Effects of school-based physical activity interventions on mental health in adolescents: The School in Motion cluster randomized controlled trial

This study investigated the effects of two different school-based physical activity interventions, called M1 and M2, on the mental health of adolescents. Both interventions aimed to increase students' physical activity by approximately 120 minutes per week over a 29-week period, compared to a standard school curriculum (control group).

Here's a breakdown of the interventions:

**Intervention M1:** This program focused on integrating physical activity into various aspects of the school day. It consisted of:

* 30 minutes of "physically active learning" (integrating movement into academic lessons).

* 30 minutes of dedicated "physical activity" (structured exercise time).

* One 60-minute "physical education" (PE) lesson per week.

* The total additional physical activity aimed for was 120 minutes per week.

**Intervention M2:** This program focused on enhancing engagement and social aspects within physical activity. It consisted of:

* One "physical education" (PE) lesson per week.

* One "physical activity" lesson per week.

* Both lessons were specifically designed to facilitate students' interest, responsibility, and social relationships, aiming to make physical activity more appealing and sustainable.

* The total additional physical activity aimed for was 120 minutes per week.

**Control Group:** Students in the control group continued with their standard school curriculum, which included regular physical education classes but without the additional structured physical activity components of M1 or M2.

The primary outcome measured was **mental health**, assessed through a self-report questionnaire. A secondary outcome was **physical activity levels**, measured objectively.

The study included 2084 adolescents (n = 2084) from 29 lower secondary schools across Norway.

**Age:** The students were approximately 14–15 years old.

**Gender:** 49% of the participants were female.

**Setting:** The study was conducted in a school-based setting in Norway.

**Baseline characteristics:** The abstract mentions specific subgroups that showed effects: those with the highest levels of psychological difficulties at baseline and immigrant adolescents. This implies a diverse group of students, including some who were already experiencing mental health challenges.

**Mental Health:** Mental health was assessed using the **self-report version of the Strengths and Difficulties Questionnaire (SDQ)**. The SDQ is a widely used screening tool for psychological difficulties and strengths in children and adolescents. It typically covers five scales: emotional symptoms, conduct problems, hyperactivity/inattention, peer problems, and prosocial behavior. A "Total Difficulties" score is derived from the first four scales, with higher scores indicating more difficulties. The abstract does not specify the exact scoring range used in this study, but generally, scores can range from 0 to 40 for Total Difficulties, with scores above a certain threshold indicating potential clinical concern.

**Physical Activity:** Physical activity levels were measured objectively using **accelerometry**. Accelerometers are small, wearable devices (often worn on the hip or wrist) that detect and record movement. This method provides an objective measure of the intensity, duration, and frequency of physical activity, reducing the reliance on self-report which can be prone to bias. The abstract does not specify the type of accelerometer used or the specific metrics (e.g., minutes of moderate-to-vigorous physical activity) derived from the data.

This study employed a **cluster randomized controlled trial (RCT)** design. This is considered a high-quality study design for investigating causal relationships.

**Study Design:**

* **Cluster Randomization:** Instead of randomizing individual students, entire schools (clusters) were randomized. Students from 29 lower secondary schools were assigned to either a control group or one of the two intervention groups (M1 or M2). This means that all students within a particular school received the same assignment (either control, M1, or M2).

* **Why Cluster Randomization Matters:** This design is often used in school-based interventions to prevent "contamination." If students within the same school were individually randomized, those in an intervention group might influence or share their activities with those in the control group, blurring the lines between the groups and making it harder to detect a true effect. Randomizing at the school level minimizes this risk. However, a drawback of cluster randomization is that it generally requires a larger total sample size than individual randomization to achieve the same statistical power, because students within the same school are likely to be more similar to each other than students from different schools.

* **Controlled Trial:** The inclusion of a control group (standard curriculum) allowed researchers to compare the outcomes of the intervention groups against what would happen without the specific interventions, helping to isolate the effects of M1 and M2.

