BACKGROUND: Clinical trials to assess interventions for mobility disability are critically needed; however, data for efficiently designing such trials are lacking. METHODS: Results are described from a pilot clinical trial in which 424 volunteers aged 70 to 89 years were randomly assigned to one of two interventions-physical activity or a healthy aging education program-and followed for a planned minimum of 12 months. We evaluated the longitudinal distributions of four standardized outcomes to contrast how they may serve as primary outcomes of future clinical trials: ability to walk 400 meters, ability to walk 4 meters in <= 10 seconds, a physical performance battery, and a questionnaire focused on physical function. RESULTS: Changes in all four outcomes were interrelated over time. The ability to walk 400 meters as a dichotomous outcome provided the smallest sample size projections (ie, appeared to be the most efficient outcome). It loaded most heavily on the underlying latent variable in structural equation modeling with a weight of 80%. A 4-year trial based on the outcome of the 400-meter walk is projected to require N = 962 to 2,234 to detect an intervention effect of 30% to 20% with 90% power. CONCLUSIONS: Future clinical trials of interventions designed to influence mobility disability may have greater efficiency if they adopt the ability to complete a 400-meter walk as their primary outcome.
Copyright the Gerontological Society of America. Reproduced by permission of the publisher.

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