Discussion
RECRUITMENT
The home-based HIIT intervention appears feasible based on meeting the acceptability and implementation parameters. Specifically, we met our recruitment goal within the intended timeframe, which was to recruit 40 participants in six months. Our retention rate (98%) was higher than our 90% retention goal. We randomized two-thirds of the eligible participants. Additionally, participants attended a mean number of 90% of the motivational sessions, which exceeded our 75% goal. The mean rating across the seven consumer satisfaction items was 5.07, which exceed our goal of five. Participants rated the information from the exercise counselor and the exercise sessions as the highest and the online exercise logs the lowest.Regarding efficacy, contrary to our hypotheses and previous studies [The study was not powered to examine which psychosocial variables mediated the effect of the intervention on PA. Therefore, in order to inform future trials examining mediation, only the effect of the intervention on the psychosocial variables was examined. Self-efficacy of physical activity, enjoyment, outcome expectations (i.e., expected outcomes and value of those outcomes as a result of PA), and positive engagement (i.e., upbeat, enthusiastic, happy) increased from baseline to 12 weeks for the intervention relative to the control. This is consistent with previous studies indicating that these psychosocial variables are related to physical activity behavior change [Among the participants randomized to the HIIT sessions, there were no differences between the moderate and HIIT sessions regarding affective responses (i.e., ratings of pleasure and enjoyment). This is inconsistent with previous HIIT studies [There were several strengths to this study. First, the examination of a home-based HIIT intervention is novel given a majority of HIIT studies have been lab-based. Second, the study was strong methodologically in that participants were randomized to the conditions, there was quality control for the counseling sessions, an experienced health coach delivered the intervention, and validated measures were used. Finally, the study sample was relatively diverse when compared to the overall population of the recruitment area. Despite these strengths, there were some limitations. First, data from the objective measure of PA was not included given the lack of validity for using accelerometers to assess HIIT sessions [The HIIT intervention appears feasible based on retention, recruitment, adherence to the counseling sessions, follow-up rates, and the consumer satisfaction survey. Participants indicated they were the least satisfied with the online exercise logs suggesting that additional research is needed to improve upon the usability and acceptability of online exercise logs. Participants were the most satisfied with the exercise counseling sessions indicating that personal motivational sessions may be important for adherence to HIIT. Based on the feasibility data and the preliminary efficacy data indicating higher rates for vigorous intensity PA among the HIIT intervention relative to control, it appears that the HIIT intervention should be examined in a large trial. It will be important to examine affective responses to better understand how these responses influence adherence. Studies should explore integrating HIIT interventions within existing opportunities (e.g., physical education) and increase physical literacy [In summary, this small pilot study indicates that home-based HIIT is acceptable and feasible for low active adults. However, selection bias may have played a role in the acceptability of vigorous intensity PA. It is possible that participants who enrolled in the study were more open to vigorous intensity PA than those not interested in the study. HIIT may be advantageous to traditional PA given it takes less time to meet the PA recommendation and it may be more enjoyable than moderate intensity activity. Future studies with larger sample sizes and objective measures of PA are needed to better understand if this study is viable for dissemination to low active adults.