Does the exercise environment matter? Indoor versus outdoor aerobic exercise for adolescents with subthreshold depression: a randomised controlled trial protocol

Abstract

Introduction

Aerobic exercise, defined as rhythmic and continuous physical activity performed at moderate intensity to improve cardiorespiratory fitness, is widely recognised as an effective non-pharmacological intervention for improving the mental health of adolescents with subthreshold depression. However, whether the environment in which exercise is performed—such as indoor versus outdoor settings—modulates its psychological benefits remains insufficiently understood, as empirical evidence is still scarce. This study aims to compare the psychological effects of aerobic exercise conducted in different environmental contexts among adolescents with subthreshold depression and to identify which exercise setting may provide greater therapeutic advantages.

Methods

This assessor-blinded, two-arm, parallel-group randomised controlled trial will recruit adolescents who meet standardised subthreshold depression screening criteria. Participants will be randomly assigned to the indoor aerobic exercise group or outdoor aerobic exercise group using sex-stratified block randomisation to ensure balanced allocation between male and female participants. Both groups will participate in an 8-week moderate-intensity aerobic exercise programme (2–3 sessions per week, 40 min per session), with heart rate monitoring employed to ensure consistency of exercise intensity across conditions. Depressive symptoms, anxiety and sleep quality will be assessed at baseline (week 0), mid-intervention (week 4) and post-intervention (week 8). Data analysis will be conducted according to the intention-to-treat principle.

Ethics and dissemination

Ethical approval has been obtained from the Ethics Committee of Capital University of Physical Education and Sports (Approval No. 2025A139). The findings of this study will be disseminated through peer-reviewed publications and academic conferences.

Trial registration number

ChiCTR2500112215.

STRENGTHS AND LIMITATIONS OF THIS STUDY.

• Participants are screened using a two-stage psychological assessment combined with cardiopulmonary fitness evaluation to ensure eligibility and exercise safety.

• Outcome assessors and data analysts are blinded to group allocation to minimise assessment and analytical bias.

• Exercise type, duration and intensity are rigorously standardised across groups, allowing clearer identification of the independent influence of environmental factors.

• Conducting all intervention sessions within a semiclosed campus setting minimises external variability, although this may also limit the generalisability of the findings to broader populations.

• Outdoor exercise sessions may still be influenced by uncontrollable environmental factors such as weather and temperature fluctuations.

Introduction

Adolescent mental health problems, including depression, anxiety, behavioural disorders and emotional difficulties, have emerged as a major public health challenge worldwide, drawing increasing attention from global health organisations and researchers alike.¹ It is estimated that approximately 10%–20% of children and adolescents experience some form of mental health difficulty.^{2 3} Among these conditions, subthreshold depression—characterised by mild symptom severity, insidious onset and the absence of clear diagnostic criteria—often goes unnoticed, yet its clinical implications are far from negligible. Subthreshold depression is associated with an increased risk of developing major depressive disorder (MDD)^{4 5} and represents a significant risk factor for the development of full-syndrome depression. Evidence indicates that adolescents with subthreshold depression are 5.7 times more likely to develop MDD within 1 year compared with their non-depressed peers.⁵

Importantly, early intervention strategies, such as psychological therapies, behavioural interventions and exercise-based programmes, for individuals with subthreshold depression have been shown to substantially reduce the risk of progression to major depression. Clarke et al reported that early cognitive intervention significantly reduced the risk of progression to MDD among adolescents with subthreshold depression, with the incidence of major depression decreasing from 28.8% in the control group to 9.3% in the intervention group during the 15-month follow-up period.⁶ These findings highlight the critical value of timely early intervention for adolescents with subthreshold depression and underscore its importance in promoting youth mental health.

In recent years, a variety of intervention strategies for adolescents with subthreshold depression have been increasingly developed and refined. These include low-dose pharmacotherapy (eg, selective serotonin reuptake inhibitors (SSRIs)),⁷ exercise-based interventions (such as Tai Chi⁸), psychological therapies (eg, cognitive behavioural therapy⁹) and lifestyle modifications, including improvements in sleep¹⁰ and dietary habits,¹¹ as well as several multimodal approaches.^{12 13} Evidence from previous studies suggests that each of these strategies confers measurable therapeutic benefits,¹⁴ and several recent systematic reviews and meta-analyses have further supported the positive effects of exercise interventions on depressive symptoms among children and adolescents.^{15 16} According to the National Institute for Health and Care Excellence, exercise therapy stands out as a first-line intervention for adolescents with mild depressive symptoms due to its low cost, minimal adverse effects and relatively rapid onset of benefits compared with other treatment modalities.^{17 18} However, existing exercise-based interventions for subthreshold depression among adolescents have predominantly focused on variables such as exercise modality (eg, Tai Chi⁸, aerobic exercise,^{19 20} swimming²¹), intensity and duration.²² In contrast, considerably less attention has been given to exercise environment, an external contextual factor that may play a meaningful role in shaping intervention outcomes. To date, empirical evidence regarding whether and how the exercise environment modulates the psychological benefits of physical activity remains limited, underscoring the need for more systematic investigation in this area. Moreover, few randomised controlled trials (RCTs) have directly compared the psychological effects of indoor and outdoor aerobic exercise under standardised exercise conditions among adolescents with subthreshold depression.

