Physical function and cognitive decline in the older population: an investigation on prospective associations and measurement challenges

Rizzo, Isabella (2023) Physical function and cognitive decline in the older population: an investigation on prospective associations and measurement challenges. MPhil thesis, Birkbeck, University of London.

Abstract

Physical fitness is thought to be associated with optimal levels of cognition in older adults, and is being increasingly investigated as a modifiable risk factor for dementia. Given the growing rates of cognitive impairment in the ageing population, establishing the association between physical fitness and dementia holds great clinical significance. The current work systematically reviewed prospective cohort studies looking at the association between baseline performance-based measures of physical function and the subsequent development of cognitive decline or dementia diagnosis. Findings from our review showed longitudinal associations between most of the physical function variables that we considered and cognitive outcomes, whereby greater levels of physical activity were associated with reduced risk of cognitive decline and dementia. The importance of these findings was discussed, highlighting the value of these prospective associations for policy making and healthcare systems. Some recommendations were made to improve methodological rigour in future longitudinal research, in light of the methodological heterogeneity and inconsistent reporting that we observed in the studies that were retrieved. Another issue which emerged from our search was the predominant use of self-reported measures to assess physical fitness. The current work reviewed the literature investigating the consistency between self-reported and performance based measures of physical function, and conducted a secondary data analysis of cross sectional data, with the aim to investigate the agreement between self-reported and performance-based measures of physical functioning. To this end, a secondary data analysis was conducted looking at the correlation between a self-reported measure of physical fitness and three objective measures of physical functioning. Further, our secondary data analysis included stepwise regression analyses investigating cognitive outcomes from models based on age, a self-reported measure of physical fitness and a measure of grip strength. Our correlational analysis showed modest rates of agreement between self-reported and performance-based measures of physical fitness, which were broadly in line with the correlations found by previous research. Results from the regression models showed that grip strength did not explain any additional variance in the cognitive outcome that was not already accounted for by the PFQ, suggesting that both measures are capturing the same construct. These findings were discussed in light of the concept of convergent validity, and some suggestions were made to address suboptimal convergence between self-reported and objective measures of physical fitness. Further, the use of regression analyses investigating the amount of overlapping variance explained by self-reported and performance-based measures was suggested as a method to validate self-reported questionnaires of physical fitness in future research.

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