 As the life expectancy of older Americans increases, prevention of age-associated physical function decline and disabilities has emerged as a major clinical and public health priority. A critical factor in an older persons ability to function independently is mobility, the ability to move without assistance. Older people who lose mobility are less likely to remain in the community, have higher rates of morbidity, mortality, and hospitalizations and experience a poorer quality of life. While several studies suggest that physical activity may prevent physical disability, including mobility disability in both healthy and frail older adults, definitive evidence is lacking. A Phase 3, randomized, controlled trial is needed to fill this evidence gap. Currently data to estimate sample size needs for such a trial are insufficient and further feasibility data should be gathered before such a trial can be effectively designed and implemented. 