Research in environmental psychology has demonstrated that the exercise environment plays a significant role in emotion regulation and psychological restoration.^{23 24} Recent studies on green and blue spaces (GBS), particularly urban GBS, have further highlighted the psychological benefits of physical activity performed in natural environments.^{25 26} Compared with enclosed indoor settings, green and natural environments—such as outdoor parks and playgrounds—are more effective in enhancing positive affect, reducing perceived stress and accelerating psychological recovery.²⁷ Several potential mechanisms may explain these effects. Outdoor exercise environments typically provide greater exposure to natural light, fresh air, green spaces and broader visual stimuli, which may contribute to emotional regulation, attentional restoration and reductions in psychological stress.^{23 27} These environmental characteristics may be particularly relevant for adolescents with subthreshold depression, who often exhibit heightened stress sensitivity, reduced positive affect and impaired emotional regulation. Compared with indoor exercise, outdoor aerobic exercise may therefore provide additional psychological benefits by enhancing emotional restoration, increasing enjoyment and intrinsic motivation for physical activity and reducing perceived stress. Based on these considerations, the present study hypothesises that outdoor aerobic exercise may produce greater improvements in depressive symptoms and related psychological outcomes than indoor aerobic exercise, even when exercise type, intensity and duration are standardised across conditions. While these benefits have been well documented in general populations, they have not yet been systematically examined within exercise-based interventions targeting depressive symptoms. Whether aerobic exercise performed in different environmental contexts yields differential effects on reducing depressive symptoms and improving emotional well-being remains an important but largely unresolved scientific question.

Building on these considerations, the present study will employ a RCT design to systematically compare the effects of indoor versus outdoor aerobic exercise on psychological health outcomes among adolescents with subthreshold depression, while rigorously controlling for exercise type, intensity and duration. The aim of this study is to elucidate the potential influence of different exercise environments on the psychological benefits of physical activity. By doing so, the study seeks to provide ecologically valid evidence to support adolescent mental health promotion and to offer new practical implications for integrating school-based physical education with mental health interventions.

Objectives

Primary objective

The primary objective of this RCT is to compare the psychological effects of indoor versus outdoor aerobic exercise among adolescents with subthreshold depression. By rigorously controlling for exercise type, intensity and duration, this study will compare the effects of indoor versus outdoor aerobic exercise to determine whether different exercise environments produce differential effects on depressive symptoms, anxiety, sleep quality and exercise enjoyment.

Secondary objectives

1. To compare the effects of indoor and outdoor aerobic exercise on additional psychological health indicators—including anxiety levels, sleep quality and depressive symptoms—in adolescents with subthreshold depression, thereby assessing the multidimensional mental health benefits associated with different exercise environments.

2. To evaluate adolescents’ activity satisfaction with indoor versus outdoor aerobic exercise, in order to examine differences in acceptability and user experience across environments. This analysis aims to inform the practical integration of school-based physical education with mental health promotion programmes.

Methods

Trial registration

This study was prospectively registered in the Chinese Clinical Trial Registry (ChiCTR) on 11 November 2025 (registration No.: ChiCTR2500112215). The full registration record is available online (https://www.chictr.org.cn/showproj.html?proj=291348). The trial was registered prior to participant enrolment, and the complete study protocol and subsequent updates will be publicly accessible on the ChiCTR platform.

Trial design

This study will employ a single-centre, assessor-blinded, two-arm, parallel-group RCT design. The trial will be conducted in the senior high school division of Shandong Labor Vocational and Technical College. Adolescents aged 12–18 years who completed a two-stage screening procedure and met the operational criteria for subthreshold depression will be randomly allocated, in a 1:1 ratio, to either an indoor aerobic exercise group or an outdoor aerobic exercise group.

The trial is designed under a superiority framework, aiming to evaluate whether outdoor aerobic exercise produces greater psychological benefits than indoor aerobic exercise when exercise type, intensity and duration are rigorously standardised across groups. The primary objective is to compare the two environmental conditions in terms of their effects on depressive symptoms, with secondary comparisons focusing on anxiety levels and sleep quality.

SPIRIT compliance

This protocol was developed in accordance with the 2025 Standard Protocol Items: Recommendations for Interventional Trials (SPIRIT) guidelines. The SPIRIT 2025 checklist and figure are provided as supplementary materials.

Trial status

The study is currently in the preparation phase. Participant recruitment is scheduled to begin in early September 2026 and is expected to last for approximately 1 month. This timing is selected because, at the start of each academic year, the school conducts routine psychological screening in accordance with educational authorities’ requirements, which facilitates the efficient identification of eligible participants. Following recruitment and baseline assessments, the 8-week intervention period is planned to take place from October to November 2026. As of the submission of this protocol, the study remains in its preparatory stage; no participants have been recruited, and no interventions or data collection activities have been initiated. Any modifications to the study timeline or protocol will be promptly updated on the trial registry and reported as required by the journal. If recruitment is slower than anticipated or the target sample size cannot be achieved within the planned period, the recruitment period will be extended and additional eligible students from the 2025 student cohort will be invited to participate.

