Epidemiology
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Study of the influence of physical activity and body composition in the context of the disablement process requires analytic techniques that can address time-dependent confounding related to exposures for the occurrence of functional limitation and disability. ⋯ Marginal structural models provide a means to address time-dependent confounding, which can occur in longitudinal studies. These analyses indicate that leisure time physical activity exerts its beneficial effects through reductions in fat mass relative to lean body mass.