Using physical education to promote out-of school physical activity in lower secondary school students : a randomized controlled trial protocol

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 4.0 https://creativecommons.org/licenses/by/4.0/ Using physical education to promote out-of school physical activity in lower secondary school students : a randomized controlled trial protocol © The Authors. 2019 Published version Polet, Juho; Hassandra, Mary; Lintunen, Taru; Laukkanen, Arto; Hankonen, Nelli; Hirvensalo, Mirja; Tammelin, Tuija; Hagger, Martin Polet, J., Hassandra, M., Lintunen, T., Laukkanen, A., Hankonen, N., Hirvensalo, M., Tammelin, T., & Hagger, M. (2019). Using physical education to promote out-of school physical activity in lower secondary school students : a randomized controlled trial protocol. BMC Public Health, 19, Article 157. https://doi.org/10.1186/s12889-019-6478-x 2019 STUDY PROTOCOL Open Access Using physical education to promote out-of school physical activity in lower secondary school students –a randomized controlled trial protocol Juho Polet 1* , Mary Hassandra 1 , Taru Lintunen 1 , Arto Laukkanen 1 , Nelli Hankonen 2 , Mirja Hirvensalo 1 , Tuija Tammelin 3 and Martin S. Hagger 1,4 Abstract Background: Given the documented decline in levels of physical activity in early adolescence, promoting physical activity in young people is a priority for health promotion. School physical education (PE) is an important existing network in which participation in physical activity beyond school can be promoted to the captive young people. The objective of current article is to present the protocol for a PE teacher-delivered theory-based trial to promote secondary school students’participation in physical activity out-of-school contexts. The intervention will be guided by the trans-contextual model explaining the processes by which PE teachers’support for autonomous motivation in the classroom promotes students’motivation to engage in out-of-school physical activity. We hypothesize that school students receiving the teacher-delivered intervention to promote autonomous motivation toward physical activity will exhibit greater participation in physical activities outside of school, relative to students receiving a control intervention. Methods: The trial will adopt a waitlist-control design with cluster-randomization by school. PE teachers assigned to the intervention condition will receive a two-week, 12-h training program comprising basic information on how to promote out-of-school physical activity and theory-based training on strategies to promote students’autonomous motivation toward physical activity. Teachers assigned to the waitlist control condition will receive an alternative training on how to monitor physical functional capacity in children with special needs. PE teachers (n= 29) from eleven schools will apply the intervention program to students (n= 502) in PE classes for one month. Physical activity participation, the primary outcome variable, and psychological mediators from the trans-contextual model will be measured at pre-trial, post-trial, and at one-, threeand six-months post-trial. We will also assess teachers’autonomysupportive techniques and behaviours by observation. Discussion: The study will make a unique contribution to the literature by testing a theory-based intervention delivered by PE teachers to promote school students’participation in out-of-school physical activity. Information will be useful for educators, community stakeholders and policy makers interested in developing programs to promote students’out-of-school physical activity. Trial registration: ISRCTN39374060. Registered 19.7.2018. Keywords: Autonomy support, Behavioural intervention, Trans-contextual model, Self-determination theory, Theory of planned behaviour, Intervention development * Correspondence: [email protected] 1 Faculty of Sport and Health Sciences, University of Jyväskylä, P.O. Box 35 (L335), 40014 Jyväskylä, Finland Full list of author information is available at the end of the article © The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Polet et al. BMC Public Health (2019) 19:157 https://doi.org/10.1186/s12889-019-6478-x Background Promoting physical activity in school Epidemiological data consistently indicate that levels of physical activity decline with age [1]. Consistent with these trends, national survey data from Finland indicate a decline in physical activity during adolescent years with only 41% of 11-year olds and 17% of 15-year olds meeting current national guidelines for physical activity [2]. Given that low levels of physical activity are related to increased risk of chronic illness later in life, and increased rates of conditions such as overweight and obesity [3,4], the promotion of physical activity participation among young people is a public health priority. Physical education (PE) stands in an advantageous position for promoting the benefits of leisure physical activity as it addresses young, diverse and captive audiences [5]. Importantly, it is through PE that young people experience a variety of physical activities, and it is these experiences that may determine future involvement in physical activity during leisure time [6]. One of the primary aims of PE is to provide young people with the necessary motor skills, knowledge and competence to choose and participate in health-related physical activity in their leisure time [7]. Nevertheless, there is relatively little research outlining how PE teachers or PE programs can effectively orient young people toward participation in regular leisure-time physical activity outside of school. The present article outlines the protocol of a trial in which PE teachers will be trained to support autonomous motivation toward leisure-time physical activity