Sex-divergent trends in cardiorespiratory endurance after the COVID-19 pandemic: a 21-year interrupted time series analysis of Chinese university students

IntroductionStudies of COVID-19 and physical fitness in Chinese university students rarely extend beyond 2022, and none has formally tested whether post-pandemic trends differ by sex.MethodsUsing 21 years of routinely collected fitness data (2005–2025), we examined indicator-specific change and sex differences in population-level trends in a repeated cross-sectional design. The cardiorespiratory analytic sample comprised 297,565 valid records from 91,686 full-time undergraduates at one comprehensive university in Zhejiang Province, China. Six indicators were classified by data availability and suitability for interrupted time series (ITS) analysis; cardiorespiratory endurance (1000-m run for men, 800-m for women) was pre-specified as the primary outcome and body weight was analysed within the same framework as a negative control. The unit of analysis was the annual grade-standardised mean, giving 18 time points per sex. Single-breakpoint models estimated the immediate level change at 2020 (β2) and the change in annual trend thereafter (β3); a sex × trend interaction tested whether post-2020 trends differed between women and men; Benjamini–Hochberg correction was applied across five confirmatory tests.ResultsRun times in 2020 rose above counterfactual projections in both sexes (women +9.60 s, 95% CI 2.58–16.62, p = 0.011, q = 0.019; men +13.02 s, 95% CI 4.76–21.28, p = 0.005, q = 0.019), although this immediate change was attenuated and no longer significant when fourth-year students, tested in May 2020 rather than December, were excluded. Post-2020 trends differed by sex (interaction p = 0.030, q = 0.037) and this difference was robust to that exclusion. In men the post-2020 trend did not differ significantly from the pre-pandemic trend (β3 = −0.47 s/year, p = 0.609), with the 2025 annual mean remaining about 14.9 s above the counterfactual projection. In women the trend was significantly steeper (β3 = +2.25 s/year, p = 0.011, q = 0.019), widening the gap to about 26.0 s by 2025. Body weight rose steadily across the whole period but showed no change attributable to the pandemic (level change p = 0.31 and 0.70; trend change p = 0.57 and 0.92 in women and men), which is consistent with the cardiorespiratory result reflecting something other than an artefact of the model.DiscussionCardiorespiratory endurance was the only indicator analysed by ITS that showed a sustained pandemic-associated change, and its subsequent trend diverged by sex. These are aggregate population trends rather than individual trajectories, but they suggest that post-pandemic aerobic programming may need to be sex-specific, and that continuity of aerobic activity deserves explicit attention in emergency preparedness planning.