Association between physical activity bout length and physical functioning in older adults using absolute and relative intensity thresholds: A longitudinal cohort study.

BACKGROUND: Defining moderate-to-vigorous physical activity (MVPA) relative to individual capacity, instead of absolute intensity cut-points, may provide a more meaningful representation of physical activity in relation to age-related decline in physical functioning. We examined whether associations between MVPA energy expenditure (MVPAEE) and changes in physical functioning differ when MVPA is defined using absolute versus relative intensity cut-points, and whether bout length modifies these associations in older adults. METHODS: This four-year follow-up study included 295 community-dwelling participants aged 75, 80, and 85 at baseline. Physical functioning was assessed using six-minute walking distance (6MWD) and five times sit-to-stand test (FTSTS) time. MVPAEE was measured using thigh-worn accelerometers combined with chest-worn electrocardiography. MVPAEE accumulated in bouts of varying lengths was defined using both absolute intensity cut-points and cut-points relative to individual capacity. Associations between baseline MVPAEE and changes in physical functioning were examined using linear regression models and compositional data analysis. RESULTS: Physical functioning declined during the follow-up. Higher relative MVPAEE was associated with a smaller decline in 6MWD (β = 0.27, 95% CI [0.08, 0.46]). Compositional data analyses showed that a higher proportion of MVPA relative to light-intensity activity energy expenditure was associated with a smaller decline in 6MWD (βilr = 0.25-0.31), independent of bout length. Absolute MVPAEE and bout-specific accumulation were not associated with changes in 6MWD or FTSTS. CONCLUSIONS: The total volume and intensity of physical activity relative to individual's capacity, rather than bout length, may be key factors in counteracting short-term declines in physical functioning in older adults.