Dose-response association between different screen time behaviors and cardiorespiratory fitness among elementary and junior high school students: A population-based cross-sectional study in a Japanese city

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Background/objective: Excessive screen time has been associated with lower cardiorespiratory fitness (CRF) in youth, but evidence on the dose-response association remains limited. This cross-sectional study examined screen time and CRF among elementary school (ES) and junior high school (JHS) students. Methods: Of 870 ES and 876 JHS students recruited in Unnan City, Japan, in 2023, the analytical sample comprised 664 ES students (aged 9-12 years; 51.8% boys) and 679 JHS students (aged 12-15 years; 47.6% boys). CRF was assessed by the 20-m shuttle run and expressed as z-scores. Screen-based behaviors (gaming and video viewing or social networking service [SNS] use) were assessed by questionnaire. Multiple linear regression and restricted cubic spline analyses were adjusted for sex, grade, breakfast consumption, sleep duration, and sports club participation. Results: Among ES students, gaming for 2-<4 h (B = -0.28, 95% CI: -0.53, -0.04) or ≥4 h (B = -0.43, 95% CI: -0.70, -0.16) was associated with lower CRF than no gaming, with an inverse trend (P for trend = 0.001). Similarly, ≥4 h of video/SNS use was associated with lower CRF (B = -0.33; P for trend = 0.001). Among JHS students, no individual category was significant, but both showed inverse trends (P for trend = 0.013 and 0.014, respectively). Restricted cubic splines showed no detectable non-linearity. Conclusion: Longer recreational screen time was associated with lower CRF in both groups-significantly lower in higher categories among ES students and significant inverse trends among JHS students. Longitudinal and experimental studies are needed to establish causality.

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