Sample size

The sample size was estimated using G*Power V.3.1.9.7. A two-sided significance level of α=0.05 and a statistical power of 1 − β=0.80 were applied. The study design includes two groups (indoor aerobic exercise vs outdoor aerobic exercise), with each participant undergoing assessments at two time points (pre-intervention and post-intervention). The sample size calculation was based on detecting a moderate effect size (Cohen’s f=0.25) for differences in depressive symptom outcomes between the indoor and outdoor aerobic exercise groups from baseline to post-intervention. The sample size estimation was primarily based on depressive symptoms assessed by the Self-Rating Depression Scale (SDS), which was defined as the primary outcome of the study. The effect size was conservatively set at a moderate level (Cohen’s f=0.25), informed by previous exercise-intervention studies.²⁸ The correlation among repeated measures was assumed to be 0.5, and the nonsphericity correction factor (ε) was set to 1. Under these assumptions, the minimum required total sample size was 48 participants. To account for potential absences, withdrawals and loss to follow-up, the final target sample size was set at 80 participants, with 40 participants per group.

Research process

The study will be conducted in four sequential phases—participant recruitment and screening, baseline assessment, exercise intervention and post-intervention assessment—to ensure scientific rigour, procedural standardisation and the reliability of study outcomes. First, students meeting the criteria for subthreshold depression will be identified through an initial screening with the SDS and a follow-up evaluation using the Hamilton Depression Rating Scale (HAMD-24). Eligibility will be further confirmed through a physical fitness assessment before enrolment. Subsequently, all participants will complete baseline assessments, including measures of depressive symptoms, anxiety levels and sleep quality. Following baseline assessment, participants will enter an 8-week indoor or outdoor aerobic exercise intervention according to their group allocation. The intervention will be delivered and supervised by qualified physical education instructors. Exercise intensity, attendance and any potential adverse events will be documented throughout the intervention to ensure both fidelity and participant safety. To continuously monitor psychological status and minimise potential risks during the intervention, the SDS will be administered again at the end of week 4. This mid-intervention assessment will be used solely for safety monitoring and progress tracking and will not be included in the study’s outcome measures. On completion of the intervention, all participants will undergo a post-intervention assessment identical to the baseline evaluation. Finally, all collected data will be compiled, processed and analysed to determine the effects of different exercise environments on the psychological health of adolescents with subthreshold depression. The overall research procedure is presented in figure 1.

Figure 1. Study flow diagram. PACES, Physical Activity Enjoyment Scale; PSQI, Pittsburgh Sleep Quality Index; SAS, Self-Rating Anxiety Scale; SDS, Self-Rating Depression Scale.

Randomisation and allocation concealment

The randomisation sequence will be generated by an independent statistician using a computer-based random number programme, employing sex-stratified block randomisation to ensure balanced allocation of male and female participants between the two groups. Allocation assignments will be placed in sequentially numbered, sealed and opaque envelopes, prepared in advance and securely maintained by a research coordinator who will not be involved in participant recruitment, data collection or outcome assessment. This process will ensure adequate allocation concealment throughout the trial.

Blinding and unblinding procedures

Due to the inherent and unavoidable differences between the indoor and outdoor exercise environments, it is not feasible to blind participants or intervention implementers in this study. However, to minimise measurement and analytical bias, outcome assessors and data analysts will remain blinded to group allocation. All assessment data will be coded and entered without any identifiable grouping information, and assessors and analysts will not have access to allocation records or on-site intervention details. If a participant experiences a serious adverse event during the intervention period that requires medical management and for which knowledge of the assigned intervention group is essential for clinical decision-making, an emergency unblinding procedure may be initiated with approval from the principal investigator. The unblinding process will follow a predefined protocol and will include detailed documentation of the reason, timing and personnel involved, with immediate notification to the data monitoring personnel. Apart from the necessary medical management, all other members of the research team will remain blinded to avoid introducing additional bias into the trial.

Trial setting

The study will be conducted in the senior high school division of Shandong Labor Vocational and Technical College. The school is a semiclosed vocational high school with approximately 3000 students enrolled. The semiclosed campus environment minimises external environmental variation, fluctuations in daily activity patterns and other potential sources of interference, thereby enhancing experimental control and ensuring greater consistency in intervention delivery. All screening procedures, baseline assessments, exercise interventions and post-intervention evaluations will take place within the school grounds. The primary locations include the indoor gymnasium, outdoor track-and-field area and the school’s psychological counselling room. The entire study will be implemented collaboratively by uniformly trained physical education teachers, psychology instructors and research assistants to ensure standardised intervention delivery, assessment procedures and data collection.

Recruitment

Participants will be recruited based on the results of the school’s annual routine psychological screening. Following enrolment of new students each academic year, the school’s psychological counselling office administers the SDS to all students as part of the institution’s standard mental health education programme. This screening is conducted independent of the present study. With permission from the school administration, the research team will recruit students who meet the SDS criteria for subthreshold depression. Participation is entirely voluntary; students may indicate their interest after receiving full study information, and written informed consent will be obtained from their legal guardians. Subsequently, students who express willingness to participate will undergo a follow-up assessment using the HAMD-24, along with evaluations of physical fitness and cardiorespiratory function to confirm final eligibility. The entire recruitment process will adhere to the principles of voluntary participation and confidentiality and will be conducted in strict compliance with ethical requirements. No incentives or financial compensation will be provided.

