The interplay between perceived fatigability, intrinsic capacity, and physical activity: network analysis in a British birth cohort study

Background: Fatigability—an individual’s susceptibility to fatigue when performing standardized activities—may arise from diminished functional reserves and contribute to reduced physical activity (PA) levels, potentially reinforcing the disablement process. In this study, we used network analysis to examine the associations among physical and mental fatigability, different domains of intrinsic capacity (IC), and PA. Additionally, we investigated whether fatigability mediates the association between IC and PA. Methods: We ran analyses of cross-sectional data on participants from the MRC National Survey of Health and Development at age 68-69 years (n=1537). Physical and mental fatigability were assessed using the Pittsburgh Fatigability Scale, and PA was self-reported. We quantified 5 IC domains: vitality, locomotion, cognition, psychology, and sensory, using performance-based and self-reported measurements. Networks were estimated based on mixed graphical models stratified by sex. Results: In both sexes, greater physical and mental fatigability were consistently associated with lower scores in 2 IC domains (psychology and locomotion) and PA.The network structure showed that physical fatigability mediated the relation between locomotion and psychology domains and PA.The same applied to mental fatigability, but to a lesser extent and only in women. Conclusions: Perceived physical fatigability is a potentially important factor on the pathway between lower physical and mental resources and activity behavior in older adults. Future work is needed to study the temporality of these associations.