In the present study, FIT FIRST 10 will run over 20 weeks, with cardiometabolic fitness as the primary outcome and project acceptability as the co-primary outcome. In this study the investigators will be testing the dose-response of the FIT FIRST concept by having two experimental groups. This will be a cluster RCT with a 1:1:1 recruitment of control schools, intervention schools with 3 weekly 40-min FIT FIRST 10 lessons, and intervention schools with 1.5 weekly 40-min session. There will be recruited a total of 1000 children, with 500 8-9-year-olds from 2nd and 3rd grade in each group from a minimum of 40 classes from 16 schools. There will be subgroup analyses of children with low socioeconomic status and ethnic minority background. Intervention effects will be tested as on health profile, cardiometabolic and musculoskeletal fitness, motivation for physical activity, acceptability of the programme for stakeholders as well as the implementation potential. The study will be running in Q1 and Q2 in 2023.

It is hypothesized that the FIT FIRST 10 concept will improve the well-being, increase sports club participation as well as increase fitness and health levels among 8-9-year-old children with low fitness, low socioeconomic and/or ethnic minority background. However, it is also hypothesized that the effects on well-being and fitness levels caused by the intervention may be most significant among ethnic minority children not enrolled in sports clubs.

Background Physical activity (PA) is an important part of a healthy lifestyle for children and youth, as it improves physical fitness, motor competences and well-being, and reduces the risk of developing overweight and lifestyle diseases later in life. Current recommendations from the Danish Health Authorities are that young people should participate in at least 60 min of moderate-to-vigorous physical activity (MVPA) per day, of which at least 3 times 30 min per week should involve vigorous physical activity (VPA).

As children's total amount of daily PA is a product of their activities in and outside of school, it can be of value to look for synergetic effects that can be achieved by having developed abilities and motivation for activities in school that can be used in leisure time sports clubs.

Based on the beforementioned knowledge, investigators have developed the FIT FIRST programme. FIT FIRST was originally an acronym for a 40-week intervention study for 8-9-year-olds: Frequent Intense Training through Football, Interval Running and Circuit Strength Training. This study showed that 3x40 min per week of FIT FIRST ball game and circuit strength training activities was very effective in improving fat percentage, blood pressure, muscle strength, postural balance and bone mineralization. In 2018, the FIT FIRST 10 concept was developed for 6-9-year-olds that covered 10 sports using the same principles.

In the present study, FIT FIRST 10 will run over 20 weeks with cardiometabolic and musculoskeletal fitness as the primary outcomes and project acceptability as co-primary outcomes. In this study the investigators will be testing the dose-response of the FIT FIRST 10 concept by having two experimental groups. This will be a cluster RCT with a 1:1:1 recruitment of control schools, intervention schools with 3 weekly 40-min FIT FIRST 10 lessons, and intervention schools with 1.5 weekly 40-min session. There will be recruited a total of 2400 children, with 800 8-9-year-olds from 2nd and 3rd grade in each group from approximately 60 classes from 24 schools. There will be subgroup analyses of children with low socioeconomic status and ethnic minority background.

Intervention effects will be tested as in Study I on health profile, cardiometabolic and muscular fitness, motivation for physical activity, acceptability of the programme for stakeholders, and implementation potential. The study will be running in Q1 and Q2 in 2023.

Measures:

Fitness levels and health profile:

Yo-Yo IR1 Children's test (total distance in meters) will be used to evaluate cardiometabolic fitness. For muscular fitness, the standing long jump test (in meters), the handgrip strength test (in kilograms), the arrowhead agility-test (in seconds) and the stork balance stand test (in seconds) will be used.

To evaluate the effects on health, measurements of resting heart rate (beats per minute) and blood pressure (millimeters of mercury) will be recorded.

Also, the participants' body composition and height will be measured using Tanita scales and InBody 230 Bioimpedance. The measurements of the participants' body composition includes measures of their body mass (in kilograms), skeletal muscle mass (in kilograms) and fat percentage. To investigate the prevalence of overweight and obesity, BMI z-scores (kg/m^2) will be calculated by using the participants' relative weight adjusted for child age and sex.

Well-being, body image, self-worth and quality of life:

KIDSCREEN 27, which is a validated questionnaire measuring physical well-being, mental well-being, social well-being and school well-being in the school environment will be used. Children's body perception and satisfaction will be measured using the Children's Body Image Scale, a pictorial scale developed for 7-11 year old children. Furthermore, the short form of the Physical Self-Inventory (PSI-S) will be used to evaluate overall selfworth (i.e., perceptions of one's physical self) and specific sub-components (physical appearance, strength, physical condition and sport competence).

Evaluation of physical activity and sport club participation:

A questionnaire will provide information in terms of types of activities undertaken outside school.

Process evaluation:

The RE-AIM framework will be used to ensure a holistic assessment of intervention and implementation outcomes. A mixed-method approach, including document analysis, participant observations, objective measurements, survey instruments and interviews will be applied. The RE-AIM framework has been used extensively to assess school-based PA interventions and to guide process evaluations. The RE-AIM have been in several studies. The framework constitutes five key elements, i.e. Reach, Effectiveness, Adoption, Implementation and Maintenance.

Also, the ecological model of implementation, by Durlak and Dupree will be used.

Inclusion Criteria:

• part of enrolled classes

Exclusion Criteria:

• none