in lower-secondary school students (the PETALS trial). The trial aims to capitalize on school PE as an existing network to promote out-of-school physical activity in secondary school students. The trial will adopt a cluster randomized design and implement an intervention based on psychological theory to train participating teachers in techniques that support school students’motivation to participate in physical activity in their leisure time outside of school. Trial effectiveness will be evaluated in terms of effects on participating school students’post-intervention out-ofschool physical activity participation. The theoretical basis for the intervention will be described next, followed by the study objectives. Theoretical basis for the intervention The identification of factors that determine physical activity participation, and the processes by which they affect action, is paramount in providing formative evidence on which to base effective behavioural interventions [8]. The application of psychological theory, particularly theories of motivation and attitudes, has been at the forefront of providing an evidence base for the factors that drive participation in physical activity [9,10]. However, only recently has this evidence has been applied to understand how teachers can promote students’physical activity outside of school [10]. Such evidence is essential as it provides guidance on the content of interventions likely to be effective in promoting physical activity participation. Self-determination theory is a prominent theory of motivation that has been applied to understand participation in health behaviours like physical activity [11,12]. Central to the theory is the construct of self-determined or autonomous motivation. This form of motivation reflects an individuals’general reflection on the causes of their action. Self-determined or autonomously-motivated individuals engage in actions such as physical activity out of interest, choice, and the sense of personal involvement they feel when engaged in the physical activities. In contrast, individuals who feel that their actions are less self-determined are likely to feel that their actions are controlled by external contingencies and engage in activities because they feel pressured, forced or obliged to do so. Research has indicated that autonomously-motivated individuals are more likely to persist with activities and more likely to gain positive or adaptive outcomes [13]. There is increasing research demonstrating that autonomous motivation is related to uptake and persistence with health behaviours [14] particularly physical activity [15,16]. The proposed mechanism by which autonomous motivation leads to adaptive outcomes is through greater interest, effort, and involvement in the task [17]. Given that autonomous motivation has been shown to be related to persistence on adaptive behaviours, researchers and interventionists have sought to identify the contexts and conditions that promote and give rise to autonomous motivation. In particular, the focus on motivational ‘environment’or ‘climate’provided by the actions of significant others with leadership roles (e.g., coaches, teachers, instructors, bosses) has been shown to be influential in developing autonomous motivation. Specifically, leaders’autonomy-supportive behaviours such as provision of choice and support for self-directed action have been shown to promote autonomous motivation and persistence in individuals operating in that environment [18,19]. In addition, individuals’perception that significant others in their environment are autonomy supportive have been shown to be strongly related to autonomous motivation as well as other adaptive outcomes in multiple contexts [14,20–22]. In educational contexts, therefore, teachers can take the lead role in fostering autonomous motivation toward learning activities in the classroom by adopting autonomy supportive behaviours. Autonomy support focuses on style and delivery of lesson content rather than the content itself [23]. Interventions that have adopted autonomy support techniques and interactive styles have been found to be effective in multiple contexts in producing positive motivational and behavioural outcomes. Importantly, evidence exists that autonomyPolet et al. BMC Public Health (2019) 19:157 Page 2 of 15 supportive interventions can produce long-term changes in motivation and behaviour in academic settings [24,25]. While there is considerable research demonstrating links between the use of autonomy-supportive intervention techniques and student autonomous motivation and adaptive outcomes in class, comparatively less research has focused on the role that autonomy support in educational settings has on students’behaviour outside the class (e.g., participation in sport and physical activities during leisure time). Recent theory has proposed the potential mechanisms by which autonomy support in an educational context like PE may lead to participation in physical activities outside of school. Capitalizing on multiple theories of motivation, particularly self-determination theory [12] and the theory of planned behavior [26]the trans-contextual model (TCM) was developed [6]. The model (Fig. 1) outlines how teachers’autonomy support for in-class activities in PE context transfers to autonomous motivation toward, and future intentions to engage in, leisure-time physical activity in an out-of-school context. According to the model, teachers’promotion of students’autonomous motivation toward physical activities in PE will lead individuals to strategically align their motivation, beliefs, and intentions toward similar activities in related contexts with those motives. A review and meta-analysis of studies adopting the model provided support for model predictions across multiple