Short-term changes in sedentary behaviour during adolescence: Project STIL (Sedentary Teenagers and Inactive Lifestyles).

Hypothesis: Changes in body composition and body image during puberty will impact on sedentary behavioural choices.

Aim: The study aimed to investigate the effects of changes in pubertal status, body composition and body image on changes in time spent being sedentary.

Participants And Methods: Eighty-three school students aged 10-15 years were studied at 6-month intervals. BMI and percentage body fat (%BF) were calculated from anthropometric measures. Sedentary behaviour and sleep time were computed for weekday and weekend days using momentary-time-sampling diaries. Body image was measured using the Physical Self-Perception Profile for Children. Pubertal status was assessed from self-reported secondary sexual characteristics.

Results: During weekdays, boys' sedentary behaviour increased by a significantly greater amount for those who increased pubertal level compared with those who did not. No significant correlations were found between changing pubertal status, body composition, and body image for either gender. For girls, but not boys, a significant amount of the variance of the change in weekend day sedentary behaviour was explained by changing %BF, independent of changing pubertal status.

Conclusion: Only a part of the hypothesis was supported by these data. Changes in sedentary behaviour were not associated with behavioural choices triggered by body compositional changes.