School- and home-based hybrid exercise intervention to prevent summer fitness loss in children: protocol for a feasibility study of the keep moving in a structured summer programme

Abstract

Introduction

Schools are key settings to intervene for the purpose of health promotion in children. However, the effectiveness of school-based interventions remains limited, potentially related to the obesogenic behaviours observed during holidays as posited by the structured days hypothesis. This study aims to evaluate the feasibility, process evaluation and preliminary efficacy of the keep moving in a structured summer (KEMOSS) intervention among Chinese children.

Methods and analysis

The KEMOSS intervention is an 8-week hybrid programme comprising two digitally supported home-based phases (weeks 1–2 and 7–8) and a structured school-based phase (weeks 3–6). The school-based phase will be a full-day scheduled programme that replicates the routine of regular school days, with a coach-led structured physical activity lesson and a free-play lesson embedded in the morning and afternoon, respectively. During the home-based phases, participants will be encouraged to complete prescribed exercises independently with support from mHealth reminders and instructional materials and to achieve a minimum of 10 000 steps per day. Feasibility, process evaluation and preliminary efficacy outcomes will be assessed using a mixed-methods approach. Quantitative data will be analysed using SPSS, while qualitative data from interviews and focus groups will be analysed thematically using NVivo.

Ethics and dissemination

Ethical approval has been obtained from the Ningbo University Ethics Committee (TY2026028). Findings will inform the refinement of the intervention and the design of a future definitive trial. Results will be disseminated through peer-reviewed publications and conference presentations.

Trial registration number

ChiCTR2600126738.

STRENGTHS AND LIMITATIONS OF THIS STUDY.

• The keep moving in a structured summer programme incorporates a hybrid delivery model, combining a full-day school-based structured summer programme with digitally supported home-based activities across the summer holiday period.

• The study incorporates a mixed-methods feasibility and process evaluation to inform refinement of the intervention and the design of a future definitive trial.

• The trial is not powered to detect definitive intervention effects on health and fitness outcomes.

• The step-count data will be collected using a consumer-grade wearable device, which may provide less accurate measurements than validated means.

Introduction

Regular physical activity (PA) is widely recognised for its beneficial effects on cardiorespiratory fitness (CRF), body composition, muscular strength and mental well-being in children and adolescents.^1–4 Schools provide an ideal setting for PA interventions due to their extensive resources (eg, staff, space and facilities) and broad reach across this population.¹ Nevertheless, the effectiveness of school-based PA interventions has been limited.^{1 5 6}

One potential explanation for the limited effectiveness of school-based PA interventions is the decline in fitness that occurs when children are out of school, particularly during the summer holidays.⁷ In this period, children are more likely to engage in obesogenic behaviours, including reduced PA, increased sedentary time and unhealthy dietary intake.^7–9 This phenomenon is known as the structured days hypothesis (SDH), which posits that structured environments, such as school days, are characterised by routine, segmentation, adult supervision and compulsory activities that collectively promote healthier behaviours.⁷ In contrast, unstructured periods, such as summer holidays, are typically more autonomous and less regulated, during which both physical and social factors may encourage unhealthy behaviours.⁷ Interventions targeting behaviour change and increase PA during the summer period may therefore help mitigate these adverse effects.^10–12

Despite this, evidence on PA interventions designed to prevent summer-related fitness loss in children remains limited and is predominantly derived from studies conducted in the US.^{10 12 13} Furthermore, existing interventions have focused only on body mass index (BMI)^{10 12 13} and CRF,¹³ with less attention paid to other key health indicators, such as muscular strength.

China accounts for approximately 13% of the global adolescent population, yet less than a quarter (22%) of school-aged children meet the recommended 60 min of moderate-to-vigorous physical activity (MVPA) daily.¹⁴ Of concern, only 17% of Chinese students took part in leisure-time PA programmes,¹⁵ and 8% engage in MVPA outside of school hours.¹⁶ This is particularly problematic during the summer holidays, when compulsory physical education is no longer available. However, no PA interventions have been conducted during the summer holidays among Chinese children.