Screening process

Stage 1: SDS preliminary screening

After the enrolment of new students in September 2026, the school’s psychological counselling office will conduct routine mental health screening for all freshmen, as required by the education authorities. Participants will be screened using the SDS. The present study will use the results of the school’s routine screening as the basis for preliminary identification, without imposing any additional assessment burden on students or administering extra evaluations to those not included in the study. According to the Chinese norm of the SDS, a standard score of 53 is commonly used as the cut-off point for depressive symptoms.²⁹ Although individuals scoring below 53 are generally not considered clinically depressed, some may still present subthreshold depressive symptoms. Therefore, students with SDS scores <53 will proceed to the second-stage screening using the HAMD-24. Candidates will be ranked in descending order based on their SDS scores to determine the sequence of follow-up assessments.

Stage 2: HAMD-24 assessment

Building on the preliminary screening, two trained mental health professionals will conduct face-to-face, structured interviews using the HAMD-24. Students will be considered eligible for subthreshold depression if they (a) obtain a HAMD-24 total score between 8 and 20—indicative of subclinical depressive symptoms—and (b) exhibit at least two core depressive symptoms. These students will be added to the candidate pool. The final sample size will be determined according to the results of the sample size estimation. Only students who meet the eligibility criteria and whose legal guardians provide written informed consent will proceed to the intervention and assessment phases.

Stage 3: cardiopulmonary fitness assessment

To ensure participant safety during the exercise intervention, all candidates who pass the first two screening stages will undergo a physiological fitness evaluation to exclude individuals unsuitable for moderate-intensity exercise. Prior to testing, researchers will review participants’ medical history and any past exercise-related symptoms to determine their suitability for physical assessment. A standardised stair-climbing test will be used to obtain an initial estimate of cardiopulmonary fitness. Previous studies have demonstrated that this method is both feasible and safe for children and adolescents.^{30 31} During the test, participants will climb a vertical height of approximately 12 m (equivalent to four floors) at a standardised pace. Researchers will monitor participants’ respiration, heart rate and subjective discomfort throughout and after the test. If a participant experiences symptoms such as chest tightness, dizziness or shortness of breath, or if their breathing does not return to a steady state within 90 s after test completion, their cardiopulmonary fitness will be deemed inadequate. To avoid potential exercise-related risks, such individuals will not be enrolled in the intervention phase.

Informed consent

Prior to the initiation of the study, the principal investigator or trained research staff will provide all eligible participants and their legal guardians with detailed information regarding the study’s objectives, procedures, potential risks and possible benefits. Explanations will be delivered through both written materials and oral communication to ensure full understanding of the study content. All personal information, psychological assessment data and exercise records related to the study will be kept strictly confidential and used solely for research purposes. No data will be disclosed to school teachers, classmates or any individuals not involved in the research. All study data will be fully anonymised prior to publication to safeguard participant privacy. After participants and their legal guardians have clearly understood the study procedures, written informed consent will be obtained from the guardians, along with written assent from the participants themselves. Participants will be officially enrolled only when both the guardian and the participant provide consent. This study does not involve the collection of biological specimens, genetic testing or any additional laboratory analyses; therefore, no supplementary consent procedures are required. An example of the participant consent form is provided in online supplemental file 1.

Patient and public involvement

Patients or members of the public were not involved in the design, conduct or evaluation of this study. No patient or public advisory group was established, and neither students nor their legal guardians contributed to the design of recruitment materials or outcome measures. After study completion, a lay summary of the findings will be made available to participants and their legal guardians on request.

Eligibility criteria

Inclusion criteria

Participants will be eligible for inclusion if they meet all of the following conditions:

1. Students aged 12–18 years enrolled at the participating school.

2. A SDS score below 53 in stage 1 screening, and a HAMD-24 score between 8 and 20 in stage 2 assessment, meeting the operational criteria for subthreshold depression.

3. Ability to participate in an 8-week indoor or outdoor aerobic exercise intervention as required.

4. Results from the cardiopulmonary fitness and physical condition assessments indicate that the student is physically suitable for moderate-intensity exercise.

5. Willingness to participate, with written informed consent provided by the legal guardian and written assent provided by the participant.

Exclusion criteria

Participants will be excluded if any of the following conditions are present:

1. A previous diagnosis of severe psychiatric disorders, including MDD, bipolar disorder, psychotic disorders or other severe mental illnesses.

2. Presence of physical conditions that may compromise exercise safety, such as serious cardiovascular disease, uncontrolled asthma or other exercise-limiting somatic illnesses.

3. Participation in regular structured physical training within the previous 6 months or identification as a school sports speciality student, which may influence intervention outcomes.

4. Evidence of suicidal risk or markedly unstable emotional state during psychological assessment.

5. Occurrence of chest tightness, dizziness, shortness of breath or other significant discomfort during the stair-climbing test or failure to return to stable breathing within 90 s after completing the test.

6. Inability to commit to completing the study protocol as required or anticipated long-term absence from school during the intervention period.

Site and staff eligibility

Site eligibility

The study will be conducted within the school campus attended by the participants. Eligible sites must provide appropriate environments for both indoor and outdoor aerobic exercise, including a well-ventilated indoor gymnasium, a standardised outdoor running track and necessary safety protection facilities. All locations must meet the operational standards required for routine teaching and physical training activities and must remain relatively isolated during the intervention to prevent interference from other school events.