**Randomization:** The 29 schools were randomly assigned to one of the three groups (Control, M1, M2). Randomization is crucial because it helps ensure that, on average, the groups are similar at the start of the study in all characteristics (known and unknown) except for the intervention they receive. This minimizes the risk that any observed differences in outcomes are due to pre-existing differences between the groups rather than the intervention itself.

**Blinding:** The abstract does not explicitly state whether blinding was used.

* **Participant/Intervention Blinding:** It is highly unlikely that participants (students) or those delivering the intervention (teachers) could be blinded to which group they were in, as the interventions involved distinct changes to their school day. This is a common challenge in behavioral and educational interventions.

* **Outcome Assessor Blinding:** It is possible, and good practice, for the researchers analyzing the mental health data (SDQ scores) to be blinded to the group assignments of the participants. This would prevent their expectations from influencing the data interpretation. However, the abstract does not confirm this. Lack of blinding for participants and teachers can introduce bias, as participants might report outcomes differently if they know they are receiving an intervention (placebo effect) or if teachers treat groups differently.

**Duration:** The intervention period lasted for **29 weeks**. This is a relatively long duration for a school-based intervention, which is a strength. A longer duration allows for potential effects to manifest and provides a more realistic test of sustainability compared to very short interventions.

**Statistical Approach:** **Linear mixed effects models** were used to examine the effects of the interventions.

* **Why this matters:** This statistical method is appropriate for analyzing data with a hierarchical structure, such as students nested within schools (clusters). It accounts for the fact that students within the same school are not entirely independent observations, which is important for accurate statistical inference in cluster RCTs. It also allows for the inclusion of covariates and the examination of interaction effects (how the intervention effect might differ across subgroups).

**What this design can and cannot prove:**

* **Can Prove:** As a cluster RCT, this study can provide strong evidence for a **causal relationship** between the specific physical activity interventions (M1 and M2) and changes in mental health *within the population studied* (Norwegian adolescents aged 14-15) and *under the conditions of the study*. The randomization helps establish causality by minimizing confounding factors.

* **Cannot Prove:**

* **Generalizability:** While strong for causality, the findings may not be directly generalizable to adolescents in different countries, different age groups, or different school systems without further research.

* **Mechanism of Action:** The study shows *if* an effect occurs, but it doesn't deeply explore *why* or *how* the interventions might work (e.g., through specific physiological changes, social interaction, or improved self-efficacy).

* **Long-term Effects:** While 29 weeks is a good duration, it doesn't provide insight into the very long-term effects of these interventions beyond the study period.

**Major Methodological Weaknesses Flagged:** The authors explicitly state that "More research is needed due to missing values and the results should therefore be interpreted with caution." Missing data can introduce bias if the missingness is not random (e.g., if students with poorer mental health are more likely to drop out or not complete questionnaires). The abstract does not specify the extent or nature of the missing values.

The study's primary analysis found no overall effect on mental health for the entire study population, but subgroup analyses revealed specific beneficial effects.

**Overall Study Population:**

* No effects were found for the overall study population regarding improvements in mental health. This means that, on average, across all students, the interventions M1 and M2 did not lead to a statistically significant improvement in SDQ scores compared to the control group.

**Subgroup Analyses (Interaction Effects):** Interaction effects indicated that the interventions might have different impacts on specific subgroups of adolescents.

* **Intervention M1:**

* **Subgroup with highest levels of psychological difficulties at baseline:** M1 showed favorable results in this subgroup.

* **Effect size (b):** -2.9

* **95% Confidence Interval (CI):** -5.73 to -0.07

* **p-value:** p = .045

* *Interpretation:* This indicates a statistically significant reduction in psychological difficulties (lower SDQ scores) for students who started with higher levels of difficulties and participated in the M1 intervention. The negative 'b' value means a decrease in the difficulty score.

* **Immigrant subgroup:** M1 showed favorable results in this subgroup.

* **Effect size (b):** -1.6

* **95% Confidence Interval (CI):** -3.53 to 0