For this investigation a total of ≃300 children (grades 5 and 6; 10-12 years of age) from 6 randomly selected schools in Valencia (Spain) will be enrolled during 8 weeks in one of the experimental groups (EG): EG1 (15' of RT Functional HIIT circuit program), EG2 (the same EG1 program but with a gamification approach) or CG (control group). Outcomes will include age and sex, maturity, PA profile, MF (assessed using an ad hoc battery), anthropometry and body composition (weight, height, sitting height, waist circumference and BMI) as well as the enjoyment and motivation during physical education. An interactive app in a digital device will be used for the gamification story, the point's registration with the personal children's avatar and content learning by flipped classroom. The hypothesis of the RETRAGAM study posits that children who participate in the gamified RT intervention will experience greater improvements in MF, body composition, motivation, and enjoyment compared to those who carry out the same intervention without gamification, as well as the control group (CG). Thus, the primary aim of the RETRAGAM study is to examine the effectiveness of a school-based RT intervention on MF, body composition, motivation, and enjoyment in children aged 10 to 12 years. Additionally, the study will compare the effects on the main outcomes among several intervention groups (i.e., RT, RT with gamification approach and the CG).

Study design, participants and selection criteria The RETRAGAM study is a randomized controlled trial conducted in public primary schools in Valencia, Spain, with approval from The Human Research Ethics Committee of the Universitat of Valencia (Id: UV-INV_ETICA-2681359). The study aims to investigate the impact of a resistance training intervention with and without gamification on children aged 10-12 years.

To select the participating schools, random schools from a total of 167 primary schools in Valencia will be contacted by phone to inquire about their compliance with the inclusion criteria. Researchers will explain the study's main objectives, and once 6 schools meeting all the requirements are identified, they will be enrolled in the study. Each school will then be randomly assigned to one of three groups: EG1, EG2, or the CG. Within each school, one grade 5 and one grade 6 class will be randomly selected for the study. Subsequently, researchers will conduct a meeting with the school staff, to discuss the study's main objectives and the intervention procedure.

Among the three intervention schools selected, one will implement a 15-minute RT circuit program (EG1, n≃100), another will apply the same program but including gamification strategies (EG2, n≃100). The remaining school will serve as the control group (CG, n≃100). In the intervention schools, the first 15 minutes of PE lessons, typically allocated for warm-up, will be replaced by the RT circuit program. In the CG school, children will continue with their regular PE sessions.

To determine the sample size for the study, effect sizes for the primary outcome, muscular fitness, were estimated. While the health benefits of muscular fitness are widely acknowledged, there is a lack of consensus regarding the clinical significance of changes in MF in young people. Based on a prior study (Lubans et al., 2016), an effect size d=0.4 for MF is anticipated. Accordingly, a minimum sample of 162 children (54 per intervention group × 3 groups) is required to detect between-group changes in MF with a power of 80% and α=0.05. To account for potential dropout in a school context (up to 10%), the recruitment target is 145 children. However, to ensure an ample number of participants, ≃300 students will be recruited (≃100 per intervention group).

Interventions

Considering that school-based RT interventions should be realistic, adapted to the school timetable, and focused on improve MF, body composition, enjoyment and motivation in primary school children 10-12 years aged, the following four 8-week school-based interventions programs were chosen for this study:

The two-interventions included in the RETRAGAM study have been designed by the researcher based on suggestions regarding the desirability of exercises to specifically develop MF following the RT guidelines for children and adolescents (Faigenbaum et al., 2009b; Lloyd et al., 2014) and considering previous developed interventions in the same range of age (Villa-González et al., 2022).

EG1: 15 min RT (FunctionalHIIT) circuit program High intensity interval training (HIIT) describes physical exercise characterized by short and intermittent efforts of vigorous activity, interspersed with resting periods at passive or low-intensity exercises (Amaro-Gahete et al., 2018). During each FunctionalHIIT training the teacher will demonstrate performance movements and will give feedback to students considering the correct execution providing standardized verbal encouragement.

Volume: ≃10 min/day three times per week at moderate to high intensity (7-10 in the Borg's CR-10 scale) (Borg, 1990) a total of ≃30 min/week, which will be the half than the minimum children's PA recommended by the WHO (60 min/week at MVPA).

Intensity: Participants were familiarized with the Borg's CR-10 scale (Borg, 1990) before the start of the intervention. During the study the children will instruct to follow the movements targeting firstly 6-7 and 9-10 at the end. After each FunctionalHIIT training, all students will rate their perceived exertion during exercise. Although there are previous studies that control intensity using the Borg scale in children of these ages (Chen et al., 2017; Engel et al., 2019; Moran et al., 2019), some random students belong to all groups (i.e., EG1, EG2 and CG) will wear a heart rate monitor (Polar Ignite 2 monitor) to control different intensity parameters (i.e., HR, HRMax, HRV).

