Randomised Controlled Feasibility Trial

The trial targets approximately 120 expressions of interest to yield a final sample of about 60 participants. The study achieved an overall retention rate of 90%. The protocol follows SPIRIT 2025 guidance and plans reporting according to CONSORT 2025 guidance for randomised feasibility trials. This is the first feasibility RCT to deliver co-designed, individualised vigorous-intensity training to adolescents through mHealth with real-time feedback. Outcomes were assessed at baseline, after 8 weeks, and at 4-month follow-up. Participants are informed of their group allocation only after completing baseline assessments. No formal sample size calculation was performed because feasibility is the main purpose of the trial. Blinding of participants and interventionists is not possible, which is identified as a limitation. MOTAFIT is a three-arm feasibility RCT with 1:1:1 randomisation to MOTAFIT, active control, and control groups. The study is designed to assess whether a definitive later trial would be feasible, rather than to statistically test effectiveness. The primary aim is to explore uncertainties in conducting a future definitive randomised controlled trial of IonaFFV.