Staff eligibility

All personnel involved in the study must possess relevant professional backgrounds and qualifications. Instructors responsible for delivering the exercise intervention must have experience in physical education, exercise training or related fields and be capable of monitoring movement technique, exercise intensity and participant safety in real time. Psychological assessments will be conducted with support from certified staff members of the school’s psychological counselling office. All assessors will hold appropriate mental health credentials and will receive standardised training to ensure proficiency in administering the SDS, HAMD-24 and other assessment instruments.

Intervention and comparator

Two exercise-environment intervention arms will be implemented in this study: an indoor aerobic exercise group and an outdoor aerobic exercise group. Both groups will engage in 8 weeks of moderate-intensity aerobic exercise, with training scheduled twice per week during the first 2 weeks and three times per week during the subsequent 6 weeks. Participants with an attendance rate of ≥80% will be considered adherent, and their data will be included in the final analysis.

Each training session will last 40 min, consisting of 5–10 min of warm-up (eg, light jogging, dynamic stretching), 30 min of moderate-intensity aerobic exercise and 5 min of cool-down (eg, static stretching and breathing regulation). Exercise intensity will follow the American College of Sports Medicine (ACSM) Guidelines for Exercise Testing and Prescription, 11th edition (2021), which define moderate intensity as 64%–76% of maximal heart rate (HRmax), corresponding to a target heart rate of 129–153 beats per minute.³² HRmax will be estimated using the formula ‘220−age’. In addition, the Borg Rating of Perceived Exertion scale (12–14) will be used to simultaneously monitor subjective exercise intensity and help maintain moderate-intensity exercise during the intervention sessions.

To ensure accurate monitoring of training intensity, approximately 20 students per group will wear heart rate monitors during each session. For the remaining participants, exercise intensity will be assessed by the instructors based on movement rhythm, breathing patterns and observable exertion. The aerobic exercise programme will include a combination of sport-based activities and fitness components, such as physical games, badminton, aerobic fitness routines, rope skipping, resistance exercises and walk-run intervals, arranged following the structure of standard physical education lessons.

All exercise sessions will be delivered and supervised by two qualified physical education instructors who hold postgraduate degrees in physical education-related fields, possess senior high school or higher-level physical education teaching certificates and have more than 5 years of experience in school-based physical education and exercise instruction. The same instructors will supervise both intervention groups using a standardised intervention protocol to ensure consistency in intervention delivery and reduce potential instructor-related bias. Training for both groups will be scheduled during comparable time periods in their respective environments to minimise the influence of temporal factors on intervention outcomes. In cases of adverse weather conditions preventing outdoor sessions, the corresponding session will be rescheduled or adjusted accordingly. A detailed plan of the aerobic exercise intervention is presented in table 1.

Table 1. Exercise schedule and environmental settings during the 8-week intervention.

Week	Exercise type	Environment	Frequency	Intensity
1	Physical games	Indoor/outdoor	Two sessions/week	HRmax 64–76%
2	Physical games	Indoor/outdoor	Two sessions/week	HRmax 64–76%
3	Badminton	Indoor/outdoor	Three sessions/week	HRmax 64–76%
4	Aerobic fitness exercises	Indoor/outdoor	Three sessions/week	HRmax 64–76%
5	Rope skipping	Indoor/outdoor	Three sessions/week	HRmax 64–76%
6	Strength training	Indoor/outdoor	Three sessions/week	HRmax 64–76%
7	Walk-run alternation	Indoor/outdoor	Three sessions/week	HRmax 64–76%
8	Physical games	Indoor/outdoor	Three sessions/week	HRmax 64–76%

HRmax, maximal heart rate.

Indoor aerobic exercise group

Training for the indoor aerobic exercise group will take place in the school gymnasium. The indoor environment provides a relatively enclosed space that effectively minimises external disturbances such as wind, noise and fluctuations in temperature. The smooth flooring, stable lighting and controlled environmental conditions ensure a consistent exercise experience for participants, helping to maintain uniform training rhythm and exercise intensity across sessions. The quieter setting also facilitates clear communication of instructions by the instructors, enabling more precise organisation of activities and effective safety supervision.

Outdoor aerobic exercise group

Training for the outdoor aerobic exercise group will be conducted on the school’s athletic field and other open-air activity spaces. Compared with indoor settings, the outdoor environment offers greater spatial openness and richer visual stimuli. Participants are exposed to natural sunlight, fresh air and broader visual fields, which may foster more positive emotional experiences. However, the outdoor environment is also influenced by weather conditions, wind, temperature fluctuations, noise and other external factors, which may lead to variations in exercise experience due to changes in sunlight or environmental disturbances. Therefore, during outdoor training sessions, instructors will adjust exercise pacing, safety reminders and session arrangements based on real-time environmental conditions. To ensure consistency between groups, both indoor and outdoor exercise interventions will maintain the same exercise modalities, frequency, duration and target intensity. Outdoor exercise sessions will only be conducted under appropriate environmental and weather conditions. In the event of extreme or unsuitable weather conditions, both indoor and outdoor exercise sessions will be suspended simultaneously to reduce potential environmental confounding.

