Adolescence is a crucial period for the onset of mental illnesses. Identifying early risk markers and potential intervention targets is vital. Physical fitness, assessed objectively through wearable technology, offers a promising avenue for investigation, circumventing the biases inherent in self-report measures. While studies have shown links between physical activity and mental health within specific diagnostic categories, questions remain about the transdiagnostic nature of these relationships and their specificity across different symptom dimensions (psychosis-like experiences (PLEs), internalizing, and externalizing symptoms). This study aims to explore these relationships in a large, diverse adolescent population using data from wearable technology, specifically Fitbits, within the Adolescent Brain Cognitive Development (ABCD) study.

Existing literature demonstrates associations between physical health behaviors and mental health outcomes across various psychopathologies. Lower cardiovascular fitness and increased sedentary behavior have been linked to increased psychosis and internalizing symptoms (depression, anxiety), while moderate-to-intense physical activity has shown associations with reduced symptom severity across different symptom dimensions. However, these studies often examine symptom dimensions in parallel, neglecting the interplay of symptoms within individuals. Furthermore, the use of self-report data introduces potential biases. The unique contribution of different fitness metrics (cardiovascular fitness, sedentary behavior, physical activity) to specific symptom dimensions is also unclear, hindering the development of targeted interventions. This study leverages the ABCD study’s large dataset and objective fitness data from wearable technology to address these gaps.

This study utilized data from 5070 adolescents (ages 10.3–13.5, 48.41% female) participating in the ABCD study and a sub-study using Fitbit Charge HR2 devices. Physical fitness was assessed using resting heart rate (RHR), time spent sedentary, and time spent in moderate physical activity. Mental health symptoms were measured using the Prodromal Questionnaire-Brief Child Version (PQ-CB) for psychosis-like experiences (PLE), and the Child Behavior Checklist (CBCL) for internalizing and externalizing symptoms. Multilevel models were employed to analyze the relationships between fitness metrics and symptom dimensions, controlling for sex, age, household income, and body mass index. The models accounted for the random effects of familial relatedness. Follow-up analyses explored symptom subdomains (anxiety and depression within internalizing symptoms). A Bonferroni correction (p < 0.017) was applied to adjust for multiple comparisons.

Higher resting heart rate (lower cardiovascular fitness) was significantly associated with greater internalizing symptoms (t = 3.63, p < 0.001). More sedentary behavior significantly predicted higher PLE severity (t = 5.49, p < 0.001). Greater time spent in moderate physical activity was significantly associated with lower PLE (t = -2.69, p = 0.007) and internalizing symptom severity (t = -6.29, p < 0.001). Follow-up analyses examining internalizing subdomains revealed that the association between resting heart rate and internalizing symptoms was primarily driven by depression symptoms. The association between moderate activity and internalizing symptoms was also significantly related to depression symptoms. These findings held even after controlling for age, sex, BMI, and household income. The effect sizes, while modest, were comparable to those reported for other established risk markers of psychopathology.

This study demonstrates significant associations between objectively measured physical fitness metrics and specific mental health symptom dimensions in adolescents. The findings highlight the transdiagnostic nature of the link between physical and mental health, with some fitness metrics showing stronger relationships with certain symptom clusters than others. The association between sedentary behavior and PLE severity suggests a potential early risk factor for psychosis. The beneficial effects of moderate physical activity on both PLE and internalizing symptoms underscore its role in promoting mental health. The specificity of these relationships emphasizes the importance of tailored interventions that address individual needs based on specific symptom profiles. The use of wearable technology provides a practical and objective way to assess physical activity and its relationship to mental health.

This study provides strong evidence for the link between physical fitness and mental health in adolescents, particularly highlighting the importance of considering specific symptom dimensions when designing interventions. The findings support the use of wearable technology to objectively assess physical activity and identify individuals at risk. Future research should investigate the underlying biological mechanisms linking physical fitness to mental health and explore the longitudinal relationships between these factors to establish causality and develop targeted, effective interventions.

The cross-sectional nature of the data limits the ability to establish causality. The sample, while large and diverse, may not be perfectly representative of the entire US adolescent population, particularly regarding household income and racial composition. The reliance on a single type of wearable technology and a limited timeframe for data collection might also influence the results. Furthermore, the study did not examine the effect of specific types of physical activity or the intensity of activity.