrained by a custom-fit automotive three-point belt system with an electromechanical motorized seat belt retractor (EMSR). The EMSR activated 200 ms prior to initiation of the crash pulse and provided a pre-tensioning shoulder belt load of approximately 300 N, with a rise time to peak load of 100 ms. The restraint system was designed such that the EMSR could be active or inactive. Photo-reflective targets were attached to a tight-fitting head piece on each subject and adhered to skeletal landmarks on the spine, shoulders, sternum, and legs as well as along the shoulder belt. Surface electromyography (EMG) electrodes were placed bilaterally on the subject’s neck (sternocleidomastoid, upper trapezius, cervical paraspinous), torso (latissimus dorsi, erector spinae), and lower extremities (rectus femoris). Subjects participated in a set of eight randomized trials, varying in arm position (up/down) and EMSR activation (on/off). Maximum head and spine excursion was calculated and normalized by the subject’s seated height. Motion capture data was utilized to calculate inverse dynamics of the head and upper neck. Muscle activity was compared for bilateral differences, and correlation with age, kinematics, and kinetics. This research was approved by the Children’s Hospital of Philadelphia, Drexel University, and Rowan University Institutional Review Boards. For head and spine kinematics, the pediatric subjects (9-14 years) had significantly greater forward normalized head and spine excursions, as compared to the young adult (18-30 years) volunteers. No effect of age was seen on the lateral normalized displacement of the head and spine. Young adults also experienced significantly greater upper neck forces and moments than the pediatric subjects. EMSR activation, simulating the pre-tensioning of a seat belt, significantly reduced forward and lateral head and spine kinematics. Upper neck kinetics was significantly reduced by EMSR activation when the subjects were seated in the arms up condition. EMSR activation also reduced variability in lateral head displacement across age groups. Muscle response in the far-side impacts demonstrated bilateral differences at both oblique and lateral impact angles. While there were significant relationships between muscle response and occupant dynamics, there was no biomechanically relevant effect of age on the integrated EMG at maximum lateral head excursion. These results suggest that the greater pediatric spine flexibility seen in the sagittal plane does not extend coronally. Although pre-tensioning is primarily indicated as a frontal impact countermeasure, these data demonstrate its efficacy in reducing head excursion that leads to injury in far-side impacts. Low-speed human volunteer tests provide insight into occupant motion at these impact angles in the presence of active musculature. These results are useful for the development of rear seat safety restraints and further advancement of pediatric computational models.Ph.D., Biomedical Engineering -- Drexel University, 201