Overview of findings
This study found a significant positive correlation between physical activity and high school students' school adjustment, and physical activity positively predicts high school students' school adjustment, which verifies hypothesis 1 and is consistent with the results of previous studies [41]. Improving and developing adolescents' adaptive ability in school is crucial to their future socialization growth, personality improvement, etc. Better school adaptive ability can help high school students establish good partnerships in school, promote adolescents' personality development and value formation, and lay the foundation for lifelong development [42]. If school adjustment is poor, it can lead to a range of problematic behaviors such as relationship distress, social withdrawal, social disorders, internet addiction, and aggressive behavior [43]. One study found that moderate- to high-intensity exercise significantly reduced depressive responses related to school adjustment, so adolescents who exercised more were less likely to have school adjustment problems [44]. One study found that students who participated in regular physical activity had a comprehensive understanding of their bodies, and their self-esteem, sense of accomplishment, and other psychological indicators were higher, which positively affected school adjustment [45]. Regular physical activity not only strengthens the body and shapes the personality but also enables adolescents to stimulate their sense of teamwork, develop interpersonal communication skills, and enrich their skills and techniques for dealing with and solving problems during the practice of participation in physical exercise, to comprehensively improve the adaptive ability of social interaction, handling, and interpersonal relationships, and to reduce the externalization of problematic behaviors.
The study results showed that psychological resilience mediates the relationship between physical activity and school adjustment in high school students, validating Hypothesis 2. The strength model of self-control explains how physical activity increases levels of self-control. The strength model of self-control states that self-control resources work similarly to muscles, and just as muscle capacity can be temporarily depleted and become more vital when it recovers after appropriate rest, the resources needed to perform self-control can be enhanced through long-term training [25]. Evidence from brain science suggests that acute aerobic exercise of moderate to low intensity increases cerebral blood flow by raising heart rate, which improves brain metabolism while also recruiting more neurons in the prefrontal lobe to participate in the self-control task [46, 47], which will facilitate the recovery of the excess resources for self-control, resulting in more tasks to continue the self-control task in the subsequent period, which will, in turn, enhance self-control. As a psychological function of changing one's responses to bring thoughts, emotions, and behaviors in line with social norms and helping to achieve long-term goals, well-developed self-control leads to better interpersonal interactions. It enhances the flexibility of individuals in adapting to their environments [48]. It has been found that the higher an individual's level of self-control, the higher their life satisfaction [49], and they will be better at managing their goals; through better goal-pursuit strategies, they will be able to effectively organize their lives [50], better resist external temptations, focus on current learning activities [51], abstain from recreational activities, and be able to better cope with stress, with a higher level of tolerance for frustration, and thus adapt better. In other words, individuals with higher self-control can inhibit impulses, shift their attention from negative to positive aspects, focus on learning through self-restraint as well as self-management, and control activities unrelated to their learning, as well as better control their emotions and reduce problematic behaviors, thus adapting to school life. Therefore, self-control is one of the most important psychological mechanisms for adapting to school and the environment.
This study also found that psychological resilience mediates the relationship between physical activity and school adjustment in high school students, supporting Hypothesis 3. The prevention model of psychological resilience development suggests that every type of stress that is not excessive in intensity is viewed as having the potential to enhance adaptive success when it is not excessive [52], which occurs below the optimal level of stress; in other words, while repeated exposure to uncontrollable stress eventually creates stress resistance [53], exposure to manageable stress will achieve this more quickly [54]. Resistance and exposure to controllable stress will accomplish this more quickly [53]. Furthermore, this agrees with the Unified Theory of Physical Activity proposed by Salmon, which suggests that the unique value of physical activity may lie in its controllable stressor [54]. Combined with the above theory, when physical activity is regarded as a controllable and moderate stressor, frequent exposure to such stress will lead to changes in the physiological and psychological responses of the individual, and the individual's anti-stress threshold will increase rapidly, thus improving the individual's psychological resilience and enabling the individual to cope with the stress in the face of adversity in a relaxed manner. Relevant studies also support the idea that physical activity may enhance a person's psychological resilience by increasing his or her stress response [55]. In addition, the resource conservation model suggests that the resources an individual possesses are closely related to adaptation; the more resources an individual possesses, the better he or she adapts [16]. In general, adolescents with positive perceptions of adversity can usually persevere and stay focused on their goals, thereby successfully coping with the effects of adversity and adapting better to school life [56]. Individuals with high levels of psychological resilience have better coping styles, resilience, and other positive psychological resources, and even in unfavorable environments, they effectively cope with the negative impacts of stress and better adapt to the environment [57]. In addition, individuals with high psychological resilience have an excellent and stable social support system. When necessary, they can actively seek and obtain support and help from others to better cope with stressful events, thus improving their adaptive capacity [58].
