Introduction: Evidence for associations between physical activity (PA) or sedentary time (ST) and vascular health in paediatric populations is of low quality due to the predominance of self-report measures of PA time and intensities, small sample sizes, and a lack of control of confounding variables. This thesis examined associations between device-derived PA and ST with vascular outcomes and a clustered cardiometabolic risk (CMR) score in a population sample, with adjustment for covariates. Methods: Cross-sectional analysis of 4,277 children (2,226 girls) aged 10.6 ± 0.2 y from the Avon Longitudinal Study of Parents and Children. Clustered CMR was measured at age 9 y. Vascular outcomes (flow mediated dilation, distensibility coefficient, and pulse wave velocity) were measured age 10 y. Light and moderate to vigorous PA (MVPA) and ST were measured via accelerometers at age 11 y. Multiple linear regression analyses were used to examine associations between exposures and outcomes, with PA and ST entered as compositional exposure variables and non-compositional variables (min∙day-1). Results: Neither light PA, MVPA or ST were significantly associated with any of the vascular outcomes in the adjusted compositional and non-compositional models. The proportion of time spent in MVPA and ST (relative to the remaining activity behaviours) were inversely (b=-0.126; P=0.001) and positively (b=0.136; P=0.016) associated with CMR in the whole group analysis, respectively. MVPA was negatively associated with CMR in both boys (b=-0.144; P=0.011) and girls (b=-0.110; P=0.032), but only girls had a positive association between ST and CMR (b=0.199; P=0.005). In the non-compositional models, MVPA was inversely associated with CMR in the whole group analysis (b=-0.002; P=0.012). In the girls, ST was positively (b=0.001; P=0.035), and LPA was inversely (b=-0.001; P=0.035) associated with CMR. Conclusion: Longer exposure to CMR factors during adolescence may be needed to establish relationships between PA and ST with vascular outcomes. These findings support interventions that promote MVPA and minimise ST for reducing CMR in children. Prospective studies are required to understand the causal directions.

Purpose: This study had 2 objectives: (1) to examine whether the validity of the supramaximal verification test for maximal oxygen uptake ( V˙O2max ) differs in children and adolescents when stratified for sex, body mass, and cardiorespiratory fitness and (2) to assess sensitivity and specificity of primary and secondary objective criteria from the incremental test to verify V˙O2max. Methods: in total, 128 children and adolescents (76 male and 52 females; age: 9.3-17.4 y) performed a ramp-incremental test to exhaustion on a cycle ergometer followed by a supramaximal test to verify V˙O2max. Results: Supramaximal tests verified V˙O2max in 88% of participants. Group incremental test peak V˙O2 was greater than the supramaximal test (2.27 [0.65] L·min-1 and 2.17 [0.63] L·min-1; P