1 Center for Injury Research and Prevention, The Children's Hospital of Philadelphia, Department of Pediatrics, University of Pennsylvania, Philadelphia, PA Center for Injury Research and Prevention, The Children's Hospital of Philadelphia, Department of Bioengineering, University of Pennsylvania, Philadelphia, PA Department of Anesthesiology and Critical Care Medicine, The Children's, Hospital of Philadelphia, Philadelphia, PA Injury Biomechanics Research Laboratory, The Ohio State University Columbus, OH.