The goal of this clinical trial is to learn if the addition of physical exercise and mindfulness training, delivered by a mobile application, to multimodal pain rehabilitation could lead to a mor effective rehabilitation process and improved outcomes. The main questions it aims to answer are:

Does 6 weeks of app-based physical exercise and mindfulness in combination with 6 weeks of multimodal hospital based pain rehabilitation give inferior results compared to 12 weeks of multimodal hospital based pain rehabilitation? Thus the study is an inferiority trial of a scaled down pain rehabilitation period with the addition of physical exercise and mindfulness training done at home with directions from a mobile application.

Participants randomized into the active intervention will begin training for 6 weeks with the app, and continue through out the rehabilitation period. Those randomized into the control group will immediately begin pain rehabilitation as usual. All participants will wear an accelerometer bracelet for a week, before, in the middle and after the 12 week study period, and fill out a questionnaire and training logs.

BACKGROUND Long-term pain, defined as lasting longer than 3 months, is common in childhood. Approximately 8-10% of all children and adolescents experience long-term physical and psychological suffering and an affected level of functioning due to pain. A recently published long-term follow-up study indicates a bleak future outlook with a clearly increased risk of low educational attainment, lack of success in professional life, and poorer mental health. Why adolescents develop long-term pain is not fully understood. The biopsychosocial pathogenesis is likely multifactorial, and dysfunctional ways of coping with stress appear to play a crucial role in both the onset and maintenance of pain.

There is considerably less scientific support for the treatments recommended for children and adolescents compared to the state of knowledge for adults. In its 2020 guidelines, the WHO also emphasized the need for studies on innovative methods of delivering physiotherapy and psychological interventions, including mobile apps, to support patients and their families participating in pain rehabilitation. Studies of new ways of delivering interventions can also shed light on how treatments that have shown promising effects in adults can best be adapted to children and young people.

The only treatment with robust evidence is interdisciplinary multimodal rehabilitation with elements of physiotherapy and psychotherapy. However, multimodal rehabilitation is demanding and only available to a limited number of patients. In the field of behavioral medicine research, a paradigm shift is underway, with many clinicians and researchers moving away from a myopic focus on symptoms and instead exploring methods that are based on patients' resources and strengths. Two examples of resource-based interventions that could streamline the rehabilitation process are graded physical activity and mindfulness, which have in common that they can be prescribed and carried out via digital technology.

The idea of complementing pain rehabilitation with a combination of daily physical activity and mindfulness is theoretically supported by Self-Determination Theory (SDT). According to SDT, psychological resources and strengths are built up in interaction with the environment if the individual feels free to move in a desired direction (autonomy), feels capable of influencing their situation (competence), and experiences a sense of belonging (connectedness). A high degree of autonomy also forms the foundation for the development of intrinsic motivation, which is something young people with long-term pain need if changes in lifestyle and strategies are to be sustainable over time.

The CACTUS project aims to modify these interventions into effective digital complements to multimodal pain rehabilitation and to study the design and new way of delivering the interventions in a randomized controlled pilot study. The project will also study the families' motivation for the offered pain rehabilitation via qualitative methodology. The project's design is based on experiences and expertise that the research group has built up during work on several previous studies of psychological interventions, which have been based on both hospital-based rehabilitation and various digital platforms.

RESEARCH QUESTIONS AND HYPOTHESES The overall aim of the CACTUS project is to study the potential of digitally prescribed physical activity and mindfulness - which have shown promising results in adult patients - as integrated parts of multimodal pain rehabilitation for adolescents aged 13-18 years, and to generate an increased understanding of how motivation for participation can be strengthened.

A first hypothesis is that adolescents can benefit as much from daily physical activity and mindfulness exercises as adults, but that adaptations are required in the way the interventions are prescribed to reach and motivate patients. A second hypothesis is that it is entirely possible to let adolescents exercise on their own with the support of a mobile app. A third hypothesis is that physical activity and mindfulness have synergistic effects if they are combined in time and space. This is because physical activity strengthens learning and concentration, which can catalyze the acquisition of new perspectives and strategies, while mindfulness can contribute to maintaining motivation for daily movement and reduce the risk of giving up in the face of temporary setbacks.