studies [10]. In particular, the analysis supported links between autonomy support from teachers and autonomous motivation in school, consistent with previous research [27,28]. In addition, the research supported trans-contextual links between autonomous motivation in school and autonomous motivation, beliefs (attitudes, subjective norms, and perceived behavioural control), and intentions toward participation in physical activity outside of school, and actual participation in physical activity outside of school. The trans-contextual model provides a theoretical framework for developing interventions in educational contexts such as school PE to promote motivation toward, and actual participation in, related activities such as physical activity outside of school. The model implies that strategies aimed at fostering autonomous motivation will promote in-school and out-of-school autonomous motivation toward physical activity, and promote adaptive beliefs and intentions toward out-of-school physical activity and actual physical activity participation. These proposed effects are supported by research demonstrating the trans-contextual effects and confirming the relevant mechanisms involved [10]. The most effective means to support autonomous motivation in school is for teachers to display autonomy-supportive behaviours during PE lessons. Consistent with the model, effects of interventions aimed at promoting autonomy support in students on out-of-school physical activity is expected to be mediated by autonomous motivation in both contexts, beliefs, and intentions. Together, these mediators provide a demonstration of how the intervention functions in promoting physical activity behaviour. In other words, it provides a framework on how the intervention works. The primary focus of this study is to test the effectiveness of an intervention based on the trans-contextual model in promoting out-of-school physical activity. Fig. 1 Theoretical framework: Trans-contextual model Polet et al. BMC Public Health (2019) 19:157 Page 3 of 15 However, we will also control for key demographic, environmental, and psychological variables that may moderate or affect intervention effects. With respect to demographic variables, we will control for effects of student’s age, gender, nationality, ethnicity and parental education level, given the potential for these variables to affect levels of physical activity and engagement in school. With respect to environmental variables, the autonomy support offered by parents and peers towards out-of-school physical activity, parental affection, and parental control may affect students’performance and engagement in school [29,30] and will also be considered covariates in our analysis of intervention effectiveness. Finally, we will also control for individual difference characteristics that have been shown to affect students’motivation in previous research. These variables include grit [31] and self-discipline [32], two factors shown to be related to long-term effort and perseverance on tasks. Finally, we will also control for the extent to which students habitually perform physical activity out-of-school. The intervention may have less effect on students who have strong exercise habits, as they are already likely to exercise regularly and are unlikely to respond to motivational messages. Objectives The purpose of the current protocol article is to report the development of a school-based intervention based on the trans-contextual model to promote secondary school students’physical activity participation by fostering autonomous motivation (the PETALS trial). The trial will adopt a cluster-randomized waitlist-control design, and participants will be teachers of lower-secondary school PE classes and their students. The intervention will involve initial training of PE teachers of lower-secondary PE classes on the use of autonomy support strategies in their regular lessons, followed by an implementation period in which teachers apply their training in regular PE classes. Effects of the intervention will be evaluated through changes in subsequent follow-up measures of participating students’physical activity levels and trans-contextual model variables relative to pre-trial baseline measures. We will also evaluate effects of the intervention on PE teachers’autonomy-supportive behavior measured using self-report and observation. We will also control for support for autonomy from parents and peers. Other salient demographic and individual difference variables will be also controlled for. We expect the research will provide formative evidence of an effective, replicable, low-cost behavioural intervention, which will help in developing long-term participation in physical activity in young people. In addition, key deliverables of the research will be a set of training materials and an intervention manual, which will provide step-by-step accessible instructions on how to implement the intervention and can be disseminated to schools with no specialist knowledge and minimal cost. Methods Trial design The study will adopt a cluster-randomized, wait-list control, single-arm intervention design with randomization by school. The trial comprises two phases: a teacher-training phase and an implementation phase. The teacher-training phase will comprise a two-week, 12-h training program in which secondary school teachers will receive the autonomy-support training program developed for the present study. The teacher-training program will be preceded by the pre-trial data collection occasion during which baseline measures of primary and secondary outcome variables will be taken. The training will be delivered by experienced teacher trainers as part of the teachers’ regular in-service training. The implementation phase will comprise a one-month