Background Students in early adolescence have a high incidence of mental health problems. Studies have found an association between physical activity and mental health in adults, but the association among children and adolescents is not clear.

Objective This study investigates the relationship between mental health and physical activity among students in early adolescence, aiming to provide evidence for the prevention and control of mental problems among students in this stage.

Methods By using stratified cluster random sampling method, a questionnaire survey was conducted among 859 fourth grade students from 12 elementary schools in 3 urban and 3 suburban areas in Shanghai. The survey included students' general information, the total and mental health problems, and physical activity time, and calculated the total and dimensional (emotional symptoms, conduct problems, hyperactivity, peer problems, and prosocial behaviors) positive rates of mental health and the daily moderate-to-vigorous-intensity physical activity time. Chi-square test was used to compare the differences in mental health status among students with different family economic, educational backgrounds and family relationships. Logistic regression models were used to analyze the associations between mental health problems and physical activity.

Results The total positive rate of mental health problems among the participants was 10.7%, and the rate of boys was higher than that of girls (13.4% vs. 8.0%, χ²=6.58, P < 0.05). The dimensional positive rates from high to low were peer problems (22.9%), hyperactivity (15.8%), emotional symptoms (9.0%), conduct problems (7.6%), and social impairment (5.2%). The median timeM (P₂₅, P₇₅) of daily moderate-tovigorous-intensity physical activity of the students was 76.9 (8.4, 107.1) min; 48.7% of the students spent less than 60 min on moderateto-vigorous-intensity physical activities per day, and the proportion in boys (52.2%) was higher than that in girls (45.1%) (χ²=4.36, P < 0.05). The rank-sum test results showed that the daily time of moderate-to-vigorous-intensity physical activity was correlated with the positive rate of emotional symptoms, conduct problems, hyperactivity, peer problems, and total mental health problems (P < 0.05). The chi-square test results showed that the factors such as mother's education level, father's and mother's monthly income, and family relationships were related to the positive rate of total mental health problems (P < 0.05). The logistic regression analysis results showed that spending less than 60 min on moderate-to-vigorous-intensity physical activities per day was associated to reporting hyperactivity, total mental health problems, and social impairment, and the ORs (95%CI) were 1.633 (1.095-2.436), 2.811 (1.676-4.714), and 2.307 (1.153-4.618) respectively.

Conclusion Among the fourth grade students in Shanghai, the positive rate of boys' mental health problems is higher than that of girls', and the overall rate of peer problems is high. Nearly half of the students spend less than 60 min on moderate-to-vigorous-intensity physical activities per day. Doing moderate-to-vigorous-intensity physical activity for less than 60 min per day is possibly associated to the positive rate of students' mental health problems.