Motor vehicle collisions are the leading cause of death for children ages 5 to 14. Enhancement of child occupant protection is partly dependent on the ability to accurately assess the interaction of child-size occupants with restraint systems. Booster seat design and belt fit are evaluated using child anthropomorphic test devices, such as the Hybrid III 6-year-old dummy., A biofidelic abdomen for the Hybrid III 6-year-old dummy is being developed by the Ford Motor Company to enhance the dummys ability to assess injury risk and further quantify submarining risk by measuring abdominal deflection. A practical measurement system for the biofidelic abdominal insert has been developed and demonstrated for three dimensional determination of abdominal deflection. Quantification of insert deflection is achieved via differential signal measurement using electrodes mounted within a conductive medium. Signal amplitude is proportional to the distance between the electrodes. A microcontroller is used to calculate distances between ventral electrodes and a dorsal electrode in three dimensions. This system has been calibrated statically, and its performance demonstrated in a series of sled tests. Deflection measurements from the instrumented abdominal insert indicate performance differences between two booster seat designs, yielding an average peak anterior to posterior displacement of the abdomen of 1.0 ± 3.4 mm and 31.2 ± 7.2 mm for the seats, respectively. Implementation of a 6-year-old abdominal insert with the ability to evaluate submarining potential will likely help safety researchers further enhance booster seat design and interaction with vehicle restraint systems , and help to further understand child occupant injury risk in automobile collisions.