Finally, the present study found that psychological resilience and self-control were chain mediators in the relationship between physical activity and school adjustment. Psychological resilience positively predicted self-control, which validates previous research [59], which suggests that students with high psychological resilience have greater self-control. This result supports the strength and resource conservation models of self-control. According to the power model of self-control, self-control depends on limited internal resources, which are rapidly depleted when an individual exercises self-control, resulting in a state of "self-exhaustion" [25]. In the depleted state, it is difficult for individuals to control themselves, leading to self-control failure. However, high school students can increase their psychological resilience through physical activity, and the resource conservation model classifies resources into four categories, among which enduring intrinsic resources include psychological resilience and the acquisition of enduring intrinsic resources will have a long-term and more significant impact on the acquisition of resources in the future [60]. Then, the present study concluded that high school students increased their psychological resilience through physical activity. The increase in psychological resilience not only enables students to better adapt to school life but also enriches the individual's psychological resources so that the individual will have more chances to acquire new psychological resources through resource inputs and to increase their stock so that the individual will have more psychological resources for self-control, and thus adapt to school life. Therefore, the higher the level of psychological resilience of high school students, the more psychological resources they can utilize internally, and the stronger the individual's self-control ability.
In summary, this study analyzed the relationship between physical activity and school adjustment and its internal mechanism in depth by constructing the chain mediation model, combining the power model of self-control and the theory of resource preservation, to provide a reference for the promotion of physical activity, psychological resilience, self-control, and school adjustment in high school students. The various influences on school adjustment do not work independently but are interconnected. This has some implications for the promotion of adolescents' school adjustment: High school students' school adjustment can be improved by increasing physical activity, mental flexibility, and self-control; schools, teachers, and parents should pay attention to the cultivation of high school students' awareness of and habits in physical activity, and promote adolescents' physical and mental health and school adjustment through physical activity; we should not start from the pure physical activity, mental flexibility, and self-control factors, but should pay attention to these factors, and should not focus on the physical activity, mental flexibility and self-control factors alone. Instead of focusing only on physical activity, mental flexibility, self-control, and other factors, the interaction of these factors should be emphasized. As far as possible, attention should be paid to the different factors affecting the individual's school adjustment to promote the better adaptation of high school students to school life.
Limitations
Although this study explored the underlying mechanisms of the relationship between physical activity and school adjustment among high school students and achieved the positive findings described above, the current study has limitations. First, the present study was cross-sectional and explored only predictive results based on relevant research strategies, making it difficult to draw strict causal inferences that could be further clarified in the future through follow-up or experimental studies. Second, this study measured physical activity through the use of a self-report questionnaire, IPAQ, which is a subjective measurement of physical activity; therefore, future research needs to improve the research paradigm and research methods by combining observation, measurement, experiment, and tracking with advanced scientific and technological means, and further research should consider the use of more objective measurement devices (e.g., accelerometers, pedometers) to measure physical activity amount, to deeply explore the mechanism of influencing high school students' mental activity. Finally, this study only analyzed the mediating roles of total physical activity Meto values, mental flexibility, self-control, and total school adjustment scores, and the correlations among the sub-dimensions of each variable need to be further investigated.