Pediatric anthropomorphic test devices (ATDs) do not contain instrumentation below the knee, making it difficult to directly measure forces applied to the leg during impact. The goal of this study was to evaluate the anthropometric characteristics and stiffness response of the pediatric ankle in order to gain valuable data for the development of a more biofidelic ATD ankle. Forty-two children between the ages of 4-12 years old were placed into 2 groups (n=21, 4-7 years and 8-12 years). Anthropometry measurements were taken bilaterally on the lower extremity. Ankle range-of-motion (ROM) measurements were taken using a universal goniometer. Lastly, active stiffness of the ankle was measured using a Biodex Isokinetic Dynamometer. Increased plantar flexion (PF) and dorsiflexion (DF) motion in the younger group was seen. Significant differences were noted between groups both in active and passive DF and in stiffness at 30°/sec and 60°/sec. Stiffness data were then used to develop specific stiffness targets for use with the development of an instrumented 6 and 10 year old ATD leg. The information gained from this study will allow researchers to gain insight into the injury mechanism and forces experienced by the leg, with the overall goal of decreasing injury risk to children.