period during which teachers will apply their training in their regular PE classes and it is followed by post-trial data collection occurrence. Thereafter, primary and secondary outcome variables will be collected at one-, three-, and six-month follow-up data collection occasions. Teachers allocated to the waitlist control condition receive a 4-h training program in which they will be instructed on how to apply a monitoring system for physical functional capacity in children with special needs [33]. Secondary school teachers (N=29) from 11 secondary schools and their students (N=502) in the city of Jyväskylä in central Finland will be invited to participate in the study. Participants and eligibility criteria Qualified full-time PE teachers teaching regular PE lessons in lower secondary schools will be eligible to participate in the study. Participating teachers will be asked to select one of their PE classes to be invited to participate in the study. Students in grades 7–9 (typical ages 13–15 years) in lower secondary schools will be eligible to participate. Students with existing physical or mental health condition that prevents participation in PE lessons, regular physical activity or completing surveys will be excluded. The proposed participant flow diagram through the trial is presented in Fig. 2. Recruitment process and informed consent All available lower secondary level school PE teachers in the city will participate in the teacher-training phase of the study, irrespective of their participation, as the city Education Department has accepted the teacher-training phase to be part of PE teachers’regular in-service training. We will recruit PE teachers and their students for the study via established links with schools and with Polet et al. BMC Public Health (2019) 19:157 Page 4 of 15 support from the Education Department. Initial contact will be made with the head teacher of the school provided with details of the study aims and methods and the commitment required by the school. Once head teacher has consented their school to participate in the study, eligible teachers from each school will be invited to participate and provided with information on the study, and the benefits and requirements of participation, and given the opportunity to ask questions. Teachers agreeing to participate to the study will complete an opt-in informed consent form. Students of the PE teachers will be recruited to the study by referral from their teacher. Invitation letters, study information, and opt-out consent forms, with the exception of opt-in consent form for participation in the accelorometry component of the study measures, will be sent to eligible students’parents or legal guardians via the schools’online administration and communication software or via email or post. Students whose parents or guardians decline to give consent for their child to participate in the Fig. 2 Participant flow diagram Polet et al. BMC Public Health (2019) 19:157 Page 5 of 15 study will be exempted, and will be provided with alternative activities while participating students complete study measures at data collection time points. Procedure and data collection methods The pre-trial baseline data collection will be scheduled for the third week after the beginning of the 2018–2019 school year. The following data will be collected: questionnaires administered to participating teachers and students comprising self-report measures, a one-week physical activity surveillance for participating students using accelerometers, and audio recordings of a selected PE class of each participating PE teacher. At pre-trial, all consenting teachers and students will complete a questionnaire containing demographic, psychological, and behavioural measures. Audio recordings of participating teachers’classes will also take place during the baseline data collection. Physical activity behaviour will be collected from a subsample of students from the intervention and wait-list control groups using accelerometers for the week after the pre-trial data collection occasion. In addition, parents or legal guardians of participating students will complete self-report measures of demographic information, provision of autonomy support towards out-of-school physical activity, parental affection, and parental control they provide for their children at the baseline data collection. Pre-trial data collection will be followed, consecutively, by the teacher training and implementation phases of the trial. In the teacher training phase, teachers allocated to the intervention group will receive the autonomy-support training program and teachers allocated to the wait-list control group will receive control education program over the same period. The completion of the training program will be followed by a 1 month implementation phase. In this phase, teachers in the intervention group will apply the techniques they learned in the training program in their regular PE classes. Following the implementation phase, a post-trial data collection occasion will be scheduled. Data collection will comprise administration of measures identical to those at pre-trial with the exception of the baseline measures of behavioural automaticity, grit, self-discipline, parental affection, parental control, parental autonomy support and demographic measures. Accelerometer data and audio recordings of teachers’lessons will also be collected from the same subsample of students and teachers, respectively. Follow-up data collection occasions are scheduled for one-, three-, and six-months post-trial. Accelerometer data from the subsample of students and audio recordings of teachers’lessons will not be collected on the one-month follow up occasion. All assessments will be completed at the threeand six-month follow up data collection