Interventions conducted during the summer can be challenging, with obstacles such as variability in family schedules and competing academic demands. Process evaluation, aimed at providing a more detailed understanding to inform policy and practice, is an essential component in the design and assessment of complex interventions such as school-based programmes.^{17 18} Incorporating process evaluation enables researchers to ‘open the black box’ of implementation by examining delivery processes, mechanisms of impact and contextual influences.¹⁷

The keep moving in a structured summer (KEMOSS) intervention is a multi-centre collaborative initiative involving the local education bureau, primary schools and universities, developed to address declines in children’s health and academic performance during the summer holiday period. The present study represents the feasibility phase of the KEMOSS intervention.¹⁹ It is an 8-week hybrid intervention combining two mHealth-supported home-based phases (weeks 1–2 and 7–8) and a structured school-based phase (weeks 3–6). The primary aim of this study is to assess the feasibility and process evaluation of the KEMOSS intervention. A secondary aim is to assess the preliminary efficacy of the intervention in children.

Methods and analysis

Trial design

The current study is a feasibility randomised-controlled trial, conducted during the 2-month summer holiday period among Chinese primary school children aged 9–11-years-old. Participants from a primary school will be recruited and randomised (1:1 ratio) to either the KEMOSS group or treat-as-usual control group. The study protocol was developed and will be reported in accordance with the Standard Protocol Items: Recommendations for Interventional Trials (SPIRIT) guidelines,²⁰ with the SPIRIT checklist provided in the online supplemental file 1. The intervention will be described in accordance with the Template for Intervention Description and Replication checklist.²¹ The completed trial will be reported in accordance with the Consolidated Standards of Reporting Trials (CONSORT) extension for pilot and feasibility trials.²² A protocol schematic detailing an overview of the intervention is shown in figure 1. By the time we submit the protocol, the intervention has not yet started. Participants will be recruited in late June, and we expect to complete the study in September 2026.

Figure 1. Overview of the keep moving in a structured summer (KEMOSS) intervention. PA, physical activity; (1) see table 1 for details of outcome assessments and (2) see figure 2 for details of intervention components.

Setting and recruitment of participants

This study will be conducted within a primary school in Ningbo, China. The school will be recruited from the nearby primary schools of Ningbo University. All grade 4 and 5 students (9–11-years-old) across the school will be contacted for participation in the KEMOSS intervention through face-to-face presentations 1 month before their summer holidays (June). Interested participants are allowed to contact the researchers or physical education teachers for further enquiries.

Participants will be eligible to take part in the trial if they are available for the intervention during their 2-month summer holiday. Students with any health or medical condition that would limit their ability to participate safely in vigorous PA will be excluded. Eligibility will be assessed in consultation with the students’ physical education teachers and through review of the annual medical examination reports provided by their parents or guardians where appropriate. Written informed consent will be obtained from parents/guardians of participating children, alongside age-appropriate assent from the children prior to the commencement of the intervention.

Sample size

No formal sample size calculation has been performed since this is a feasibility trial. A total of 60 children will be recruited in the current study. This aligns with guidance for feasibility studies, which typically recommend 24–50 participants to assess recruitment, retention, implementation and outcome variability.²³

Randomisation and blinding

Participants will be randomised in a 1:1 ratio to the KEMOSS or control group using a computer-generated randomisation sequence and stratified by sex. The randomisation will be conducted by a technician independent of the study. Due to the study design, participants and researchers will not be blinded. To minimise potential contamination, participants will be asked not to share intervention materials or activities with children in the other study group during the intervention period. Outcome assessment will be conducted by research assistants blinded to the group allocation.

Intervention

Informed by feedback from patient and public involvement (PPI) activities with parents and school teachers, participants in the KEMOSS group will undertake an 8-week hybrid intervention during the summer holiday period (figure 2), comprising two 2-week home-based phases and a 4-week school-based phase delivered in between them.

Figure 2. Intervention components and details of the KEMOSS intervention. HIIT, high-intensity interval training; KEMOSS, keep moving in a structured summer; PA, physical activity.

