Children today spend many hours sitting at school and at home, often in front of screens. Prolonged sitting in childhood increases the risk of future health problems such as diabetes, heart disease and poor mental wellbeing. MAFF is a simple school-to-home programme that teaches short, practical movement breaks in the classroom and encourages families to repeat them together at home through structured weekly challenges. Over 12 weeks, we will compare classes using MAFF with classes continuing as usual to assess whether children sit less, move more and improve their wellbeing. If effective, MAFF will provide schools and communities with an easy-to-implement toolkit to promote healthier and more active families.

Sedentary behaviour is an independent risk factor for cardiometabolic disease and poor mental health, even among individuals meeting physical activity recommendations. In children, prolonged sitting is associated with impaired metabolic regulation, increased adiposity risk, lower psychological wellbeing and behavioural tracking into adolescence and adulthood. Early intervention targeting sedentary patterns is therefore essential for primary prevention.

Most school-based interventions focus on increasing moderate-to-vigorous physical activity (MVPA) during school hours, with limited transfer to home environments and weak sustainability. Many also require specialised personnel or equipment, limiting scalability and equity.

The MAFF (Methodology for Family Physical Activity) model is grounded in the Socio-Ecological Model and the COM-B framework (Capability, Opportunity, Motivation-Behaviour). It targets:

• Capability: structured micro-activity routines delivered in class.
• Opportunity: embedding movement within existing school and home routines.
• Motivation: intergenerational engagement, social reinforcement and repeated micro-success experiences.

Its innovation lies in the structured school-to-home transfer mechanism and the focus on sedentary behaviour fragmentation through short, repeated movement bouts integrated into daily life.

Preliminary feasibility observations in school settings indicate high acceptability of micro-activity breaks among teachers and students, with minimal curricular disruption. Informal caregiver feedback suggests that structured home challenges are manageable and motivating. These observations support feasibility assumptions.

The trial will be prospectively registered in an international clinical trial registry.

Study Design A cluster randomized controlled trial (CRT) with class-level randomization will be conducted to minimize contamination. Classes will be stratified by school cycle (primary vs preparatory) before allocation.

Approximately eight classes (n≈200 children) will be recruited. The intervention will last 12 weeks, followed by a 3-month follow-up. Parental consent and child assent will be obtained.

Allocation will be performed by an independent researcher using computer-generated random sequences.

Intervention

The MAFF intervention includes:

1. Two weekly 10-15 minute micro-sessions delivered during school hours.
2. Two weekly structured home-based movement challenges involving caregivers.
3. Adherence logging with fortnightly feedback.
4. Fidelity monitoring using structured checklists. The intervention requires no specialised equipment and minimal curricular disruption. Control classes will continue usual practice.

Outcomes and Measurement

Primary outcomes:

• Daily sedentary time
• MVPA

Secondary outcomes:

• Screen time
• Sleep (validated scale)
• Psychological distress (DASS-21)
• Family cohesion (FACES IV - Portuguese version)
• Exploratory anthropometrics (BMI percentile; waist circumference) Accelerometry will be collected in a stratified subsample (n≈60) using 7-day wear protocols at baseline and post-intervention (and follow-up if feasible). Valid wear-time criteria will be predefined. Anthropometric assessments will follow standardized procedures conducted by trained assessors.

Process Evaluation A RE-AIM framework will assess reach, adoption, implementation fidelity, dose delivered/received, and maintenance at follow-up. Adherence will be quantified through logs and session tracking.

Statistical Analysis All analyses will follow the intention-to-treat principle. Multilevel mixed-effects models will account for clustering (children nested within classes). Baseline covariates (cycle, sex, socioeconomic status) will be included where appropriate.

Primary analyses will test group × time interaction effects on sedentary time and MVPA. Secondary analyses will examine mediation (family cohesion; adherence) and moderation (sex; school cycle). Sensitivity analyses using multiple imputation will address missing data. Effect sizes and 95% confidence intervals will be reported.

Power Calculation A priori sample size estimation assumed a moderate effect size (Cohen's d = 0.40) for sedentary time reduction, α = 0.05 and power = 0.80. Assuming an intraclass correlation coefficient (ICC) of 0.03 and an average cluster size of 25 students, eight clusters (≈200 participants) provide adequate power. Allowing for 15% attrition, the study remains sufficiently powered under intention-to-treat analysis.

Data Management and Quality Assurance Data will be stored securely in compliance with GDPR. Predefined analysis protocols will be established prior to statistical modelling. Measurement staff will be trained and standardized.

Contribution to the Evidence Base By integrating ecological behavioural theory, cluster randomized design, objective measurement and mediation analysis, this project generates causal evidence on scalable sedentary behaviour reduction strategies in children and strengthens the preventive public health evidence base.

The project directly aligns with preventive health priorities aimed at reducing behavioural risk factors early in life.

Inclusion Criteria:

• Enrolled in participating Portuguese public schools
• Written informed consent provided by a parent or legal guardian
• Assent provided by the child
• Able to participate in light-to-moderate physical activity as part of normal school activities

Exclusion Criteria:

• Medical conditions or physical limitations that contraindicate participation in physical activity
• Cognitive, developmental, or behavioural conditions that would prevent understanding or adherence to the intervention procedures (as assessed by school staff)
• Lack of parental/legal guardian consent
• Concurrent participation in another structured physical activity or behavioural intervention study

De-identified participant-level data, including daily sedentary time, screen time, moderate-to-vigorous physical activity (MVPA), sleep, psychological distress, family cohesion, and anthropometric measures, will be shared with qualified researchers upon reasonable request after publication of main study results. Data will be provided via a secure repository, and use will require ethical approval and adherence to study guidelines.

IPD and supporting information will be available after publication of main study results, starting approximately [Month Year] and remaining accessible for at least 5 years.

Qualified researchers with a scientifically valid proposal and ethical approval may request access. Access will include de-identified participant-level data, study protocol and CSR. Data will be shared via a secure repository, with access granted after review by the study team.