Participant timeline

Participants will enter the study after completing the two-stage screening process (SDS preliminary screening and HAMD-24 assessment) and passing the physical fitness evaluation. The full study timeline includes screening, baseline assessment, an 8-week intervention period and post-intervention assessment. The key time points are as follows:

1. Screening phase (week 0):

   All students will complete the SDS screening, followed by HAMD-24 assessments for those who meet the preliminary criteria. Cardiopulmonary fitness and physical condition evaluations will then be conducted to determine final eligibility.

2. Baseline assessment (end of week 0):

   Eligible participants will complete baseline assessments, including depressive symptoms, anxiety, sleep quality and physical condition.

3. Mid-intervention monitoring (week 4):

   A safety monitoring assessment (eg, SDS) will be conducted during week 4 to observe psychological fluctuations and identify potential safety risks. These data are used solely for safety monitoring and will not be included in the outcome analysis.

4. Intervention period (weeks 1–8):

   Participants will receive an 8-week indoor or outdoor aerobic exercise intervention according to group allocation. Training will be held twice per week during the first 2 weeks and three times per week during the following 6 weeks, with each session lasting 40 min. Attendance, exercise intensity and adverse events will be documented.

5. Post-intervention assessment (end of week 8):

   At the end of the intervention period, participants will complete the same psychological and sleep assessments conducted at baseline to evaluate pre-post changes.

This study does not include long-term follow-up assessments, as it is designed to evaluate the immediate effects of the intervention. The participant timeline is illustrated in figure 2 and was developed in accordance with the SPIRIT 2025 statement. The completed SPIRIT checklist is provided as online supplemental file 2.

Figure 2. SPIRIT schedule. SPIRIT, Standard Protocol Items: Recommendations for Interventional Trials.

Outcomes

Primary outcome

The primary outcome is the change in depressive symptom severity from baseline to post-intervention (week 8), assessed using the SDS total score.

Secondary outcomes

The secondary outcomes include:

1. Anxiety symptoms, assessed using the Self-Rating Anxiety Scale (SAS).

2. Sleep quality, assessed using the Pittsburgh Sleep Quality Index (PSQI).

3. Physical activity enjoyment/satisfaction, assessed using the Physical Activity Enjoyment Scale (PACES).

Anxiety symptoms and sleep quality will be assessed at baseline (week 0) and post-intervention (week 8). Physical activity enjoyment/satisfaction (PACES) will be assessed at post-intervention only.

Outcome measures

Self-Rating Depression Scale

The SDS is a 20-item self-report instrument designed to assess depressive symptoms. Each item is rated on a four-point scale (‘occasionally’, ‘sometimes’, ‘frequently’, ‘constantly’). Developed by Zung in 1965, the SDS is valued for its simplicity and ability to reflect individuals’ subjective experiences of depressive symptoms and treatment-related changes.³³ Commonly used cutoffs are: <53 (normal), 53–62 (mild depression), 63–72 (moderate depression) and >72 (severe depression). The cut-off of 53 applied in this study follows established standards frequently used in Chinese adolescent mental health research. As a self-report measure, standardised administration is essential to reduce potential biases in adolescent responses.

Hamilton Depression Rating Scale

The HAMD, developed by Hamilton in 1960, is one of the most widely used clinician-rated instruments for evaluating depression severity.³⁴ The present study will employ the 24-item version (HAMD-24). Most items are scored on a 0–4 scale, whereas several items use alternative scoring ranges depending on symptom characteristics. Although originally developed for adults, the HAMD-24 has shown good applicability and psychometric performance in Chinese adolescent populations.^{35 36} In this study, two trained assessors will independently conduct face-to-face structured interviews and behavioural observations to improve scoring accuracy and reliability. Total scores <8 indicate the absence of depressive symptoms, scores of 8–20 indicate mild depressive symptoms, scores of 21–34 indicate moderate depression and scores >35 indicate severe depression. Any discrepancies in scoring will be resolved through discussion and consensus. Participants included in this study met the subthreshold depression criteria of scoring 8–20 and exhibiting at least two core depressive symptoms.

Self-rating anxiety scale

The SAS, developed by Zung in 1971, assesses anxiety severity and contains 20 items rated on a four-point frequency scale (‘none or rarely’, ‘some of the time’, ‘a good part of the time’ ‘most or all of the time’).³⁷ Conventional cutoffs include <50 (no anxiety), 50–59 (mild anxiety), 60–69 (moderate anxiety) and >70 (severe anxiety). The cut-off criteria used in this study are consistent with those applied in adolescent mental health screening research in China.

Pittsburgh sleep quality index

The PSQI, developed by Buysse et al in 1989, is a widely used instrument for evaluating subjective sleep quality over the previous month.³⁸ The scale includes 19 self-rated items grouped into seven components scored from 0 to 3, yielding a global score ranging from 0 to 21. Higher scores indicate poorer sleep quality, with scores >5 generally reflecting sleep disturbance.

All outcome measures will be assessed at baseline (week 0) and post-intervention (week 8).

Physical activity enjoyment scale

The PACES is a widely used self-report instrument designed to assess participants’ enjoyment and satisfaction during physical activity.³⁹ The scale consists of 18 items rated on a seven-point Likert scale, with higher scores indicating greater enjoyment and more positive exercise experiences. PACES has demonstrated good reliability and validity in adolescent populations and has been frequently applied in exercise-intervention research. In this study, PACES will be administered at post-intervention (week 8) to evaluate participants’ subjective enjoyment and acceptance of indoor versus outdoor aerobic exercise environments.

