Background: The use of devices such as smartphone applications (app) to increase the level pf physical activity (PA) decreases over time. Adaptive intervention trials have been recommended to test technology-based interventions due to the possibility of adapting interventions based on individuals’ responses. Objective: To investigate the effects of a smartphone app combined with behavior change techniques on the level of PA in adults and older adults. Secondarily, we investigated the time in sedentary behavior and in moderate-to-vigorous physical activities (MVPA). Methods: This is a single-blinded, pilot, Sequential Multiple Assignment Randomized Trial. Participants were randomized into 3 groups during a 24-week intervention (G1 – app with tailored messages; G2 – app with tailored messages + gamification I; CG – control group with educational information). In the 6th week of intervention, participants from G1 and G2 were classified into responders and non-responders according to their performance in the average number of daily steps; non-responders were re-randomized among the other groups, adding a second type of gamification (G3 – app with tailored messages + gamification II). After another 6 weeks, participants were re-assessed and advised to keep monitoring the number of steps with the app, but without interference from the researchers. The effects of the interventions were analyzed using linear mixed models. Results: Fifty-three participants (44.0 ± 12.7 years) were included in the study (CG: 17, G1: 17, G2: 19). Responders were 62.5% and 46.7% in G1 and G2, respectively (P=.376). Regarding the level of PA, participants from G1 increased the average number of daily steps in all assessments (final vs initial, B: 797.2 steps/day, 95%CI: 475.3 to 1119.1, P<.001; follow-up vs initial, B: 2097.6 steps/day, 95%CI: 1577.2 to 2618.1, P<.001). All participants reduced the time in sedentary behavior in the final assessment compared to the initial (B: -70.8 min/week, 95%CI: -88.8 to -52.9, P<.001), without differences among groups. Regarding the time in MVPA, all participants presented with variations along time. Regardless of the initial group and allocation in the second randomization, participants from G1 and G2 assessed as responders in the 6th week of intervention showed a constant increase in the average count of daily steps (week 6 vs. week 1, B: 1548.0 steps/day, 95%CI: 1407.4 to 1688.6, P<.001; week 12 vs. week 1, B: 1720.3 steps/day, 95%CI: 1568.8 to 1871.7, P<.001; week 12 vs. week 6, B 172.3, 95%CI: 20.8 to 323.8, P<.05). Conclusions: The adaptive interventions protocol using a smartphone app with behavior change techniques increased participants’ level of PA. It is possible that stepping up the behavior change techniques and the progressive offering of new stimuli contribute to the change in behavior towards PA. Clinical Trial: Brazilian Registry of Clinical Trials (ReBEC) RBR-8xtc9c. Universal Trial Number (UTN) U1111-1218-1092.
