This study is a randomized controlled trial (RCT) designed to evaluate the long-term effectiveness of "Exercise Snacks" (fragmented, high-intensity bouts of exercise) in reducing mobile phone addiction among adolescents.

Participants will be randomly assigned to either an intervention group or a control group. The intervention spans 5 months and is followed by a 1-month follow-up period (Month 6). The intervention group will perform short bursts of exercise (e.g., 1-minute sprints, squats) multiple times daily during school breaks. The program is divided into three progressive phases: adaptation, enhancement, and consolidation. The study aims to determine whether this sustained "snack-style" exercise regimen can significantly lower mobile phone addiction scores, improve physical fitness, and enhance psychological traits such as self-control and resilience over a semester-long period.

Background and Rationale: Adolescents face increasing risks of mobile phone addiction, which is associated with sedentary behavior. "Exercise Snacks"-isolated bouts of vigorous exercise performed periodically throughout the day-offer a time-efficient solution. This study applies the COMB model and Self-Determination Theory to a long-term, semester-based intervention.

Study Design: This is a single-blind, randomized controlled trial conducted over a 6-month period (5 months of intervention + 1 month of follow-up).

Intervention Protocol: The intervention group follows a progressive "Exercise Snacks" program:

Adaptation Phase (Month 1 / Weeks 1-4): Focus on habit formation. Participants perform "Sprint Snacks" (e.g., stair climbing) 3 times daily and "Strength Snacks" (e.g., squats) 2 times weekly. Intensity is monitored to ensure safety and correct posture.

Enhancement Phase (Months 2-3 / Weeks 5-12): Focus on physiological adaptation. Frequency increases to 4 daily Sprint Snacks and 3 weekly Strength Snacks. A weekly collective High-Intensity Interval Training (HIIT) session is introduced to maximize cardiorespiratory benefits.

Consolidation Phase (Months 4-5 / Weeks 13-20): Focus on psychological integration. Daily snacks continue. Collective HIIT sessions increase to twice weekly. A cognitive-behavioral guidance component is added (bi-weekly) to help students identify phone use triggers and internalize healthy behaviors.

Follow-up Phase (Month 6 / Weeks 21-24): The structured intervention ceases. Participants are encouraged to maintain self-guided exercise. Final assessments are conducted at the end of Month 6 to evaluate the retention of intervention effects and any "rebound" in mobile phone addiction.

Outcome Measures: The primary outcome is Mobile Phone Addiction (SAS-SV), assessed at baseline, Month 3, Month 5 (post-intervention), and Month 6 (follow-up). Secondary outcomes include physical activity levels (PARS-3 and wearables), self-control (BSCS), resilience (RSCA), and mental health status.

Inclusion Criteria:

1. Adolescents aged 12-18 years enrolled in the participating middle/high school.
2. Owner of a smartphone with daily usage time > 2 hours.
3. Score on the Smartphone Addiction Scale-Short Version (SAS-SV) indicating a risk of addiction (e.g., score > 31 for males, > 33 for females).
4. Physically capable of participating in high-intensity exercise (PAR-Q screening negative).
5. Provided written informed consent (from both student and guardian).

Exclusion Criteria:

1. Diagnosed with severe physical disabilities or cardiovascular diseases that contraindicate high-intensity exercise (e.g., congenital heart disease).
2. Currently participating in other professional sports training or weight loss programs.
3. Diagnosed with severe psychiatric disorders (e.g., severe depression, schizophrenia) requiring medication.
4. Taking medications that affect heart rate or cognitive function.

De-identified individual participant data (IPD) that underlie the results reported in the article will be shared. This includes the text, tables, figures, and appendices. Data will be anonymized to protect participant privacy.

Beginning 3 months following article publication and ending 5 years following article publication.

Data will be available to researchers who provide a methodologically sound proposal for meta-analysis or verification purposes. Proposals should be directed to the Principal Investigator at penny@link.cuhk.edu.hk. To gain access, data requestors will need to sign a data access agreement.

Due to the nature of the behavioral intervention (exercise training), it is not possible to blind the participants or the instructors to the group allocation. However, outcome assessors and data analysts are blinded to the group assignment to minimize detection and reporting bias. All data will be coded, and the statistical analysis will be conducted without knowledge of group identity.