Adherence

Adherence to the exercise intervention will be monitored using standardised attendance logs maintained by trained instructors. A session will be recorded as completed if the participant attended and completed the full 40 min training session as scheduled. Adherence will be defined as participation in ≥80% of all planned sessions across the 8-week intervention period. Reasons for missed sessions—including illness, scheduling conflicts, physical discomfort or withdrawal—will be documented. Participants who do not meet the adherence threshold will be included in the intention-to-treat (ITT) analyses but excluded from the per-protocol (PP) analyses.

Harms and stopping rules

Harms

Throughout the intervention period, all exercise sessions will be supervised on-site by qualified physical education instructors, who will continuously monitor participants’ heart rate, exercise performance and subjective discomfort. If any abnormal symptoms—such as dizziness, chest tightness or shortness of breath—occur during training, the session will be stopped immediately and an initial assessment will be conducted by the instructors. If necessary, participants will be escorted to the school infirmary for further medical evaluation.

All adverse events will be systematically documented, including mild discomfort (eg, muscle soreness, fatigue) and any other abnormal physiological reactions. In the event of a SAE, the research team will report the incident to the principal investigator and the institutional ethics committee within 24 hours, and clinical professionals will intervene promptly to conduct appropriate evaluation and management.

To monitor psychological safety, an SDS assessment will be administered during week 4 as part of mid-intervention safety surveillance. If a participant’s SDS score shows a marked increase, psychological counselling or referral recommendations will be provided, and the decision regarding continuation in the intervention will be made on a case-by-case basis. Participants requiring additional psychological support during or after the study will be referred to the school psychological counselling service or appropriate healthcare providers as needed. If clinically significant psychological deterioration is identified during safety monitoring, participants and their legal guardians will be informed promptly and appropriate support or referral will be arranged. No special compensation is planned for study participation, and any routine medical care required will be provided according to standard institutional procedures.

Stopping rules

Individual participants will be withdrawn from the study under any of the following circumstances:

1. Development of severe physical symptoms—such as persistent chest pain, syncope or pronounced dyspnoea—that contraindicate continued exercise participation.

2. Safety monitoring indicating significant deterioration in psychological status (eg, SDS ≥53 with a substantial increase from baseline).

3. Occurrence of a SAE, after evaluation by clinical experts and the research team, resulting in a recommendation to discontinue participation.

4. Voluntary withdrawal by the participant or their legal guardian.

If multiple SAEs occur during the study, the principal investigator will consult with the institutional ethics committee to determine whether the trial should be suspended or terminated. All safety procedures have been reviewed and approved by the ethics committee and will be conducted in accordance with relevant research and clinical safety guidelines.

Ancillary and post-trial care

Participants who experience adverse events during the study will receive appropriate medical evaluation and care through the school infirmary or a designated medical facility. Participants requiring additional psychological support after the intervention will be referred to the school counselling centre or appropriate mental health services. Participation is voluntary, and withdrawal from the study will not affect access to routine educational or healthcare services.

Methods: data collection, management and analysis

Data management

A data monitoring group, consisting of two members of the research team and one independent psychology expert, will oversee the completeness, accuracy and integrity of all collected data. This group functions solely for data oversight and does not constitute an independent data monitoring committee. All data will be backed up immediately after collection and stored on encrypted, password-protected hard drives. The HAMD-24 assessments will be completed using paper-based forms, whereas all other questionnaires and process-related data will be collected through a standardised mobile-based survey system.

Paper records will be stored in locked filing cabinets accessible only to authorised personnel. Electronic data will be stored on a password-protected computer equipped with secure firewalls to prevent unauthorised access or data breaches. Prior to analysis, all datasets will undergo de-identification to ensure that no personally identifiable information can be traced back to individual participants.

The principal investigator and authorised members of the research team will have full access to the final dataset. No contractual agreements or external restrictions will limit the research team’s access to or use of the study data. To protect the privacy of adolescent participants, data access rights will remain strictly confined to approved investigators.

Data analysis

All statistical analyses will be conducted using SPSS V.26.0 (IBM, Armonk, New York, USA). Prior to analysis, the normality and homogeneity of variance of continuous variables will be examined to determine the appropriateness of parametric tests. Baseline differences between the indoor and outdoor groups will be assessed using independent-samples t-tests for continuous variables and χ² tests for categorical variables.

The primary analysis will use analysis of covariance, with post-intervention scores as the dependent variable, group (indoor vs outdoor) as the fixed factor and baseline scores as the covariate. The same analytical approach will be applied to SDS, SAS and PSQI. PACES will be analysed separately because it is assessed only at post-intervention. Effect sizes will be presented as partial eta squared (η²p) with 95% CIs, in alignment with Consolidated Standards of Reporting Trials (CONSORT) guidelines. Within-group pre-post changes will be evaluated using paired t-tests or non-parametric alternatives if normality assumptions are not met. These analyses will be interpreted as supportive rather than primary. The primary analyses will be conducted according to the ITT principle, whereas PP analyses will be performed to assess the robustness of the findings.

