Background. To evaluate development and progression of functional limitation and retain comparability with established approaches, we raised the measurement ceiling of commonly used self-report and performance-based measures of function. This study evaluated the utility and concurrent validity of these expanded measures. Methods. The study population consisted of 3075 black and white men and women aged 70 to 79 years, with no reported mobility limitations or disability, participating in the Health, Aging, and Body Composition, or Health ABC study. Self-report measures were expanded by ascertaining ease of performance and including more demanding levels of some tasks. A single foot stand and narrow walk supplemented an established performance battery. For walking endurance, we developed the Long Distance Corridor Walk (LDCW), which includes distance covered in 2 minutes and the time to walk 400 m. Results. The expanded self-report items identified one half of the men and one third of the women as exceptionally well functioning and 10% to 13% of men and 21% to 36% of women with lower capacity. The supplemented and rescored performance battery discriminated function over the full range. The LDCW further differentiated walking capacity at the high end and also identified a subgroup with limitations. The self-report and performance measures were significantly, but weakly, correlated (0.13–0.35) and were independent predictors of walking endurance. Conclusions. Well-functioning persons in their 70s exhibit a broad range of functional capacity readily ascertained by expanded self-report and performance tests. Significant associations among these measures support their concurrent validity, but generally weak correlations indicate they tap different, but important, dimensions of physical function.

Over the past three decades, considerable effort has been dedicated to quantifying the pace of ageing yet identifying the most essential metrics of ageing remains challenging due to lack of comprehensive measurements and heterogeneity of the ageing processes. Most of the previously proposed metrics of ageing have been emerged from cross‐sectional associations with chronological age and predictive accuracy of mortality, thus lacking a conceptual model of functional or phenotypic domains. Further, such models may be biased by selective attrition and are unable to address underlying biological constructs contributing to functional markers of age‐related decline. Using longitudinal data from the Baltimore Longitudinal Study of Aging (BLSA), we propose a conceptual framework to identify metrics of ageing that may capture the hierarchical and temporal relationships between functional ageing, phenotypic ageing and biological ageing based on four hypothesized domains: body composition, energy regulation, homeostatic mechanisms and neurodegeneration/neuroplasticity. We explored the longitudinal trajectories of key variables within these phenotypes using linear mixed‐effects models and more than 10 years of data. Understanding the longitudinal trajectories across these domains in the BLSA provides a reference for researchers, informs future refinement of the phenotypic ageing framework and establishes a solid foundation for future models of biological ageing.