Discussion
This mixed method study explored inactive adults’ adoption, acceptability and sustained use of three different digital interventions aimed at increasing PA. The activity tracker with PAI was perceived as the most successful of the three digital interventions due to its sufficient usability, it was widely adopted and used over time due to its perceived effect on motivation and increase in PA levels. Although acceptability and sustained use of online social support were high, the intervention was not perceived as motivational for PA. Online training was the intervention with the lowest adoption, usability and sustained use. Personalized feedback and goal setting through PAI seemed to be major factors for a positive behavioral change among the study participants. These behavioral change components were not included in the two other interventions.Personalized feedback from digital devices, like smartphone apps, is considered as an important facilitator for lifestyle behavior change (We expected home-based online training and online social support to give an additional effect to the users. However, as these two interventions were delivered as an addition to PAI, which was perceived as the most effective and useful intervention, their added impact and effect was limited. If home-based online training and online social support were delivered as stand-alone interventions, the impact on motivation and experienced effect may have been higher.This study was partly conducted during the COVID-19 lockdown or when socializing with others was limited, and it was expected that home-based online training would provide a good alternative to do PA in this time of limited PA resources. A study on the acceptability of online workout classes by a population in isolation during the pandemic reported good feedback on the impact on PA (The purpose of the online social support was to offer a closed online forum where participants could exchange experiences, discuss and motivate each other to perform PA, reach PAI point goals and create a social support environment. Although the intervention had the highest usability score and a good score on adoption and sustained use, was perceived to have a limited additional value relative to PAI on the motivation to do PA. Studies on the impact of peer-support interventions to promote PA through social media showed inconsistent results. One study reported that more activity and feedback from the users in a social support group on Facebook increased the participants PA level slightly (Digital interventions for increasing PA have shown positive effects mostly in younger populations (