Missing data will be handled according to the ITT principle using multiple imputation (m=20), incorporating baseline scores, group allocation and available outcome data. Sensitivity analyses will be performed by comparing results derived from imputed and complete-case datasets. Adverse events recorded during the intervention will be summarised descriptively. Given the minimal risk of the intervention, no interim analyses are planned. All primary and secondary outcome analyses were prespecified before study initiation.

Data confidentiality

All participant data will be de-identified and handled confidentially. Access to identifiable information will be restricted to authorised study personnel only. Research data will be retained for at least 5 years after study completion and then securely destroyed according to institutional regulations.

Auditing

Given that this study involves a low-risk behavioural intervention, an independent data monitoring committee is not required. No independent external auditing is planned for this study, as the intervention is low-risk and conducted entirely within a school setting. Oversight of the study will be provided by the ethics committee of Capital University of Physical Education and Sports. The principal investigator will conduct regular internal monitoring to ensure adherence to the approved protocol, verify data quality and maintain the accuracy and completeness of study records.

Protocol amendments

The current study protocol is V.1.0 (10 November 2025). Any substantial amendments to the study protocol, including changes to eligibility criteria, outcome measures, intervention procedures or statistical analyses, will require prior approval from the ethics committee of Capital University of Physical Education and Sports. Approved amendments will be documented, updated in the ChiCTR) and communicated to relevant study personnel, participants and journal editors when appropriate.

Ethics and dissemination

Ethical approval for this study has been granted by the ethics committee of the Capital University of Physical Education and Sports (Approval No.: 2025 A139). Written informed consent will be obtained from all legal guardians and written assent will be obtained from all participants prior to enrolment. The results of this study will be disseminated through publication in peer-reviewed journals and presentations at national and international academic conferences. Summary findings will also be made publicly available on the ChiCTR. Participants and their legal guardians may request a lay summary of the study findings if desired.

Discussion

Adolescents with subthreshold depression represent a high-risk transitional population for the future development of MDD, highlighting the importance of early and targeted intervention strategies. This study protocol outlines a RCT designed to examine whether different exercise environments—specifically indoor versus outdoor aerobic exercise—produce differential effects on the psychological health of adolescents with subthreshold depression. Although substantial evidence has demonstrated that aerobic exercise is an effective non-pharmacological intervention for improving mood among adolescents,^{40 41} scientific research on the environmental context in which exercise takes place remains limited. Findings from environmental psychology suggest that engaging in physical activity in natural or outdoor settings tends to elicit more positive emotional responses and greater psychological restoration.^{42 43} However, existing studies have primarily focused on exercise type, duration and intensity, while the role of the exercise environment as an independent factor in psychological interventions has not been adequately investigated. In particular, few studies have directly compared indoor and outdoor exercise interventions among adolescents with subthreshold depression within school-based settings, and the potential mechanisms through which exercise environments influence psychological outcomes in this population remain unclear. Therefore, the present study holds important theoretical and practical value for understanding how contextual factors influence the mental health benefits of exercise in adolescents. By directly comparing two commonly used school-based exercise settings, this trial aims to address a meaningful research gap and clarify whether exercising in different environments leads to distinct psychological outcomes.

This study incorporates several methodological strengths. First, a multistage screening procedure—including SDS prescreening, HAMD-24 diagnostic assessment and cardiopulmonary fitness evaluation—ensures that participants not only meet the criteria for subthreshold depression but are also physically capable of safely engaging in moderate-intensity aerobic exercise. Second, the intervention protocol is standardised according to ACSM guidelines, ensuring that exercise structure and intensity are matched between the indoor and outdoor groups, thereby enhancing the comparability of the two environments. Third, comprehensive safety monitoring is implemented throughout the trial, including a mid-intervention SDS assessment to support feasibility and participant protection. Additionally, the statistical analysis plan incorporates ITT principles and multiple imputation for missing data, which strengthens the robustness and reliability of the study findings.

By integrating a rigorous study design with focused attention to the exercise environment, this trial has the potential to offer valuable practical insights for the fields of education and public health. The findings may help schools, physical education instructors and mental health practitioners better understand how different exercise environments influence adolescents’ emotional and behavioural responses. Furthermore, the results may provide preliminary evidence for developing environment-specific school-based exercise interventions and guide future longitudinal and mechanism-based studies exploring how environmental factors interact with physical activity to influence adolescent mental health. Moreover, the study is expected to contribute to theoretical advancements in both exercise psychology and environmental psychology by further elucidating the interactive mechanisms between physical activity and environmental factors. Ultimately, the results of this trial aim to provide scientific evidence to inform the development of more context-sensitive and youth-appropriate exercise interventions, thereby supporting mental health promotion among adolescents with subthreshold depression.

Limitations

This study has several limitations. First, the trial is conducted in a single school setting, which may limit the generalisability of the findings to adolescents from other regions or educational backgrounds. Second, although the intervention protocol is standardised, outdoor exercise sessions remain subject to environmental variability such as weather and temperature, which may influence participants’ exercise experiences. Third, the limited availability of heart rate monitors prevented uniform monitoring of exercise intensity across all participants. Finally, psychological outcomes rely primarily on self-report instruments, which may be affected by mood fluctuations or social desirability biases. These limitations should be considered when interpreting the findings and may guide future research.