Changes in physical fitness and body composition of athletes after the COVID-19 lockdown: a systematic review, meta-analysis, and meta-regression, with assessment of the certainty of evidence

This systematic review with meta-analysis analysed the effects of COVID-19 lockdowns on physical fitness and body composition in athletes. A comprehensive search was conducted in three databases (PubMed, Web of Science, and Scopus) up to January 2025 (included). Studies were included based on PICO criteria, involving adult athletes, original articles, and any quantitative assessment of physical fitness and/or body composition conducted within one month before and two weeks after the lockdown. The Joanna Briggs Institute Critical Appraisal Checklist was used to assess the risk of bias, while the Cochrane Grading of Recommendations Assessment, Development, and Evaluation (GRADE) approach evaluated the certainty of evidence. A total of 14 studies (261 athletes) with a low risk of bias met the inclusion criteria. Narrative synthesis revealed that the effects of lockdowns on athletes' physical fitness and body composition were varied, with consistent impairments (e.g., endurance-related fitness), relative stability (e.g., body mass, CMJ height, maximal strength), and mixed results (e.g., sprinting). A meta-analysis of 11 studies indicated a non-significant effect of lockdown on body mass (effect size [ES]=-0.115, 95% confidence interval [CI] -0.214 to 0.164, P=0.797). Similarly, 10 studies showed a variable, non-significant reduction in CMJ height (ES=-0.303, 95% CI -0.655 to 0.045, P=0.097). However, CMJ relative peak power (six studies) demonstrated a trivial-small negative effect (ES=-0.199, 95% CI -0.341 to -0.058, P=0.019). These findings should be interpreted with caution as the certainty of evidence was very low. While evidence remains limited, targeted and individualised training might help mitigate some of the detraining effects observed during a lockdown, particularly in endurance-related fitness outcomes.