School-based phase

The school-based phase will consist of a 4-week full-day structured programme designed to replicate the routine and schedule of regular school days; however, no formal academic teaching will be delivered. Instead, participants will engage in supervised self-directed learning activities, including reading in the school library and completing summer holiday assignments. In addition, participants will be provided with a nutritionally balanced lunch developed in accordance with government dietary guidelines.

Two 40-min PA lessons will be scheduled each day, one in the morning and one in the afternoon. The morning PA lessons will consist of structured activities, beginning with a 10-min warm-up incorporating Baduanjin, a form of traditional Chinese martial arts suitable for individuals with varying health conditions. This will be followed by 6 min of bodyweight resistance-based high-intensity interval training (HIIT), approximately 20 min of themed exercise activities and a 4-min cool-down period. The themed activities will include basketball, street dance, boxing and game-based exercises, with each theme delivered over 1 week (see online supplemental file 2 for further details). Session intensity will be monitored using heart rate data collected via the Polar Verity Sense (PVS 4J, Malaysia).

The Baduanjin and themed exercise components will be delivered by qualified university students majoring in sports-related disciplines recruited from Ningbo University, while the HIIT sessions will be delivered by trained research assistants. The afternoon PA lessons will consist of supervised free-play activities, during which participants will be free to choose sports and activities according to their interests. A range of sports equipment, such as basketballs, footballs, badminton and table tennis, will be made available, and participants will have access to the school’s sports facilities. All non-PA sessions will be supervised by school teachers.

Home-based phases

Two home-based phases incorporating mHealth-supported PA components will be implemented at the beginning and end of the summer holiday period, each lasting 2 weeks. This design was recommended by both teachers and parents during the PPI phase, as it allows participants time to rest following the end of the school term and to prepare for the transition into the new academic term.

During the home-based phases, participants in the KEMOSS group will be required to complete the Baduanjin and HIIT sessions five times per week under the supervision of their parents or other family members. In addition, participants will be encouraged to achieve a minimum of 10 000 steps per day, a threshold associated with meeting the recommended 60 min of MVPA per day in children.²⁴ Daily step counts will be monitored using a Xiaomi Band (Xiaomi Corp., China).

Instructional videos for both Baduanjin and HIIT sessions will be provided to support correct exercise performance and facilitate independent participation. Participants will be encouraged to complete the prescribed activities at any time of day to accommodate varying family schedules. To support adherence, parents will be required to maintain a training checklist, and the research team will send regular reminders for children and their parents to complete everyday tasks. The daily step-count data will be collected on a weekly basis by the research team.

Control group

The control group will receive no intervention during the intervention period and will be encouraged to continue their summer plans. To assess potential contamination, focus groups with control-group participants will explore whether they were exposed to any KEMOSS intervention components during the intervention period and, if so, the extent of such exposure.

Intervention outcomes

The primary aim of this study is to examine the feasibility and process evaluation of the KEMOSS intervention. A secondary aim is to assess its preliminary efficacy. Details regarding what data will be collected, how, from whom and at what time points are presented in table 1. All coaches and teachers, along with 10 purposively selected parents, will be invited to participate in semi-structured interviews. Four focus groups will be conducted with children (n=24), with two groups comprising intervention participants and two comprising control participants. Children will be grouped by similar age and balanced by sex where feasible. Interviews and focus groups will be conducted by researchers with qualitative research experience. Open-ended interview and focus-group guides will be developed by the first author based on the feasibility and process evaluation measures and refined through discussion within the research team. Children will be reminded not to share comments made during the focus group. The proposed sample is considered sufficient to provide rich and diverse data for assessing feasibility.²⁵ Efficacy outcomes, including BMI, CRF, handgrip strength and standing long jump,²⁶ will be assessed in the primary school sports hall at pre-intervention (T0) and post-intervention (T2) by trained research assistants independent of the programme.

Table 1. Details for the outcome assessment of the KEMOSS intervention.

