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Background The transition from middle to high school is characterized by surging academic stress, a precipitous decline in moderate-to-vigorous physical activity (MVPA), and alarming increases in sedentary time (ST). While family support is known to buffer stress, the specific types of support required may shift dramatically across developmental stages. Guided by Stage-Environment Fit Theory and the Stress-Buffering Hypothesis, this study investigated whether the buffering effects of family tangible support versus emotional support on device-measured MVPA and ST differ between middle and high school students. Methods A stratified sample of 523 adolescents (298 middle school, 225 high school) from diverse regions in China was recruited. MVPA and ST were objectively measured using ActiGraph GT3X+ accelerometers over seven consecutive days. Self-reported questionnaires assessed chronic academic stress and family support types. Multi-Group Structural Equation Modeling (MG-SEM) was employed to test the moderated effects. Results Academic stress robustly predicted decreased MVPA and increased ST across the entire sample. However, the buffering mechanisms exhibited profound developmental heterogeneity. For middle school students, family tangible support significantly buffered the detrimental effects of stress on both MVPA ( β = 0.21, p 0.01) and ST ( β = −0.18, p 0.01). Conversely, for high school students, tangible support became entirely ineffective. Instead, family emotional support emerged as the sole significant buffer against stress-induced physical inactivity ( β = 0.24, p 0.001) and excessive sitting ( β = −0.22, p 0.01). Conclusion The efficacy of family interventions is heavily contingent upon developmental timing. Combatting the dual burden of physical inactivity and sedentary behavior in high-pressure educational systems requires a transition from tangible parental involvement in early adolescence to autonomy-respecting emotional support in late adolescence.
Li et al. (Mon,) studied this question.