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.

Ambient temperature has been associated with daily step counts, but evidence characterizing continuous, non-linear relationships in older adults remains limited, particularly in large cohorts with repeated daily measurements. This study examined the association between daily mean temperature and objectively measured daily step counts in the Kyotango Longevity Cohort, conducted in a region of Japan recognized for its high longevity. This cross-sectional analysis included 4344 person-days from 769 community-dwelling adults aged ≥65 years (mean age 74.0 years; 59.3% women). A generalized additive mixed model examined the non-linear association while accounting for repeated measurements and relevant covariates. A significant non-linear association was observed (effective degrees of freedom = 3.3; χ² = 21.5; p < 0.001). Predicted step counts peaked at 14,941 steps/day (95% confidence interval, 14,567–15,350) at an estimated optimal temperature of 20.4 °C (95% confidence interval, 17.6–22.3) and declined toward colder and warmer conditions. Compared with the optimum, predicted step counts were 828 steps/day lower at −10 °C and 1448 steps/day lower at +10 °C. No significant asymmetry was observed. Daily mean temperature showed an inverted U-shaped association with daily step counts. These findings may inform climate-adaptive strategies to support physical activity and healthy longevity.