Participant descriptives.

The benefits of physical activity (PA) and the negative impacts of sedentary time (SED) on both short- and long-term health in youth are well established. However, uncertainty remains about how PA and SED jointly influence maximal oxygen uptake (). Therefore, the aim of this study was to determine the joint influence of PA and SED on using compositional analyses. 176 adolescents (84 girls, 13.8 ± 1.8 years) completed an incremental ramp test and supramaximal validation bout on a cycle ergometer, with PA and SED recorded for seven consecutive days on the right hip using a ActiGraph GT3X accelerometer. Time spent in Sleep, SED, light, moderate and vigorous PA was analysed using a compositional linear regression model. Compositions with 10 minutes more time in vigorous PA (> 27.5 mins⋅day-1) compared to the average 17.5 mins⋅day-1 were associated with a + 2.9% - 11.1% higher absolute and scaled whilst compositions with less (> -10 mins⋅day-1) VPA were associated with a reduced absolute and allometrically scaled (-4.6% - 24.4%). All associations were irrespective of sex, maturity, and training status. The proportion of time spent sedentary had little impact on absolute and scaled (0.01–1.98%). These findings therefore highlight that intensity of PA may be of greater importance for increases in than reductions in SED and should be incorporated into future intervention designs.