Longitudinal development of oxygen uptake in 6- to 12-year-old children: the Health Oriented Pedagogical Project (HOPP)

Longitudinal studies using directly measured peak oxygen uptake (V̇O₂peak) throughout primary school are scarce, and the interpretation of developmental changes depends on how oxygen uptake is scaled for body size and composition. This study examined the longitudinal development of absolute, body-mass-relative, allometrically scaled, and fat-free-mass-scaled V̇O₂peak from first to sixth grade. Fat-free-mass-scaled V̇O₂peak was the prespecified primary outcome. Data were obtained from the Health Oriented Pedagogical Project, a six-year school-based physical activity study in south-eastern Norway. Of 351 children with parental consent, 330 completed at least one valid maximal treadmill test. Because valid V̇O₂peak data from 2016 were available from only three of nine schools following technical data loss, the inferential analyses included measurements from 2015, 2017, 2018, 2019, and 2020. Linear mixed models included test year, school allocation group, sex, and the test year × group interaction as fixed effects, participant ID as a random intercept, and test year as the repeated factor. School-level models were conducted as sensitivity analyses. The primary model included 904 observations from 314 children. Fat-free-mass-scaled V̇O₂peak was associated with test year, allocation group, sex, and test year × group (all p ≤ 0.030). From fifth to sixth grade, the adjusted mean decreased by 5.73 mL·kg FFM⁻¹·min⁻¹ in the intervention group and by 5.11 mL·kg FFM⁻¹·min⁻¹ in the control group. The difference between the group-specific changes was − 0.62 mL·kg FFM⁻¹·min⁻¹ (95% CI − 4.25 to 3.00; p = 0.736). Body-mass-relative and allometrically scaled V̇O₂peak also decreased significantly in both groups, whereas absolute V̇O₂peak remained broadly stable. Boys had higher adjusted values for all four expressions, although the sex difference was substantially attenuated after scaling to fat-free mass. In school-level sensitivity analyses, changes in the three body-size-adjusted expressions were negative at all nine schools. V̇O₂peak adjusted for body mass or fat-free mass decreased from fifth to sixth grade in both allocation groups, whereas absolute V̇O₂peak remained stable. The final-year changes did not differ between intervention and control schools. Scaling method, body composition, and school-level heterogeneity materially affected the interpretation of paediatric aerobic fitness. The study design could not separate growth-related changes from pandemic-related behaviour or time-varying participant composition. ClinicalTrials.gov NCT02495714; retrospectively registered 20 June 2015.