Outcome variables	What to measure	How to measure	From whom	Timing
Feasibility outcomes (primary outcome)
Acceptability	To what extent is the intervention judged as suitable or attractive to recipients?	Interview on the perceived appropriateness	Children and teachers	T2
Demand	To what extent is the intervention likely to be used?	Interview on the intent to continue use	Children and family members	T2
Practicality	To what extent can the intervention be carried out using existing means, resources and circumstances?	Narrative report on the cost and resources used for the implementation of the intervention	Researchers	T2
Integration	To what extent can the intervention be integrated within the school and family?	Interview on perceived fit and sustainability of the intervention	Teachers and family members	T2
Expansion	To what extent can the intervention be expanded?	Interview on fit with the goals of and positive and negative effects on the school and families	Teachers and family members	T2
Process evaluation (primary outcome)
Reach	The number of participants contacted	Intervention records	Researchers	T0
Recruitment	The number of participants included at T0	Intervention records	Researchers	T0
Retention	The number of participants who completed the intervention	Intervention records	Researchers	T2
Adaptation	Changes made from the protocol for facilitating the implementation of the intervention	Intervention records	Researchers	T1
Fidelity	The extent to which the intervention is delivered as planned	Monitoring of exercise intensity in the school-based phase (PVS); complement of the home-based tasks via training checklist completed by parents	Children	T1
Dose delivered	Number of school-based physical activity lessons delivered	Intervention records.	Coaches	T1
Dose received	Number of school-based sessions completed	Attendance as documented in the intervention records	Children	T1
Response	To what extent is the intervention satisfying	Interview on the satisfaction of the intervention	Teachers, children and family members	T2
Adverse event	Injuries related to the intervention occurred	Intervention records	Children	T1
Barriers	Factors impeding the implementation of the intervention	Interview on the barriers to intervention delivery and engagement	Coaches, children and family members	T2
Facilitators	Factors facilitating the implementation of the intervention	Interview on the facilitators of intervention delivery and engagement	Coaches, children and family members	T2
Efficacy outcomes (secondary outcome)
Physical activity	Daily steps	Monitoring the average steps per day in both intervention and control groups	Children	T1
Body composition	Body mass index	Height and weight	Children	T0, T2
Cardiorespiratory fitness	Cardiorespiratory fitness	20-m shuttle run	Children	T0, T2
Handgrip	Upper body strength	Handgrip	Children	T0, T2
Standing long jump	Lower body strength	Standing long jump	Children	T0, T2

BMI, body mass index; PA, physical activity; PVS, Polar Verity Sense; T0, pre-intervention; T1, during intervention; T2, post-intervention.

Feasibility outcomes

Informed by Bowen et al,²⁷ five feasibility outcomes will be assessed, including acceptability, demand, practicality, integration and expansion. These outcomes will be examined through surveys or semi-structured interviews with parents/family members, teachers and professional coaches and focus groups with participating children at T2.

Acceptability will assess the extent to which the intervention is perceived as appropriate or attractive by participants. Children and their physical education teacher will be asked about their perceptions of the intervention, including whether the activities were enjoyable, appropriate for children and acceptable within their daily lives.

Demand will assess the extent to which the intervention will likely be used in the future. Children, teachers and parents will be asked to rate on a 5-point scale for their intention to continue participating in or using components of the intervention during the interviews or focus groups.²⁸

Practicality will assess the extent to which the intervention can be implemented using the available resources, facilities and circumstances. A narrative record of the resources required for intervention implementation, including personnel, facilities, equipment, cost and other relevant resources, will be maintained by the research team.

Integration will assess the extent to which the intervention can be incorporated into existing school and family contexts. Teachers and parents/family members will be asked about the perceived fit of the intervention with existing school routines and family circumstances, as well as its potential sustainability within these settings.

Expansion will assess the perceived potential for extending or scaling up the intervention beyond the feasibility trial. Teachers and parents/family members will be asked whether the intervention is compatible with the goals and needs of schools and families and about its perceived positive and negative effects and potential for wider implementation.

Process evaluation

Process evaluation will be guided by the Medical Research Council process evaluation framework¹⁷ and adapted from a previous review.¹⁸ A total of 11 process evaluation measures will be assessed, including reach, recruitment, retention, adaptation, fidelity, dose delivered, dose received, response, adverse event, barriers and facilitators. These measures will be assessed using a mixed-method approach, incorporating interviews, focus groups, surveys and intervention records.

