Are adolescents really being sedentary or inactive when at school? An analysis of sedentary behaviour and physical activity bouts.

This study analysed physical activity (PA) and sedentary behaviour (SB) performed in bouts during schooltime. Adolescents of two schools answered a questionnaire, had their height and weight measured and wore accelerometers during schooltime. Moderate-to-vigorous intensity PA was estimated in bouts of 2, 5 and 10 min, and SB in bouts of 5, 10, 30 and 60 min. Body mass index was calculated, and adolescents were classified as overweight or normal weight. Frequency, duration and volume of PA and SB were calculated for the whole sample and for those who performed at least one bout weekly. PA and SB differences between gender and weight status were tested using Mann-Whitney and t tests. A total of 415 adolescents (54% girls, 12.3 ± 1.3 years old, 34.6% overweight) performed 0.1 (± 0.2), 0.2 (± 0.5) and 0.4 (± 0.6) bouts of 10, 5 and 2 min of PA, and 10.6 (± 2.0), 6.0 (± 1.3), 1.1 (± 0.9) and 0.4 (± 0.5) bouts/day of 5, 10, 30 and 60 min of SB, respectively. Boys accumulated less SB and more PA in bouts. No differences were found for weight status.Conclusion: Adolescents hardly engaged in bouted PA and accumulated more SB in smaller bouts during schooltime. Bouted PA and SB were not associated with weight status. What is Known: • Adolescents are not engaging in enough PA and are accumulating large volumes of SB, which might negatively impact their health. • Schools play an important role in daily overall PA, with opportunities such as PE classes and recesses. But most classes are still undertaken in seated positions. What is New: • Adolescents hardly engage in bouted (> 10 m) PA when at school., and most SB is accumulated in short bouts (≤  10m). • Weight status was not associated with bouted PA or SB at school.