Understanding the “how” and “why”: A mixed methods process evaluation for the PRO-HIIT intervention

INTRODUCTION: Process evaluation completes outcome evaluation by explaining "how" and "why" an intervention is (in)effective. The aim of this study was to conduct a mixed methods process evaluation for the PRO-HIIT intervention. METHODS: The PRO-HIIT intervention replaced the traditional warm-up period with 6-8 minutes of high-intensity interval training in the physical education and activity lessons, aiming to promote physical fitness, psychological parameters and academic performances among Chinese adolescents. The process evaluation was guided by the Medical Research Council guidance for the evaluation of complex interventions. Three process evaluation domains, including twelve process evaluation measures, were assessed using both quantitative and qualitative methods. Key means for process evaluation included were training logbook recording, intensity monitoring, and semi structured focus groups. RESULTS: The PRO-HIIT intervention showed high level of retention rate and dose received. The dose delivered was slightly less than anticipated, with an average 26.5 sessions delivered over twelve intervention weeks. The average heart rate was 146 beats per minute, corresponding to 71% maximum heart rate, with the mean heart rate peak was 175 beats per minute (85% maximum heart rate). The average sessional rating of perceived exertion was 5, ranging from 3 to 8. Overall, participants and physical education teachers showed positive response towards the PRO-HIIT intervention. Session duration and work-to-rest ratio were adapted to balance the intervention satisfaction and effectiveness. Barriers to intervention delivery included competing priorities, severe weather, lack of sleep, repetition of exercises, and rating of perceived exertion administration, whereas facilitators included space efficiency, imparting knowledge, peer coaches, music, and physical education teachers perform the session with participants. No intervention-related injury occurred. CONCLUSIONS: This process evaluation provided a lens through which to facilitate the interpretation of the effectiveness of the PRO-HIIT intervention. The results provide valuable insights into how a school-based HIIT intervention can be implemented and refined.