Frequency: 3 times per week, based on a previous intervention on FunctionalHIIT in children (Engel et al., 2019) and following the RT guidelines for children and adolescents (Faigenbaum et al., 2009b; Stricker et al., 2020) and the recommendations of the Resistance Training for Children Consensus (Lloyd et al., 2014).

Type of exercise: For the FunctionalHIIT program the participants will perform eight weight-bearing exercises in circuit format (i.e., push-ups, squats, planks, etc.).

Training load variation: We consider that participants will not immediately meet the required intensity level. Instead, a gradual progression will be introduced to control the exercise dose. Initially, participants will start at an intensity level of 6-7 on the Borg's CR-10 scale, and this will increase by 1 point every two weeks until reaching 9-10 out of 10. The training duration during the main portion will increase by 5 seconds every two weeks, while the recovery interval between exercises will decrease (from 3 seconds initially to 15 seconds at the end). The exercises themselves will become progressively more challenging over the course of the training program and change every two weeks. Furthermore, considering individual characteristics, the progression will be adapted for particularly inactive or overweight children, allowing them to repeat exercises from the previous week.

Training sessions: Each training session will commence with a 3-minute dynamic standardized warm-up (3-6 RPE), including muscle activation exercises such as skipping, jumping jacks, and jumps. The 10-minute main part (7-10 RPE) will involve participants performing eight exercises in pairs or trios in a circuit format, with a focus on making as many repetitions as possible and targeting the previously indicated RPE levels. The teacher, along with the support of two selected students from the school, will encourage and monitor the technique and effort during the circuit. The training session will conclude with a 2-minute cooling-down (3-4 RPE) that includes stretching exercises.

EG2: 15 min RT (FunctionalHIIT) circuit program with gamification The EG2 training part will develop following the same protocol described in EG1.For the gamification experience, students will embark on a superhero-themed narrative journey where they must train diligently to earn points and badges, improving their avatars along the way. The ultimate goal is to defeat a super villain in the final battle after 8 weeks. The RETRAGAM study utilizes a self-created app (accessible at https://goo.su/GVgSPW) for the gamification story, point tracking, and personalization of children's avatars. As part of the flipped classroom approach, children can access the platform to preview upcoming exercises and educational content (i.e., benefits of RT, knowledge of the muscles, bones and joints involved in exercises, postural habits, etc.). To motivate students, biweekly motivational videos related to the superhero narrative will be shared, encouraging them to actively participate in the RT program. At the end of each training day, students will conduct self-evaluations, earning 0-3 points based on their level of training engagement (exercise technique, RPE compliance and comradeship). These points will be used to improve their individual avatars at the end of each week (i.e., superhero attires, equipment, power, etc.). At the end of the 8 weeks, students will have to defeat the super villain at the final battle (questionnaire on what has been learned) and they will receive a physical badge that they can wear in their t-shirt at the retest measurement session. Teacher will have a spreadsheet to record all the points that will complement the evaluation of the PE sessions. Figure 1 summarises the EG2 intervention.

CG (Control group) We will provide general advice to the CG participants though an information meeting that will indicated to maintain their lifestyle. At the end of the intervention, in order to comply with the ethical commitment, PE teachers will be instructed on the RT programme and the RETRAGAM app to allow them to carry out the intervention autonomously when they consider.

Data analysis All outcome variables will be verified for normality, and the results will be presented using mean and standard deviation or median and interquartile range, where appropriate for continuous variables, and frequency and percentage for categorical variables. We will use an analysis of covariance (ANCOVA) for between group comparisons at the baseline (EG1 vs EG2 vs CG), and the nonparametric method of Kruskal-Wallis and t-tests, as appropriate.

To assess the effectiveness of the different interventions on objectively measured MF and the other outcomes, a multi-factor ANOVA [time-point (T0, T1) x group (EG1 vs EG2 vs CG)] will be used with the outcome measures as dependent variables in separate models, the intervention as an independent variable and controlling for potential confounders (such as gender, sociodemographic characteristics, health status, BMI, PHV, PA level and MF baseline). Statistical significance will be set at p<0.05.

Inclusion Criteria:

As this is a school-based intervention study, the inclusion criteria are for the school itself. They are as follows:

1. 1 class of grade 5 and other of grade 6 in the school.
2. At least 20 children class size.
3. Includes 45 min PE sessions three times per week in both grades.
4. They cannot develop physical condition didactic unite at the same time.

Exclusion Criteria:

• Exclusion criteria include participants who have diagnosed medical problems that prevent them from participating in the intervention.