Longitudinal trajectories of balance function and their association with social participation in older adults: A 5-year community-based study

Background: Balance function declines with aging and is a key determinant of functional independence in older adults. Although social participation has been suggested as a modifiable behavioral factor, its role in shaping longitudinal trajectories of balance function remains unclear. This study aimed to characterize changes in balance performance over time and to examine the association between social participation and these trajectories. Methods: This longitudinal study included 6,898 community-dwelling adults aged ≥65 years in Hamamatsu, Japan, followed from 2019 to 2024. Balance function was assessed using one-leg standing time with eyes open (OLST). Social participation was characterized in terms of activity type, participation frequency, and group size. Linear mixed-effects models with random intercepts were applied to evaluate temporal changes in OLST and their associations with social participation, adjusting for demographic and participation-related variables. Results: Among 6,898 participants (mean age 77.8 years; 77.8% women), OLST showed a progressive decline over time, with significant reductions emerging from the third year onward (p < 0.001). Higher participation frequency was associated with longer OLST in a dose–response manner (p for trend < 0.001). Participation in larger groups (≥60 members) was associated with longer standing time compared with smaller groups (<20 members) (β = 2.86 seconds, 95% CI: 0.97–4.76). In contrast, participation in low-intensity activities (e.g., tea gatherings) was associated with slightly shorter OLST (β = −1.28 seconds). Interaction analyses indicated that trajectories of decline were generally parallel across social participation categories. Conclusions: Balance performance declined progressively over time in older adults, while social participation characteristics were associated with overall levels of balance function rather than modifying its rate of decline. These findings suggest that social participation alone may be insufficient to alter age-related trajectories of balance decline, highlighting the potential need for integrating structured physical activity into community-based programs.