Reach will be assessed as the number of eligible participants who were contacted or approached. This information will be obtained from recruitment records maintained by the research team at T0.

Recruitment will be assessed by the number of participants enrolled in the study. Recruitment data, including the number of participants who provided consent and completed baseline assessment, will be obtained from study records.

Retention will be assessed as the number and proportion of enrolled participants who complete the intervention and the final assessment at T2. Retention data will be obtained from intervention and assessment records.

Adaptation will assess any deviations or modifications made to the intervention protocol to facilitate implementation across the intervention period. The research team will document any changes made to the intervention content, delivery procedures, scheduling or implementation procedures, together with the reasons for these changes.

Fidelity will assess the extent to which the intervention is delivered as specified in the study protocol. During the school-based sessions, heart rate will be monitored using a PVS to quantify the exercise intensity achieved. Additionally, completion of prescribed home-based activities will be monitored using a training checklist completed by parents.

Dose delivered will be assessed as the number of planned school-based PA lessons that were actually delivered by the professional coaches. The number of sessions delivered, including any cancelled or modified sessions, will be documented in the intervention records.

Dose received will be assessed as the number of school-based PA sessions attended and completed by each participating child. Attendance will be recorded by researchers and documented in the intervention records.

Response will assess participants’ subjective responses to and satisfaction with the intervention. Teachers, children and parents/family members will be asked to rate on a 5-point scale during interviews or focus groups about their satisfaction with the intervention, including their perceptions of the activities, delivery and overall intervention experience.²⁸

Adverse events will be assessed as injuries or other adverse events occurring during or as a consequence of participation in the intervention. Any intervention-related adverse events will be documented in intervention records throughout the intervention period. The type, timing, severity and circumstances of any adverse event will be recorded where applicable.

Barriers and facilitators will assess factors that hinder or support intervention delivery, participation or engagement. Professional coaches, children and parents/family members will be asked during interviews or focus groups to identify factors that made intervention delivery or participation difficult or facilitated intervention implementation.

Efficacy outcomes

Efficacy outcomes related to children’s PA, body composition, CRF and muscular strength in both groups will be assessed to provide preliminary estimates of intervention effects and to inform outcome selection for a future definitive trial.

Physical activity

PA will be assessed using average daily step counts recorded via wearable devices (Xiaomi Band) in both intervention and control groups across the 8-week intervention period. Participants will be contacted by a researcher to record their daily average steps on a week-by-week basis.

Body composition

Body composition will be assessed via BMI. Height will be measured to the nearest 0.1 cm using a portable stadiometer, and body mass to the nearest 0.1 kg using a digital scale. BMI will be calculated as weight (kg) divided by height squared (m²).

Cardiorespiratory fitness

CRF will be assessed using the 20-m shuttle run test, a well-established field-based measure.²⁹ Participants will be instructed to run back and forth between two lines set 20 m apart, keeping pace with audio signals. The test will be terminated when the participant fails to complete two consecutive shuttles or voluntarily stops. Performance will be recorded as the number of laps completed.

Muscular fitness

Upper-body strength will be measured using a digital handgrip dynamometer with adjustable grip. Participants will perform the test standing with the elbow flexed at 90 degrees. One trial will be conducted in each hand, and the highest value (kg) will be recorded.³⁰ Lower-body strength will be assessed using the standing long jump. Participants will stand behind a marked line with feet shoulder-width apart and use arm swing to jump as far as possible. Three attempts will be allowed, with the best distance recorded.

Progression criteria

The CONSORT extension for pilot and feasibility trials recommends prespecifying criteria to inform decisions about whether and how to proceed from a feasibility study to a future definitive trial.²² Accordingly, progression of the KEMOSS intervention to a definitive trial will be informed by findings from the feasibility and process evaluations, together with exploratory efficacy outcomes where relevant. A traffic-light framework³¹ will be adopted, with the prespecified criteria presented in table 2. The thresholds for ‘red (stop)', ‘amber (amend)’ and ‘green (go)’ were prespecified by consensus within the research team. Alongside the qualitative findings, the progression decision will be guided by the following criteria: proceed without major modification if all criteria are rated green; modifications to the intervention will be considered if no criterion is red but at least one criterion is amber; and progression to a definitive trial will not be recommended if any critical criterion is rated red.

