This study aims to conduct a cluster RCT intervention study with adolescents in Denmark and investigate whether the FIT FIRST concept can improve young people's bone health. We hypothesize that students from the schools randomized to the intervention arm will show greater improvements in bone health than students randomized to the control arm. The experiment will be of societal relevance in relation to evidence for the effects of implementing high-intensity physical activity in school.

In 2018, the FIT FIRST 10 concept was developed for 6-9 year olds. It included 10 sports, all of which were modified to include both strength and cardio training, but based on sports that you can go to in your spare time, as opposed to many other classic school activities such as roundball, stickball and catch. In 2020, the FIT FIRST 20 concept for 10-12 year olds was developed to include 20 sports, and the FIT FIRST Teen material with more than 20 sports has been released in 2024.

In short, all versions of the FIT FIRST concept are built on the principles of using multiple sports activities modified for the school environment, with a focus on high intensity, high commitment and inclusion. The use of several sports activities that are modified for the school environment can also affect recess and leisure sports, as students are introduced to many fun activities that are easy to organize themselves and/or go to clubs in their spare time.

The implementation of the FIT FIRST concept involves training teachers to deliver the program, which has been developed by experienced staff from the University of Southern Denmark (SDU), the Sports Confederation of Denmark (DIF) and Team Denmark. These partners have developed 3-6 lessons for each of the sports, all using exercises and activities carried out in pairs or small groups. All the lessons included in the FIT FIRST Teen manual are designed to provide high-intensity strength and cardio training and to be motivating for children to participate, according to self-determination theory.

a) Design: A cluster-randomized trial design is used, where school classes are randomly divided into an intervention group that implements the FIT FIRST training concept, and a control group that continues regular schooling.

c) Practical implementation: Schools on Funen will be invited via e-mails, newsletters from the Danish Sports Confederation and social media, and can sign up with 1-2 classes. All students at the participating schools can participate in the survey. The first 10 schools that agree to participate in the study will be randomized into two arms, intervention and control. The intervention arm will implement 3 weekly 40-minute FIT FIRST Teen lessons over the course of 18 weeks, equivalent to half a school year. The teachers who will be responsible for the teaching are trained on a 6-hour theoretical/practical course and the schools receive an equipment package with balls, cones and vests for use in the teaching. The FIT FIRST intervention contains activities that are common in physical education and movement education and will therefore involve the whole class, including the students who are not test subjects in this study. The workout is designed to have elements of high-heart rate training and elements that stimulate bones and muscles. All exercises are based on sports activities you can do in your spare time.

On the test days, all students will also join the class on a trip to the University of Southern Denmark, where only test subjects with consent will participate in trial activities, while others will only participate in sports activities organized by their teacher on the university's campuses, where there are facilities such as a swimming pool and jumping center that make it attractive for the classes to visit.

The schools in the control arm will continue their normal sports curriculum and they will receive the intervention at the end of the trial as a thank you for their efforts. Here, too, the whole class comes on a trip and is allowed to borrow the university's sports facilities.

Bone density will be measured with a whole-body DXA scan before and after the 18 weeks, and bone markers in the blood will be measured via a venous blood sample. In addition, hand grip strength is measured The project will run in 2026.

Tests and measurements:

Anthropometric measurements: Measurement of height and weight is carried out with a portable stadiometer and scale, respectively.

DXA scan: A full-body DXA scan is performed to determine bone mineralization and body composition (fat percentage and muscle mass). The DXA scanner is iDXA Lunar, GE Healthcare. The DXA scan will take place at a hospital or university located close to the school. The test is performed by a radiographer or trained research staff after 0 and 12 weeks.

Blood samples: Blood sampling is performed with a cannula from a vein in the arm for analysis of the concentrations of Osteocalcin, procollagen type I N-terminal propeptide (PINP) and β-C-terminal telopeptide of type I collagen (CTX-I). The blood is collected in 4 ml EDTA glass. Thus, a total of 8 ml of venous blood is collected from each test subject over the two visits.

Muscle strength: Handgrip strength is measured with a handgrip dynamometer (CAMRY).

Intervention The classes in the intervention group will carry out 3 weekly sessions of 40 minutes duration, where they use the FIT FIRST concept with high-intensity, fun and involving training based on sports that you can go to in your spare time. It will be their own teachers who are responsible for the teaching after they have been on a 6 hour course to learn how to use the concept.

4. Statistical considerations.

a) The size of the sample (200 students from a total of 10 classes, 5 in each group) is selected based on an expected effect size of 0.025 g/cm2 on whole-body bone density and a standard deviation of 0.035 g/cm2. Intra Cluster Correlation is expected to be 0.02 g/cm214-16 and the size of the classes is expected to be 20 students on average.

5. Subjects A. Inclusion criteria

• Healthy boys and girls in 7th+8th grade aged 12-14.99 years.
• Informed consent of the parents/authority holder(s).
• Willingness to carry out training and testing.

B. Exclusion Criteria:

• Subjects who are not considered by the researchers to be suitable to follow the protocol/testing and training program.

Inclusion Criteria:

• Part of participating classes

Exclusion Criteria:

• None

Not possible within the Danish GDPR regulations