1. Center for Injury Research and Prevention, Children's Hospital of Philadelphia, Philadelphia, Pennsylvania, USA.
2. Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA.
3. Department of Mathematics and Statistics, Bowling Green State University, Bowling Green, Ohio, USA.
4. School of Nursing, University of Pennsylvania, Philadelphia, Pennsylvania, USA.
5. Wallace H. Coulter Department of Biomedical Engineering, Emory University and Georgia Institute of Technology, Atlanta, Georgia, USA.
6. Sports Medicine and Performance Center, Children's Hospital of Philadelphia, Philadelphia, Pennsylvania, USA.