| Sumario: | Background: This study aimed to identify latent moderate-to-vigorous intensity physical activity (MVPA) and sedentary behavior (SB) trajectories in older adults participating in a randomized intervention trial and to explore associations with baseline social-cognitive predictors. Methods: Data were assessed at baseline (T0, participants were inactive or had recently become active), after a ten-week physical activity intervention (T1), and a second 24-week intervention phase (T2). Latent class growth analysis was used on accelerometer-assessed weekly MVPA and daily SB, respectively (n = 215 eligible participants). Activity changes within trajectory classes and baseline social-cognitive predictor differences between trajectory classes were analyzed. Results: A "stable insufficient MVPA" (n = 197, p for difference in MVPA level at T0 and T2 (p) =.789, effect size (Cohen's d) =.03) and a "stable high MVPA" trajectory (n = 18, p =.137, d =.39), as well as a "slightly decreasing high SB" (n = 63, p for difference in SB (p) =.022, d =.36) and a "slightly increasing moderate SB" trajectory (n = 152, p =.019, d =.27) emerged. Belonging to the "stable high MVPA" trajectory was associated with higher action planning levels compared to the "stable insufficient MVPA" trajectory (M = 5.46 versus 4.40, d =.50). Belonging to the "decreasing high SB" trajectory was associated with higher action self-efficacy levels compared to the "increasing moderate SB" trajectory (M = 5.27 versus 4.72, d =.33). Conclusions: Change occurred heterogeneously in latent (not directly observed) subgroups, with significant positive trajectories only observed in the highly sedentary. Trial registration: German Registry of Clinical Trials, DRKS00016073, Registered 10 January 2019.
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