1. A Shumway-Cook, PT, PhD, is Associate Professor, Department of Rehabilitation Medicine, University of Washington, Box 356490, Seattle, WA 98195 (USA).
2. AE Patla, PhD, is Professor, Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada
3. A Stewart, PhD, is Professor, Institute for Health and Aging, University of California-San Francisco, San Francisco, Calif
4. L Ferrucci, MD, PhD, is Director, Clinical Epidemiology, INRCA, Florence, Italy
5. MA Ciol, PhD, is Biostatistician, Department of Rehabilitation Medicine, University of Washington
6. JM Guralnik, MD, PhD, is Director, Laboratory of Epidemiology, Demography, and Biometry, National Institute on Aging, National Institutes of Health, Bethesda, Md