Effects of physical exercise on the developmental trajectory of social anxiety in middle school students: a mixed growth model

Based on longitudinal data collected from Chinese middle school students across four time points, this study employed a mixed growth model to examine the impact of physical exercise on the developmental trajectory of social anxiety, explore trajectory heterogeneity, and identify the longitudinal protective mechanisms of physical exercise. The Physical Exercise Rating Scale and the Social Anxiety Scale were administered in four longitudinal surveys conducted from September 2024 in Chengdu and Leshan (Sichuan Province) as well as Hangzhou and Lishui (Zhejiang Province), with a three-month interval between each wave. Single-growth and mixed growth models were constructed to examine class membership probabilities and predictive factors. (1) Significant trajectory heterogeneity in social anxiety was identified, with two distinct classes: a “high–slow decline” group and a “low–fast decline” group. The intercept and slope were negatively correlated. (2) Social anxiety and physical exercise showed a cross-sectional negative correlation ( r ≈ − 0.30) and stability across time (PE r ≈ 0.32–0.46; SA r ≈ 0.29–0.33). (3) After controlling for gender, age, place of origin, and grade, physical exercise still significantly and negatively predicted the probability of following an adverse trajectory ( β = − 0.396, P < 0.001). Social anxiety among middle school students shows a steady decreasing trend over time. Sustained participation in physical exercise was longitudinally associated with lower social anxiety and a lower likelihood of following an adverse social anxiety trajectory. It is recommended that schools implement tiered, contextualized, process-oriented, cooperative, and enjoyable sports programs to enhance accessibility and continuity of exercise, thereby improving unfavorable social anxiety trajectories.