Lower Extremity Anthropometry, Range of Motion, and Stiffness in Children and the Application for Modification and Validation of the Anthropomorphic Test Device

The rear seat is the safest place for children to sit in a vehicle. It is estimated that approximately 92% of children are seated in the rear seat, which leaves another 8% seated in the front seat where there is increased risk for injury caused from the airbag or dashboard. However, those in the rear seat have an increased risk of injury to the lower extremity, especially in a frontal collision. The 4--7 year old (y.o.) population is vulnerable because only 45% are using the proper restraint type. The other 55% are using the wrong restraint type (i.e. lap and shoulder belt) or are completely unrestrained. Pediatric anthropomorphic test devices (ATDs) lack instrumentation below the knee, making it difficult to experimentally measure forces applied to the leg during impact. The goal of this study was to evaluate the anthropometric characteristics, range of motion (ROM) and stiffness response of the pediatric ankle to gain valuable data for the development of a more biofidelic ATD ankle. Forty-two children between the ages of 4--12y.o. were placed into 2 groups (n=21, 4--7y.o. and 8--12y.o.). Anthropometry measurements were taken bilaterally on the lower extremity. ROM measurements were taken using a universal goniometer. Lastly, stiffness of the ankle was measured using a Biodex Isokinetic Dynamometer. Dorsiflexion (DF) appears to be the most sensitive motion, as there was a significant effect of age, type of movement (passive vs. active), and sex. The 4--7y.o. group was the focus for the remainder of the study. Stiffness results revealed DF was 1.5x stiffer than the other motions tested; Plantar flexion (PF), inversion (INV), eversion (EV). Stiffness data were then used to develop stiffness targets for use with the development of an instrumented 6y.o. ATD lower extremity (ATD-MOD). The ATD was altered to have a tibia load cell and representative ankle motion and stiffness. To validate the ATD-MOD, the leg was measured for anthropometry, ROM, and stiffness. Lastly, the leg was evaluated with knee bolster airbag (KBA) tests. ATD-MOD anthropometry was similar to the volunteers. Total ROM improved from the standard 6y.o. ATD to the ATD-MOD form 130° to 100° of total motion, however, the amount of DF and PF were not directionally accurate. Motion in the ATD-MOD was 30° greater in DF and 36° less PF than the volunteers. KBA tests revealed that the most injurious scenario was with the toes on the dashboard. The airbag axially loaded and forcefully DF the ankle, resulting in a tibia index above the injury threshold. The greatest moments were recorded in the tibia and femur when the airbag struck the knees and the feet were on the floor, but the values did not exceed the injury threshold. The information gained from this study will benefit the automotive industry by providing critical information necessary to produce a more biofidelic ankle in pediatric ATDs. This information will also allow researchers to gain insight into the injury mechanism and forces experienced by the leg, with the overall goal of decreasing musculoskeletal injury risk to children.