Objective To investigate the influence of physical exercise on the development trajectories of depression among junior high school students, and examine the mediating effect of the peer acceptance.

Methods The physical activity rating scale, the ostracism experience scale for adolescents and the depression self-rating scale were used. A whole year and a two-stage follow-up investigation on 1 396 junior high school students were made. And latent growth mixture model was used to analyze the data.

Results  For junior high school students, relatively, the growth mixture model of the three-classification of depression was the optimal one (AIC = 15,862.569, BIC = 15,927.502, aBIC = 15,886.211, Entropy = 0.758, LMRLRT: P = 0.009). Regression analysis of multiple independent variables showed that the peer acceptance (T₂) had a significant impact on the depression developmental trajectory (β=0.393, P<0.001). Moreover, controlled for the mediating variable, the relative impact effect of medium-activity volume and high-activity volume of physical exercise on depression development trajectory types was 0.070 and 0.194 higher, respectively, compared to low-activity volume of physical exercise.

Conclusion For junior high school students, the depression could be divided into three development trajectory types, namely the stable-high subgroup (3.39%), stable-medium subgroup (49.04%), and the stable-low subgroup (47.57%). Physical exercise could predict the types of depression trajectories in junior high school students. Moreover, when used the low-activity volume of physical exercise as the reference, both medium-activity volume and high-activity volume of physical exercise could alleviate students' depression through their peer acceptance.