The long-term goal of the project is to further develop and evaluate the study's interventions and the new digital way of prescribing parts of the rehabilitation, which can ultimately increase accessibility and the proportion of patients who achieve positive results. A second goal is to generate knowledge and insights into what motivates young people to engage in their rehabilitation, which can help make multimodal pain rehabilitation both more effective.

The project is designed to find out whether a reduced period of pain rehabilitation, supplemented with digitally delivered physical activity and mindfulness, is as effective as a longer period of traditional treatment. Expressed as a sharp research question: Is 6 weeks of multimodal pain rehabilitation, plus 6 weeks of physical activity and mindfulness training - prescribed via a mobile application - as effective and safe as 12 weeks of regular pain rehabilitation?

STUDY DESIGN The quantitative part of the CACUTS project is thus designed as a randomized controlled non-inferiority trial. See Figure 1 below. Approximately 60 patients aged 13-18 years will be recruited consecutively and randomized to either start their rehabilitation with 6 weeks of physical activity and mindfulness and then undergo 6 weeks of multimodal pain rehabilitation, or receive care according to the usual schedule (12 weeks).

Half of the patients in the intervention group will start with a 2-day introduction and half will receive an hour-long introduction. The multimodal rehabilitation, which both groups receive in different amounts, will be provided by the same team of caregivers at the Children's Clinic, University Clinic in Linköping. The project leader will therefore be Region Östergötland.

POPULATION SIZE The investigators have assumed mean patient estimates of the primary outcome measure WASAS-Y (see Outcome measures below) and a hypothetical difference between the two groups of 10%, which is considered to be a clinically meaningful difference. With a false positive probability (alpha) of 0.05 and a power of 80%, this gave an estimated minimum total population size of 42 study participants. To create margins for dropouts, and match the study to the scope of the pain team's activities, the investigators will aim to include approximately 60 patients. The use of sequential monitoring, via Bayesian probability distribution analysis, means that the investigators will be able to reduce or increase the number of study participants along the way. This is to reduce the risk of both over- and under-recruitment, and to enable the compilation of pilot data from a limited population (10-15 patients) that can form the basis for funding the research project and also ensure the safety of the study participant. In light of how many patients the team for chronic pain (Children's Clinic, US Linköping) receives annually (approximately 40), it is estimated that it will take 2 years to complete the recruitment of 60 study participants. The planned start of the study is February 2026, and recruitment is completed in the first months of 2028.

THE INTERVENTIONS Participants who are randomized to the intervention group will be given 6 weeks of daily physical exercise and mindfulness at home via a mobile app; a fully developed and pilot-tested prototype called Phystic. They will then receive regular multimodal pain rehabilitation for 6 weeks. During the 12-week follow-up period, participants will be recommended to continue daily exercise but will be allowed to choose freely from the workouts available in the mobile app during this period.

The home exercise will be initiated in two different ways. Either via an hour-long visit to the hospital or via a more intensive two-day introduction that also includes pain education and guided physical and mental training according to the same principles as the exercises included in the mobile app. Participants in the intervention group will be prescribed a physical activity workout of the day and an extra exercise of the day with guided mindfulness five days a week via the mobile app. The app offers the option of choosing to take a Rest Day, and the participant is then encouraged to do a short exercise that stimulates relaxation and recovery. Through the mobile app's chat function, study participants will be able to ask questions and receive encouraging feedback from a selected healthcare professional (physiotherapist or psychologist).

The mobile app has been developed as an innovation project at Linköping University and is compatible with the two most common mobile operating systems (iOS and Android). The system also includes a stainless steel weight bar with shock-absorbing protection at the ends. The system has been tested in two different pilot studies of young people with obesity and cancer since autumn 2023, and interviews have shown that a majority describe a positive experience of training using the app. The instructors are mainly licensed healthcare professionals (physiotherapists, psychologists, doctors) with extensive experience working with adolescents and young adults. There are currently around 50 instructional videos of varying lengths and intensities that include fitness, strength, mobility, yoga, and guided mindfulness and breathing exercises.

The control group receives regular multimodal pain rehabilitation for 12 weeks, which means that they receive care for the same length of time as the intervention group but twice as much interdisciplinary multimodal and hospital-based treatment. Instead of a mobile app, the control group receives an activity calendar that allows for the registration of daily activities, random measurements of well-being (pain and life satisfaction), and a way to collect qualitative data (free writing based on open questions) in a way that is similar to the qualitative data collection in the mobile app.

