This initiative invites applications for support of analyses of existing data (or data whose collection will be completed by one year from the date of this FOA) from older populations with high prevalence of mobility disability, low muscle mass, and low muscle strength (weakness). Interventions targeted at low muscle mass and/or weakness may prevent or reduce mobility disability in some older persons. To assess the efficacy of such interventions against mobility disability, there is a need to test them in persons in whom muscle mass and/or strength are (or will be) sufficiently low to be likely contributors to disability. Thus there is a need for evidence-based diagnostic cut-points to define target populations for treatments. Applications submitted in response to this FOA should support development and evaluation of diagnostic cut-points based on analyses of relations of mobility disability to muscle mass and strength. These analyses should extend and expand upon analyses to date on this topic, which have identified and proposed cut-points for low muscle mass and weakness. Studies supported through this FOA should clarify relations between muscle mass and strength, impaired physical function, and mobility disability, and their implications for setting diagnostic cut-points, through analyses of data from populations with substantially more individuals with mobility disability than were included in previous analyses. Applicants are encouraged to consider combining datasets from multiple populations that contain information on all of the following: at least one objective measure of muscle mass at the same anatomic site(s) common to all included studies, at least one direct measure of muscle strength common to included data sets, gait speed, and self-reported mobility status in populations of older adults. Analyses of relations to other measures of strength, muscle power, and physical performance (e.g., Short Physical Performance Battery), and mobility disability are encouraged.