Between-class variation and compositional correlates of physical fitness, and the concordance of composite scores with BMI-defined abnormality: a multilevel analysis of annual surveillance records from one Chinese university.

Introduction: This repeated cross-sectional retrospective study examined between-class variation in university students' physical-fitness scores and whether annual composite-score trends were concordant with changes in BMI-defined abnormality. Methods: Annual surveillance records collected from 2020 to 2024 at one public university in Sichuan Province, China, were analysed. The dataset included 106,461 records from 2,591 class-year units (44,240 male, 62,221 female; mean age 20.67 ± 1.54 years; mean BMI 21.64 ± 3.52 kg/m Results: The null-model intraclass correlation coefficient was 0.178, indicating meaningful between-class clustering. After adjustment, higher within-class BMI, older age, and male sex were associated with lower composite scores. At the class level, higher mean BMI was associated with lower scores, whereas the class-size association was statistically significant but practically negligible. The within-class BMI-fitness association varied across class-year units, although heterogeneity was modest and the association was negative in almost all units. Composite scores increased in 2021, declined in 2022, and increased again in 2023-2024. In contrast, BMI-defined abnormality rose every year, from 23.3% in 2020 to 31.4% in 2024. Discussion: University fitness data have a meaningful clustered structure, and improvement in a weighted composite score does not necessarily imply parallel improvement in the BMI distribution. Surveillance reports should therefore account for clustering and present composite scores alongside BMI categories and component-specific outcomes.