occasions, identical to the post-trial data collection occasion. Immediately after the three-month follow up data collection occasion, the wait-list control group will receive the autonomy-support training program. Post-trial and one-month follow up assessments will be conducted for teachers and students from this group, using identical measures as those administered in the intervention group (Fig. 3). Retention of participants will be maximized through pro-active email and telephone communication with school teachers who will provide access to students. Where collection of data on any given collection occasion is prevented due to unforeseen circumstances, we will negotiate an alternative occasion for the data collection as close as possible to the scheduled occasion. Intervention Autonomy support intervention group Teachers in schools allocated to the intervention condition will receive the twelve-hour interactive autonomy support teacher-training program developed specifically for this study. The program aims to familiarize PE teachers with techniques and strategies aimed at promoting students autonomous motivation toward out-of-school activities. The program focuses on six sets of autonomy-supportive strategies and techniques: Taking students’perspective, using non-controlling and informational language, providing a rationale, displaying patience, providing choices, and accepting negative Fig. 3 Timeline for data collection in months and participant contacts by group Polet et al. BMC Public Health (2019) 19:157 Page 6 of 15 emotions and feelings. The techniques are adapted from the strategies identified in previous autonomy support training programs [34–37]. The training will be delivered by two trained teacher trainers with extensive experience of PE and teacher education. The trainers will undergo a familiarization session with the core research team in which they will be introduced to the training material (manual, power point presentations, supportive material) and provided with instruction on how to deliver the program prior to implementation of the training. A summary of session content and related delivery techniques are presented in Table 1. The teacher training was developed in three stages. In the first stage, we identified the most effective autonomy supportive techniques and means to deliver them to PE teachers based on current evidence. We therefore reviewed the existing literature on autonomy support techniques and training programs. Previous successful applications of autonomy support interventions in general classroom [38] and PE settings [39], as well as feasibility [36] and conceptual articles on autonomy support [34,40] were identified. In addition, we acquired teacher training material from existing autonomy support training programs used in previous interventions [35,37]. Table 1 Description of teacher training program: Content and matched behaviour change techniques for each session Session topic Content Behaviour change techniques a 1. Introduction and added value of the training to teaching practice Introduction and warm up activities Information on the added value of the training and expectations Explore current supportive style and reflection Why autonomy support matters Introduction to self-determination theory Social support (unspecified) Social support (practical) Discrepancy between current behaviour and goal Shaping knowledge Information about social and environmental consequences Imaginary reward 2. Autonomy supportive techniques: Description and benefits for students and teachers Basics of the autonomy supportive teaching techniques: Definitions and implementation examples Using autonomy supportive techniques: Benefits for students and teachers based on previous research results Demonstration of the behaviour Shaping knowledge Information about social and environmental consequences Behavioural practice/rehearsal 3. Use of autonomy supportive techniques to provide instructions How, when, and why to use autonomy supportive techniques when giving instructions (organizational, technical and tactical) Taking students’perspective Using non-controlling and informational language Providing rationale Providing choices Displaying patience Information about social and environmental consequences Instruction on how to perform a behaviour Behaviour substitution Habit reversal Framing/ reframing Self-monitoring of behaviour Behavioural experiments Behavioural practice/rehearsal Graded tasks 4. Use of autonomy supportive techniques to provide feedback, encouragement, and praise How, when, and why to use autonomy supportive techniques when providing feedback, encouragement, and praise Using non-controlling and informational language Taking students’perspective Displaying patience Generalization of a target behaviour Behaviour substitution Habit reversal Framing/ reframing Information about social and environmental consequences Instruction on how to perform a behaviour Self-monitoring of behaviour Behavioural experiments Behavioural practice/rehearsal Graded tasks 5. Use of autonomy supportive techniques to deal with discipline issues and off-task behaviours How, when, and why to use autonomy supportive techniques when dealing with discipline issues and off task behaviours Taking students perspective Accepting negative affect Providing rationale Using non-controlling and informational language Displaying patience Providing choices 6. Building personalized action plans Plan changes in own teaching practice: Specific goals and plans for change when giving instructions, provide feedback, and respond to students with low motivation Identify barriers and problem solving for using the autonomy supportive techniques in every day teaching practice Development of their individualized infographic poster Generalization of a target