Changes in accelerometer-measured physical activity and self-reported leisure time physical activity from adolescence to young adulthood: a longitudinal cohort study from the Fit Futures Study.

Background: Adolescence is associated with declining physical activity (PA) levels, and potential prevailing changes into young adulthood are indicated, but less explored. This study investigates longitudinal changes in PA from adolescence to young adulthood among males and females in a North Norwegian cohort.

Methods: In the population-based Fit Futures Study, PA was assessed with both questionnaires (Saltin-Grimby Physical Activity Level Scale) and accelerometers (ActiGraph) at ages ~ 16 (n_self-report=936; n_{accelerometer}=674), ~ 18 (n_self-report=808; n_{accelerometer}=507), and ~ 27 (n_self-report=648; n_{accelerometer}=466). We used mixed effects models to analyze longitudinal changes in accelerometer-measured PA and sedentary time, alongside mixed effects multinomial logistic regression for changes in self-reported leisure time PA.

Results: We observed a significant non-linear U-shaped trend in accelerometer-measured moderate-to-vigorous PA (MVPA) over time (p < 0.001), with an initial decline in minutes per day from age 16 (mean ± SD: 70.7 ± 25.2) to age 18 (62.3 ± 23.8), followed by an increase to age 27 (67.5 ± 30.4). At age 16, males exhibited higher MVPA than females. By age 18 and 27, MVPA levels were similar between sexes. Accelerometer-measured sedentary time decreased linearly across all three surveys (p = 0.002). We observed distributional shifts in self-reported leisure time PA over time: vigorously- and highly active proportions declined, while the moderately active proportion increased, and the proportion of sedentary participants remained stable (~ 20%). Compared to vigorously active, the odds of reporting sedentary (OR: 1.07, 95% CI: 1.03 to 1.11), moderately active (OR: 1.11, 95% CI: 1.07 to 1.15), and highly active (OR: 1.07, 95% CI: 1.03 to 1.11) increased with each year from age 16 (all p ≤ 0.001). Compared to moderately active, the odds of reporting other categories decreased over time (ORs: 0.92 to 0.96, all p ≤ 0.001).

Conclusions: We observed non-linear changes in accelerometer-measured MVPA, indicating a U-shaped trend with a decline from 16 to 18 years, followed by an increase to age 27. Self-reported leisure time PA levels declined from adolescence to young adulthood, with decreasing proportions highly and vigorously active, while the proportion moderately active increased and the proportion of sedentary was unchanged. These results indicate that from adolescence to young adulthood, not all PA changes lead exclusively to increased sedentariness.