1. Department of Family Medicine and Community Health, University of Pennsylvania Perelman School of Medicine; Philadelphia, PA
2. Center for Clinical Epidemiology and Biostatistics, University of Pennsylvania Perelman School of Medicine; Philadelphia, PA
3. Chestnut Hill Hospital; Philadelphia, PA
4. Gait Study Center; Temple University School of Podiatric Medicine; Philadelphia, PA
5. Department of Emergency Medicine, University of Maryland School of Medicine; Baltimore, MD