1.1 Participant recruitment
The study adhered to the principle of voluntary participation and employed a cluster sampling method. A total of 2,663 university students were randomly selected from 50 classes across 8 colleges in Songjiang District, Shanghai. Inclusion criteria were university students aged 17 to 26 years. Exclusion criteria included: (1) history of mental illness; (2) use of psychiatric medication within the past two months. This study adheres to the ethical requirements of the latest version of the Declaration of Helsinki. It has been reviewed and approved by the Ethics Committee of Shanghai University of Sport (Ethics Registration Number: 102772023RT075). With the consent of the relevant sports department authorities, trained researchers posted recruitment posters and distributed recruitment leaflets to recruit participants. For individuals meeting the recruitment criteria, the possibility of their inclusion was preliminarily assessed based on inclusion and exclusion criteria. Participants were informed about the study's purpose and content and signed informed consent forms, indicating their voluntary participation. The recruitment and screening period was from October 2023 to April 2024. The participant recruitment process is illustrated in Fig. 1.
1.2 Measurement Tools
(1) General Information Questionnaire
Including basic information such as the participants' age, BMI, gender, only-child status, household registration location, single-parent status, alcohol consumption, and smoking habits.
(2) Physical Activity Rating Scale-3 (PARS-3)
Based on the Chinese version of the Physical Activity Rating Scale revised by Liang De qing et al.[20], this study assesses participants' physical activity levels by considering the intensity, duration, and frequency of physical activity. The physical activity level score is calculated as the product of exercise intensity, duration, and frequency. The Cronbach's α coefficient for this scale is 0.856, and the test-retest reliability is 0.82[21].
(3) Patients′ Health Questionnaire (PHQ-9) [22]
The questionnaire includes 9 items, with response options of 'not at all', 'several days', 'more than half the days', and 'nearly every day', scored as 0, 1, 2, and 3, respectively. The total score ranges from 0 to 27, with higher scores indicating more severe depressive symptoms. The PHQ-9 has demonstrated good reliability and validity, with a Cronbach's α coefficient of 0.84[23].
(4) Cognitive Flexibility Inventory(CFI)
This study utilized the Cognitive Flexibility Inventory (CFI) developed by Dennis and Vander Wal, with the Chinese version translated by domestic scholar Wang Yang[24], to assess patients' cognitive flexibility. The questionnaire consists of 20 items and includes two dimensions: 'alternatives' and 'control'. 'Alternatives' refers to the ability to adopt multiple approaches to deal with difficulties, while 'control' indicates the individual's awareness that the challenging situations they face are controllable[24]. Each item is rated on a 5-point Likert scale, with a total score ranging from 20 to 100; higher scores indicate better cognitive flexibility. The CFI has demonstrated good reliability and validity, with a Cronbach's α coefficient of 0.88.
1.3 Data Collection
Participants completed an online survey that included a basic information form, the Physical Activity Rating Scale, Patient Health Questionnaire, and Cognitive Flexibility Inventory. Before filling out the questionnaires, the investigators explained each item, clarified that the collected data would be used solely for scientific research, and informed the participants of their right to complete the survey independently and truthfully, as well as their right to withdraw at any time. During the survey, participants were reminded to answer carefully. Upon completion, the investigators checked for patterned responses, completion times less than 3 minutes, and questionnaires with more than one-third of the items unanswered. A total of 126 invalid questionnaires were excluded, including 39 with patterned responses, 41 with completion times less than 3 minutes, and 46 with more than one-third of the items unanswered. Finally, 2,537 questionnaires were included in the analysis, resulting in an effective response rate of 95.27%.
1.4 Statistical Analysis
Statistical analyses were conducted using SPSS version 29.0. An independent samples t-test and chi-square test were employed to compare demographic characteristics between university students with and without depressive symptoms. Correlation analysis was used to explore the relationships among physical activity intensity, depressive symptoms, and cognitive flexibility. One-way ANOVA was performed to compare the differences in scores for depressive symptoms and cognitive flexibility across different levels of physical activity intensity. Post-hoc tests were conducted to compare the differences in scores for depressive symptoms and cognitive flexibility between low, moderate, and high-intensity physical activity groups. The Process 3.5 macro was used to analyze the mediating effect of cognitive flexibility, serving as a mediator between physical activity intensity and depressive symptoms.