DATA ANALYSIS Both positive and negative effects of the intervention will be evaluated using Bayesian probability distributions. From the patients' medical records, the investigators will collect data on comorbidity, medication treatment, and how many hospital visits each study participant has made during their pain rehabilitation. Information on the number of relapses during the follow-up phase (12 weeks) will also be obtained from the medical records.

ETHICAL ASPECTS Previous research has shown that the two subcomponents of the project's intervention (physical activity and mindfulness) are associated with very few risks. Physical activity is never completely risk-free, but the training sessions in the mobile app are instructed by physiotherapists with experience in instructing children and adolescents, and using the mobile app is considered to be less risky overall than many everyday situations, such as school sports or leisure sports. The fact that the participants in the intervention group have to wait six weeks for physiotherapy and psychotherapy is considered to be a limited risk, as the study participant has telephone contact with the team and receives support to begin several change processes that may be beneficial. All handling of personal data will be carried out in accordance with the rules of the GDPR. The mobile application stores information about user statistics and written text in various exercises, but in this study, it mainly takes place on the participant's device. In connection with the collection, data will be anonymized. Overall, the benefits of participating in the project are considered to far outweigh the risks.

CLINICAL SIGNIFICANCE The CACTUS project could fill some of the knowledge gaps that exist regarding the effects of physical exercise and mindfulness on young people suffering from chronic pain. If the project results show that the new treatment principles and the new technology are effective and appreciated by the patients, significantly more young people could receive rapid and effective help with their pain problems. The qualitative part of the project, which aims to better understand the motivation to participate in the rehabilitation process, could generate important insights that can form the basis for further development of care for young people with long-term pain. A long-term vision for the CACTUS project is to further develop the mobile app and its content into a fully digital and tailored treatment for mild to moderate pain problems. Such a solution has great potential to reach significantly more patients and thus make care more equitable and cost-effective. Overall, the results of the research project could contribute to a large number of young people receiving better and more person-centered pain rehabilitation.

DISSEMINATION AND IMPLEMENTATION OF PROJECT RESULTS The results from the CACTUS project will be published in at least two refereed journal articles. The results will also be presented at one or more scientific meetings. Furthermore, parts of the project results will be presented at scientific meetings arranged by the Swedish Childhood Pain Association and at the hospitals that collaborate with and refer patients to the long-term pain team in the Southeastern healthcare region.

TIMETABLE 2024 Application to the Swedish Ethics Review Authority. 2025 Preparing recruitment and fine-tuning of technical solutions. 2026 Start recruitment, follow-up, and data collection. Completion of the project. 2027 Continued recruitment, follow-up, and data collection. 2028 Completion of the project.

RESEARCH GROUP Oskar Lundgren MD, Ph.D, MSc - Principal Investigator Senior Lecturer in Pediatrics, Linköping University Teacher (step 3) in Mindfulness-Based Stress Reduction (MBSR). Chief Physician, Chronic Pain Team and Pediatric Rehabilitation Team* Project Manager, Phystic Innovation Project

Eva Shaerling, Registered Nurse Specialist Nurse in Rehabilitation Medicine Team Leader, Chronic Pain Team*

Prospective PhD Student Erik Nilsson, Registered Psychologist Prospective PhD Student

Hanna Tigerstrand Grewnert, PhD, Physiotherapist Senior Lecturer in Physiotherapy, Linköping University Advisor on physical training and involved in the Phystic project

Maria Gyris MD Chief Physician, Specialist in Pediatric Neurology Västerviks Hospital, Region Kalmar Advisor on Pediatric Neurology & Rehabilitation

Ulrika Müssener PhD Associate Professor in Community Medicine, Linköping University Advisor on Qualitative Method and Analysis

Marcus Bendtsen PhD Associate Professor in Biostatistics and Public Health, Linköping University Advisor on Study Design, Statistics, and Data Security

* HRH Crown Princess Victoria Children's and Youth Hospital Linköping University Hospital

Inclusion Criteria:

• Age 13-19 years
• The presence of long-term pain (>3 months) and a great deal of suffering or an affected level of function.
• Ability to read in Swedish
• Access to a mobile phone or tablet with internet connection

Exclusion Criteria:

• Low motivation
• Severe mental illness
• Contraindications for physical activity (medical)

Researcher handling data, computing and interpreting results will be blinded to allocation of patients.