1. Department of Physical Therapy and Rehabilitation Science, and Division of Geriatrics, University of California, San Francisco, 1500 Owens Street, Box 0736, San Francisco, CA 94158, USA
2. Department of Physical Therapy and Rehabilitation Science, University of California, San Francisco, San Francisco, California, USA
3. Department of Psychiatry and Behavioral Sciences, and Department of Epidemiology and Biostatistics, University of California, San Francisco; and San Francisco VA Health Care System, San Francisco, California, USA