The design of the MOVI-HIIT intervention is framed within the socio-ecological model of behavior modification, in such a way that it will be designed to intervene in the individual, family and school environment. It will have a duration of 8 weeks and will consist of two 5-minute daily physical activity breaks based on intervallic training, five days a week.

Students in the control group (CG) will receive mandatory lessons on Spain (one 45-minute session of Psychomotor/Physical Education), and the usual classroom teaching methodology. Teachers in the CG schools will be asked not to make any changes to their methodology during the time of the study, with the promise by the research team to share and explain the MOVI-HIIT program once the interventions are completed.

Each HIIT break will last approximately 5 minutes and will not require any specific materials. The structure of the HIIT-Rest will be as follows: 1' to describe the work to be done; 3' of work following the HIIT protocol: 6 repetitions of a functional movement such as squats, Jumping Jack or running on site for 20" at high intensity (85-90% of HR -heart rate- max) followed by 10" of recovery (65-75% of HR max); and 1' to perform a return to calm in order to lower the activation and prepare the student body to return to class activities.

The schoolchildren's exercise enjoyment will be measured with the short version of the Physical Activity Enjoyment Scale, PACES-S [10.3390/ijerph182111035]), (range from 4 to 12, where the highest rate indicates more satisfaction); and the schoolchildren's perceived effort will be assessed with the Children's Effort Rating Table, Pictorial-CERT [10.1111/j.1365-2214.2007.00767.x.], with a range from 1 to 10 (where 10 indicates the highest perceived effort).

The acceptability of the exercise program will be assessed recording adherence (rate of school's session attendance recorded in the online platform) and safety of the exercise programme (number and type of adverse events recorded by the teacher everyday after the session).

These trial feasibility outcomes will include: (1) recruitment (percentage achieved of the targeted study sample), (2) retention (drop-out rate and reasons for withdrawal), (3) outcome completion (percentage of children with both measurements at the end of the study).

An ad-hoc questionnarie will ask participants' about their satisfaction with the program (5-item pictorial scale questions, where the higher rate indicates higher satisfaction).

It will be assessed with teachers feedback via interviews on the use of the exercise platform (user-friendliness, connectivity, attractiveness, practicality) and the barriers and facilitators to implementing the programme.

Cardiorespiratory fitness (Course Navette or 20-m shuttle run test), validated to measure maximum aerobic capacity in children. It will be carried out according to the Léger protocol included in the PREFIT Battery: Assessing FITness in PREschoolers adapted to children under the age of 6.