The social ecological model provides a comprehensive framework for understanding the multiple-level determinants of physical activity. This study explores the significant individual, social, and environmental variables and their interactions in relation to physical activity in middle-aged and older adults in Taiwan. A cross-sectional study design was implemented. Healthy middle-aged and older adults were recruited (n = 697) through face-to-face and online surveys. The data collected comprised self-efficacy, social support, neighbourhood environment, and demographic characteristics. Hierarchical regression was used for statistical analysis. Self-rated health (B = 74.74, p < .001; B = 101.45, p = .022) and self-efficacy (B = 17.93, p < .001; B = 14.95, p = .020) were the significant individual variables in both middle-aged and older adults. Neighbourhood environment (B = 6.90, p = .015) and the interaction between self-efficacy and neighbourhood environment (B = 1.56, p = .009) were significant in middle-aged adults. Self-efficacy was the most significant predictor for all participants, with the positive correlations of neighbourhood environment arising only for middle-aged adults with high self-efficacy. Policy making or project design should consider multilevel factors in order to facilitate their physical activity.
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