Table 2. A priori progression criteria for determining feasibility of the KEMOSS intervention.

Criterion	Green	Amber	Red
Demand	≥ 80%	60%–79%	< 60%
Recruitment of target sample	≥ 80%	60%–79%	< 60%
Retention rate	≥ 80%	60%–79%	< 60%
Session intensity	≥ 70% HRmax	60%–69% HRmax	< 60% HRmax
Dose delivered	≥ 80%	60%–79%	< 60%
Dose received (school-based sessions)	≥ 80%	60%–79%	< 60%
Satisfaction	≥ 80%	60%–79%	< 60%
Adverse event	< 5%	5%–10%	> 10%
Data collection	≥ 80%	60%–79%	< 60%

Note: Percentages are calculated based on the number of participants eligible for each assessment. For example, ≥80% data collection indicates that at least 80% of eligible participants completed the specified outcome assessment at the relevant time point.

HR_max, maximum heart rate.

Trial management

To ensure the quality of intervention delivery, QG and YL will act as primary points of contact for participants, parents/guardians and school staff, maintaining regular communication to obtain feedback, address queries and facilitate any necessary adaptations to the intervention. YL, MHL and JYG will oversee intervention delivery, working closely with coaches to ensure consistency and quality of sessions, including monitoring exercise intensity and adherence to the planned protocol. They will also be responsible for recording attendance at the school-based phase and supporting participant engagement during the home-based (mHealth) phase (eg, sending reminders). A structured training logbook will be maintained by JYG to document session delivery, attendance, adherence to planned activities and any deviations from the protocol. This will support monitoring of process evaluation. Regular team meetings will be conducted to review progress, discuss emerging issues and ensure coordinated trial management throughout the study period.

Data analysis

Qualitative data related to the interviews and focus groups will be transcribed verbatim in Chinese and will be translated to English by the first author. A second author will check the transcribed data for completeness and accuracy. Transcripts will be analysed using thematic analysis following a six-phase guidance³²; the coding is initially deductive based on the feasibility and process outcomes, followed by an inductive process directed by the content of the data. To maximise consistency and completeness, two authors will independently conduct the thematic analysis using NVivo software (V.15) and discuss their findings until consensus is reached. The quantitative data related to feasibility outcomes (eg, satisfaction) and process evaluation (eg, dose delivered and received) will be presented descriptively. Integration of quantitative and qualitative findings will occur during the interpretation stage, with the quantitative and qualitative findings jointly interpreted to inform refinement of the intervention and the design of a future definitive trial.

Daily step counts will be aggregated to calculate mean daily steps across the intervention period. The overall mean daily steps will be summarised and descriptively compared between the intervention and control groups. Mean daily steps will also be calculated separately for the home-based phase and the school-based phase to describe PA levels across the two intervention components. Efficacy outcomes in terms of BMI, CRF, handgrip strength and SLJ will be analysed using linear mixed models following an intention-to-treat approach. The models will include time (T0 and T2), group (KEMOSS and control) and the time-by-group interaction as fixed effects, and participants will be included as a random effect, with adjustment for covariates, including age, sex, maturation and baseline outcome status, where appropriate. The models will use all available outcome data without imputation, under the assumption that data are missing at random. Parameters will be estimated using maximum likelihood estimation. All inferential analyses will be considered exploratory, and effect estimates and their 95% CIs will be reported, along with corresponding p values. Data analyses will be carried out via IBM SPSS Statistics for Windows (SPSS 28.0; IBM Corporation, Armonk, NY, US) with an alpha level of 0.05.

Ethical considerations and study dissemination

Ethics approval is obtained from the Ningbo University Ethics Committee (TY2026028). Results from the KEMOSS intervention will be disseminated via peer-reviewed journals, scientific conferences as well as the local education system. The study protocol has been registered on the Chinese Clinical Trial Registry (ChiCTR2600126738) and approved on 15 June 2026. The de-identified individual participant data generated during the current study, together with relevant supporting documentation, will be made available from the corresponding author on reasonable request following publication of the study results. Data will be shared where ethically and legally permissible and where participant confidentiality can be maintained.

