Contemporary guidelines for young people advocate both a sustained and accumulative approach to moderate-to-vigorous physical activity (MVPA) participation. In order to investigate the behavioural significance of applying these approaches, this study assessed if differences in adolescent MVPA occurred when either a sustained or accumulated criterion was adopted. Using heart rate thresholds indicative of intensity, the physical activity of 25 adolescents was assessed by monitoring heart rate over 3 days. Results indicated that differences in MVPA participation did exist when different approaches were used, in that the majority of adolescents were active with respect to an accumulated criterion yet inactive with respect to a sustained criterion. Such a disparity may be due to accumulative MVPA being more characteristic of young people's natural activity behaviour. Such findings have strong implications for practitioners seeking to counter young people's inactivity, in that the application of an accumulative approach may be more effective at initiating and establishing an habitual activity behaviour than that of a sustained approach.