The Active-Start study was a non-blinded randomized controlled trial (NCT03893149), conducted with children from three public schools classified as high vulnerable and located in a deprived area of Santiago (Chile). All students in fourth grade (aged 8–10 years) within participating schools were eligible for participation. A total of 198 children from the 5 classes participating in the study were invited to take part in the Active-Start program. Out of the 198 children, only 170 (85.8%) agreed to the informed consent and were included in the study. From these, 146 had valid data for all measurements (baseline and at the end of the intervention) and were included in the analysis (intervention group, n= 88, 41.2% girls, mean=10.12±0.79 years; and control group, n= 58, 50.9% girls, mean= 9.97±0.51 years). Exclusion criteria included (1) children with some physical pathology or medical contraindication to perform physical exercise or (2) diagnosed with learning disabilities or mental disorders. Both the intervention and control group continued to receive their standard physical education lessons (2h/week).

Out of the 21 schools that were potentially available for participation in the study, four schools showed interest and were further informed about the study. Although the rest of the schools showed interest, time constraints or other logistic issues precluded them from participating in the study. Finally, only three schools accepted to participate in the program. There were five fourth-grade classes in total. Three of them were allocated to the intervention group and the two remaining were allocated to the control group. These groups were randomly selected on the basis of a computer-generated random number list. The randomization and intervention occurred at class level because of feasibility reasons.

The study protocol was approved by the University of Santiago Ethics Committee (Code number: 938) and conformed to the principles of the Declaration of Helsinki. Parents were asked to give their written informed consent before children enrolment in the study. The parents or children could revoke this consent and withdraw from the study at any stage.

The Active-Start intervention. Students participated in the Active-Start program during the spring semester of 2018. The intervention was delivered 5 times per week before starting the first school-class (8:00–8:30 a.m.). The program lasted for 8 weeks. Each session was previously planned and described in a manual dedicated to the study. These sessions were designed by the research team and delivered by a graduate in Sport Sciences who was not directly involved in the research study and was previously trained on how to deliver the intervention so that it was standardized across the classes allocated to the intervention group. The intervention program included sports games adapted to the age of the participants, playground games, dance and other recreational and cooperatives activities. The intensity of the main part of the sessions was moderate-to-vigorous according to a previous study (García-Prieto et al., 2017) and this intensity was confirmed by accelerometry. Also, different activities favoring social interactions were performed to facilitate the interactions between the participants at the end of the sessions (cold-down). Each class allocated to the intervention group was exposed to approximately 39 Active-Start sessions.

Recruitment occurred in August 2018, with baseline and follow-up measures collected in September and December 2018, respectively. Trained researchers measured the variables and outcomes of the study under standardized conditions. All data were collected at the same time in the morning, between 8:00 a.m. and 11:00 a.m.

Bullying victimization. To assess bullying victimization, this study used a scale extracted from the Single School Well-being Questionnaire-CUBE (in Spanish, Cuestionario Único de Bienestar Escolar—CUBE) (Ministerio de Educación del Perú– MINEDU, 2013). This scale has 10 items that explore the reason for experiencing bullying, however we excluded the cyberbullying questions (two items) due to the age of the children. Children were asked how often they had been bullied at school in the past seven days for a variety of bullying types including physical bullying; verbal bullying; and social exclusion bullying. Each bullying type was assessed using a 4-point Likert scale: none, only once, twice to 4 times, and 5 or more times. We used the cut-off point of “twice to 4 times” or more to code a student as bullied in each particular category of bullying. The scale presents a Cronbach's alpha of .81.

Weight status. Body weight was measured to the nearest 0.1kg using a portable electronic scale (Seca 769, Hamburg, Germany), and height was measured to the nearest 0.1cm using a portable stadiometer (Seca 220, Hamburg, Germany). Weight status was classified using the International Obesity Task Force age-specific and sex-specific thresholds (Cole, Bellizzi, Flegal, & Dietz, 2000).

Biological maturation. Peak height velocity (PHV) is a common indicator of maturity in children and adolescents (Malina, Rogol, Cumming, Coelho e Silva, & Figueiredo, 2015). Anthropometric variables (weight, height and seated height) were used to obtain PHV according to Moore's equations (Moore et al., 2015). Years from PHV were calculated by subtracting the age of PHV from the chronological age. The difference in years was defined as a value of maturity offset.

Physical activity. Moderate-to-vigorous PA (MVPA) was assessed using a GENEActiv (ActivInsights Ltd, Kimbolton, UK) tri-axial accelerometer previous to the program intervention. The GENEActiv was attached to a polyurethane strap, and worn at the wrist, like a watch. Data were collected at a rate of 85.7Hz. Children were informed about the monitor placement and were asked to wear the monitors on their non-dominant wrist, continuously for a period of seven days (two weekend days) (Schaefer, Nigg, Hill, Brink, & Browning, 2014). Non-wear time was assessed over 60minutes windows, using moving increments of 15minutes. Data were downloaded using GENEActiv software version 1.4 and analyzed using the GGIR software package for R (cran.r-project.org). Children who achieved on average of ≥60minutes of MVPA per day were categorized as active.

Conventional summary statistics were used to describe participants in the study on key variables and demographics characteristics. First, we tested associations between bullying victimization, age, sex, weight status, PHV and MVPA at baseline by computing zero-order correlations using Pearson correlations. Intervention effects were estimated using logistic regression models and are presented as odds ratios (OR) with 95% CIs, with adjustment for baseline bullying victimization and MVPA, and with age, sex, weight status, PHV, and school as covariates. Data analyses were performed using the Statistical Package for Social Sciences (Version 23.0) software. Statistical significance was set at p < .05.