Patient and public involvement

PPI³³ included 10 children, 5 family members (parents/guardians) and 2 school teachers, who were involved in the development of the study protocol. Children were invited to provide input on the selection and design of themed PA sessions, with a focus on identifying activities that were engaging and enjoyable. Parents/guardians contributed to discussions on the feasibility of the intervention, including the timing and frequency of sessions, as well as the overall structure of the programme and the home-based (mHealth) component. School teachers provided feedback on the suitability of the intervention within a school context, including the appropriateness of HIIT, strategies to support recruitment and the availability of school resources during the summer holidays, such as space and facilities.

Feedback from the PPI group informed several aspects of the intervention design, including the adoption of a hybrid delivery model, the selection of varied activity themes, refinement of session structure and consideration of practical factors to enhance feasibility and acceptability. The involvement of multiple stakeholders ensured that the intervention was tailored to the needs and preferences of children and their families, as well as aligned with the school setting. Moreover, based on discussions with school teachers during the PPI process, the school will provide access to an air-conditioned stadium for delivery of the KEMOSS intervention. A school nurse will also be available throughout the intervention to provide immediate medical support if required. These arrangements will help mitigate potential risks associated with the KEMOSS intervention.

Discussion

This protocol describes the KEMOSS intervention, a randomised feasibility trial designed to mitigate summer fitness loss in children by maintaining structured PA opportunities during the school holiday period. Grounded in the SDH, the intervention aims to address the decline in PA and related health outcomes associated with unstructured summer holidays.

The KEMOSS intervention incorporates several novel components designed to enhance both its effectiveness and sustainability. First, the inclusion of Baduanjin, a form of traditional Chinese martial arts, provides a low-impact and safe mode of exercise suitable for warm-ups.³⁴ Baduanjin has been widely reported to confer physical and mental health benefits,^35–37 and its integration into the programme may promote not only immediate improvements in activity levels but also longer-term engagement if participants acquire and retain the skills.

In addition, the intervention will engage children in vigorous PA by incorporating bodyweight resistance-based HIIT, which is an efficient approach to eliciting high levels of exertion (≥70% maximum heart rate) within a limited timeframe.^{38 39} Vigorous PA has been shown to provide additional health benefits beyond moderate-intensity activity, particularly in children.^40–42 Moreover, the inclusion of varied, theme-based exercise sessions further aims to enhance enjoyment and engagement. By offering diverse activity formats across the intervention period, the programme seeks to appeal to a wide range of preferences, potentially supporting recruitment and adherence, while also exposing children to different forms of PA that may encourage continued participation beyond the intervention.

The hybrid design of the intervention, combining a full-day school-based phase with a subsequent mHealth-supported home-based phase, represents a key strength of the KEMOSS intervention. The full-day structured schedule was strongly suggested by parents and was supported by the school staff and teachers during the PPI phase. This approach may enhance recruitment and participation by providing a supervised, school-like environment throughout the day, which may be particularly attractive to working parents who would otherwise need to arrange additional daytime care following isolated PA sessions.

The hybrid approach enhances feasibility by accommodating variations in family availability during the summer period while maintaining continuity of structured activity. The mHealth component enables participants to engage in PA in a flexible manner. Despite mHealth interventions being demonstrated as effective in promoting PA in children,⁴³ maintaining fidelity during this phase represents a potential challenge. To address this, several strategies have been incorporated, including the provision of structured exercise tasks, instructional videos, as well as regular reminders delivered in collaboration with school staff.

Delivering interventions during the summer holiday period presents unique challenges compared with school-term programmes, including reduced routine, competing family commitments and variable access to resources. By incorporating a process evaluation alongside the intervention, the present study will provide a deeper understanding of factors influencing implementation, engagement and fidelity, as well as potential barriers and facilitators to delivery. These findings will provide important insights into the feasibility and implementation of the KEMOSS intervention and inform refinement of the programme and the design of a future definitive trial.