Exploring Which of Timing or Accumulation of Physical Activity During Adolescence Better Predicts Physical Activity in Early Adulthood

BACKGROUND: Adolescence is key for developing physical activity (PA) habits that persist into adulthood. However, whether the timing of adolescent PA matters remains unclear. This study compares the timing versus cumulative exposure to adolescent PA in predicting young adults' PA levels and trajectories. METHODS: Data were drawn from the MATCH (Monitoring Activities of Teenagers to Comprehend their Habits) study. A purposive sample of 530 participants self-reported their weekly number of days with ≥60 minutes of moderate to vigorous PA every 4 months from 2011 to 2019 (ages 11-18). Young adult PA (ages 20-23) was assessed annually from 2020 to 2024 using the International Physical Activity Questionnaire. Adolescent PA history was operationalized as either a Cumulative Index of Exposure or a Weighted Cumulative Exposure, which assigns age-specific weights. Associations between adolescent and young adult PA were estimated using generalized linear mixed models. RESULTS: Young adult PA declined over time. Weighted cumulative exposure provided a better fit than Cumulative Index of Exposure and indicated that early adolescent PA (11-15 y) protects against young adult PA decline (eg, age 11: exp(β) = 1.007, 95% CI, 1.003-1.012; 15: exp(β) = 1.016, 95% CI, 1.003-1.029), whereas late adolescent PA (≥17 y) better predicts adult PA levels at the beginning of early adulthood (17: exp(β) = 1.061, 95% CI, 1.050-1.073; 20: exp(β) = 1.275, 95% CI, 1.210-1.343). CONCLUSIONS: PA levels in early and late adolescence appear to be differently related to young adult PA. Public health strategies should promote sustained PA throughout adolescence, with particular emphasis on PA levels during the transition to adulthood.