behaviour Goal setting Action planning (and implementation intention) Instruction on how to perform a behaviour Prompts/cues Adding objects to the environment Pros and cons Social support unspecified a From behaviour change technique taxonomy (Version 1) [66] Polet et al. BMC Public Health (2019) 19:157 Page 7 of 15 Next, we developed a list of training activities and delivery techniques considered effective and relevant to the proposed intervention. We also conducted a mapping exercise guided by a recent study outlining selfdetermination theory techniques and constructs [23]to ensure that the training activities and delivery techniques precisely matched the theory-based motivational determinants (e.g., autonomous motivation, psychological need satisfaction) targeted in the intervention. The second stage involved the development of a detailed draft of the teacher training program. For each session, we developed a detailed description of the program content including aims, learning outcomes, main instructional points, examples, and interactive activities. Accompanying supportive materials (worksheets, printed examples, video demonstrations, presentation slides, and session’s summaries) were produced for illustration of the program content. The content and materials were reviewed and revised by the core research team. The third and final stage involved review and revision of the entire program and materials by external stakeholders and teacher educators. Reviewers were experienced PE teachers and teacher-training experts, and researchers with expertise in the theoretical approaches on which the program was based, the delivery techniques used, and behavioural interventions conducted in school settings.The stakeholders reviewed each session content in detail in an interactive workshop with the research team. They also provided written feedback on the materials separately. Stakeholders identified issues relating to the clarity of the aims and descriptions, relevance of the examples, and overlap and redundancy in the materials. The program content and materials were further revised by the research team resulting in a final autonomy support training program with supporting materials. 1 Wait-list control group Participating teachers allocated to the waitlist control group will receive an alternative training program comprising 4-h of education on how to apply a monitoring system for physical functional capacity for children with special needs [33]. The control intervention is delivered in a one-day workshop by two educators experienced in PE teacher training. Outcome measures All self-report outcome measures were translated from English into Finnish using a back-translation process by two bilingual researcher [41]. Primary outcome variable The primary outcome measure is students’post -intervention participation in out-of-school physical activity at the pre-trial, post-trial, one-, three and 6 month follow-up data collection occasions. Physical activity participation will be measured using the short form of the International Physical Activity Questionnaire, IPAQ [42], which will be modified to make explicit reference to out-of-school physical activity. The IPAQ comprises four items recording the frequency (number of days) and duration (hours) of participation in moderate and vigorous physical activity, walking, and sitting over the past 7 days. The physical activity score for moderate and vigorous physical activity and walking is calculated based on norms and expressed in MET-minutes per week. A score for total physical activity in out-of-school contexts is provided by the sum of the duration and frequency of vigorous, moderate and light physical activity scores. The IPAQ has acceptable concurrent validity and reliability indices [43]. Secondary outcome variables Physical activity behaviour. A subsample of participants (approx. 120) from representative school classes covering grades 7–9 will have their physical activity participation measured using accelerometry. The purpose of this secondary measure is to provide concurrent validity and comparison data to support the IPAQ used as the primary outcome measure. 2 Participants will wear the accelerometers (Hookie AM 20) for seven consecutive days after each data collection occasions at pre-trial, post-trial, and at the three-, and six-month follow up data collection occasions. These accelerometers have been shown to be valid and reliable in as a measure of physical activity in previous research [2]. Participants will be also asked to complete a short diary of their daily in-school and out-of-school physical activities for the period during which they wear the accelerometer. Data will provide duration participants spent in sedentary, light, moderate, and vigorous intensity physical activity per day. Diary data will be used to identify duration of physical activity on in-school and out-of-school contexts. It will also provide a measure of total energy expenditure in each context. For the purposes of the current study, we will compute total time spent in light, moderate, or vigorous physical activity and total energy expenditure in out-of-school contexts as criterion measure to test the concurrent validity of the IPAQ measure in each participant. The accelerometry data will be included only for those participants who have provided valid accelerometer data for a minimum of 3 days. Mediating variables All students will complete a battery of self-report measures of psychological variables based on the transcontextual model. These factors are expected to reflect the mechanisms by which the intervention affects change in the primary outcome consistent with the model. Polet et al. BMC Public Health (2019) 19:157 Page 8 of 15 26. Fishbein M, Ajzen I. 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