Policy Dosing in School Physical Education and Adolescent Fitness: A Threshold-Type Association in a Two-Wave Panel Study from Kunming, China

Degui wang^*zongqing fengjinying zhangyi weiwenjun bi

• Kunming City College, Kunming, China

Background: The minimal effective "dosage" of school physical education (PE) and its nonlinear relation to adolescent fitness are not well defined. We developed a policy-based Education Policy Dosing Index (EPDI, 0–100) from four exam-oriented domains—compulsory test intensity, elective test diversity, scoring/enforcement strictness, and competition & incentive provision—aligned with Yunnan's 2019 education–exam reform and the junior-middle-school PE examination scheme. Multiple weighting checks (equal, ±20%, PCA/entropy) were used to confirm its formative nature. Methods: We analyzed a two-wave school panel in Kunming, China, from a single public middle school, yielding 2,673 student-occasion records: the same 891 students measured at three time points (Grade 7 baseline, then two follow-ups in Grades 8–9; 2021–2023). Segmented regressions with two-way fixed effects (per 10 EPDI; reference threshold=60) used CR2 inference. We profiled the breakpoint (τ̂), compared restricted cubic splines versus linear models, and conducted robustness checks (alternative weights, 10-fold delete-group jackknife, placebo threshold, negative-control outcome). Outcomes were sex-by-grade z-scores; component-specific Ns vary slightly due to test-day absence/invalid measures. Results: EPDI showed a threshold-like association with fitness; τ̂≈66, with diagnostic breakpoints mostly 60–75 across outcomes. For every 10 EPDI units, composite PFI showed a pre-threshold slope −0.074 (95% CI −0.116 to −0.032), slope change +0.274 (0.151 to 0.397), and post-threshold slope +0.201 (0.110 to 0.291). Component effects were stronger and directionally consistent: vital capacity +0.588 (0.501–0.675), 50-m sprint (higher z=slower) −0.207 (−0.272 to −0.143); standing long jump and sit-and-reach showed moderate gains; BMI was negligible. Girls benefited at lower EPDI levels, whereas boys and Grade-8 students gained more after the breakpoint. For the composite PFI, evidence of non-linearity was modest; signals were stronger at the component level. Conclusion: An EPDI around 60 is a practical, exam-aligned planning benchmark, with diminishing marginal returns beyond the breakpoint and equity-relevant heterogeneity by sex and grade. Segmented and spline models outperformed linear models for most outcomes, and findings were robust across weighting/sensitivity checks. Estimates reflect one school over a short window and should be replicated across schools/districts with maturation and socioeconomic markers. Associations are interpreted as non-causal.