Associations Between 24-h Movement Behavior Composition and Muscle Health in Community-Dwelling Older Adults: The Kyotango Longevity Cohort Study

Background/Objectives: Daily movement behaviors are interdependent within a 24 h period. This study examined associations between 24 h movement behavior composition and muscle-related outcomes and estimated the effect of reallocating time to moderate-to-vigorous physical activity (MVPA) based on these outcomes. Methods: A total of 808 community-dwelling older adults from the Kyotango Longevity Cohort Study were included in this cross-sectional study. Waking behaviors (sedentary behavior, light-intensity physical activity, and MVPA) and sleep were assessed using accelerometers and sleep diaries. Daily time spent performing each behavior was averaged using the compositional mean to create a 24 h composition. Compositional data analysis and Firth’s penalized logistic regression were used to examine associations between movement behavior composition and muscle-related outcomes (low muscle strength, low muscle mass, and sarcopenia defined by the Asian Working Group for Sarcopenia 2025 criteria). Compositional isotemporal substitution analyses were performed to estimate time reallocations toward MVPA. Results: A higher relative contribution of MVPA was associated with lower odds of low muscle strength (odds ratio, 0.67; 95% confidence interval, 0.48–0.93; p = 0.016) and body mass index-adjusted low muscle mass (odds ratio, 0.81; 95% confidence interval, 0.66–0.99; p = 0.042). No associations were observed for height-adjusted low muscle mass and sarcopenia. Reallocating time from other waking activities toward MVPA was associated with lower odds of these outcomes, whereas no significant associations were observed for reallocations from sleep. Conclusions: A higher relative contribution of MVPA was associated with lower odds of low muscle strength and BMI-adjusted low muscle mass. Small reallocations of time from other waking activities toward MVPA may maintain muscle health in older adults.