Effects of reducing sedentary behaviour by increasing physical activity, on cognitive function, brain function and structure across the lifespan: a systematic review and meta-analysis.

This systematic review and meta-analysis investigated the effects of reducing sedentary time (ST) by replacing it with physical activity (PA) on cognitive function, brain function, and brain structure.

The **interventions** involved:

**Reducing Sedentary Time (ST):** This meant actively decreasing the amount of time spent sitting or reclining with very low energy expenditure (defined as ≤1.5 metabolic equivalents).

**Increasing Physical Activity (PA):** The sedentary time was reallocated to physical activity. This included both single bouts of PA and multiple, shorter bouts of PA. The specific types and intensities of PA varied across the included studies but generally aimed to break up prolonged sitting.

The **comparators** typically included:

**Prolonged Sedentary Time:** Participants continued to sit for extended periods without interruption.

**Inactive Control:** A group that received no intervention or maintained their usual sedentary habits.

The **outcome measures** were:

**Cognitive Function:** This is a broad term encompassing various mental processes like attention, memory, executive function (planning, problem-solving, decision-making), and processing speed. The specific tests used to measure these functions varied across the individual studies included in the meta-analysis.

**Brain Function:** This refers to how the brain works, often measured through techniques like electroencephalography (EEG) or functional magnetic resonance imaging (fMRI) to assess neural activity, connectivity, or efficiency.

**Brain Structure:** This refers to the physical characteristics of the brain, such as the volume of different brain regions (e.g., grey matter, white matter) or the integrity of neural pathways, typically measured with structural MRI.

The review specifically looked at both **acute effects** (changes observed within hours or a few days after the intervention) and **chronic effects** (changes observed after interventions lasting weeks or longer).

This meta-analysis synthesized data from 25 randomized controlled trials (RCTs), involving a total of 1,289 participants (n=1,289).

The participants were described as **healthy individuals without cognitive impairment or neurological conditions** that affect cognitive functioning. The age range was broad, spanning **across the human lifespan, from children aged 4 years and older**. This included:

Children (4-9 years)

Adolescents (10-17 years)

Young adults (18-35 years)

Middle-aged adults (36-64 years)

Older adults (≥65 years)

The abstract does not specify the exact distribution of participants across these age groups or the specific settings (e.g., lab, home, school, workplace) of the individual studies. However, the inclusion criteria ensured a diverse population in terms of age, making the findings potentially relevant to a wide range of individuals.

As a systematic review and meta-analysis, this study did not directly measure outcomes but rather synthesized measurements from the included individual randomized controlled trials (RCTs). Therefore, the specific instruments and scales varied widely across the 25 included studies.

For **cognitive function**, the individual studies likely used a variety of standardized neuropsychological tests designed to assess different domains of cognition. Common examples of such tests, which would have been used in the included studies, might include:

**Executive Function:** Tasks like the Stroop test (measuring inhibitory control and selective attention), Wisconsin Card Sorting Test (measuring set-shifting and abstract reasoning), or various working memory tasks (e.g., n-back task).

**Attention:** Tests such as the Continuous Performance Test (measuring sustained attention) or reaction time tasks.

**Memory:** Verbal or visual recall tasks, digit span tests.

**Processing Speed:** Simple reaction time tests or symbol-digit coding tasks.

The abstract does not list specific cognitive tests, indicating that a broad range of validated cognitive assessments were likely included, reflecting the diverse nature of cognitive function.

For **brain function**, the included studies would have employed neuroimaging or neurophysiological techniques. The abstract mentions "neural efficiency of the hippocampus" in one chronic study, which implies techniques such as:

**Functional Magnetic Resonance Imaging (fMRI):** Measures brain activity by detecting changes in blood flow.

**Electroencephalography (EEG):** Measures electrical activity in the brain.

**Near-Infrared Spectroscopy (NIRS):** Measures changes in blood oxygenation in the brain.

For **brain structure**, the review found no studies that met its inclusion criteria. Had there been studies, they would typically use:

**Structural Magnetic Resonance Imaging (sMRI):** Provides detailed images of brain anatomy, allowing for measurement of grey matter volume, white matter integrity, and overall brain size.

The meta-analysis combined the results from these diverse measurement tools by calculating standardized effect sizes (Hedges' g), which allows for comparison and aggregation of findings across studies that might have used different specific tests but measured similar constructs.

This study was a **systematic review and meta-analysis** of **Randomized Controlled Trials (RCTs)**. This design is considered the highest level of evidence for establishing cause-and-effect relationships.

**How they ran the study:**

1. **Systematic Search Strategy:** The researchers conducted comprehensive searches across seven major academic databases: PubMed, Scopus, CINAHL, PsycINFO, SPORTDiscus, Web of Science, and ProQuest Dissertation and Theses. This broad search aimed to capture as many relevant studies as possible, minimizing publication bias. The searches were conducted from the inception of each database up to June 17, 2024, with updated searches performed in 2022, 2023, and 2024. They also performed a "backward citation hand search" by reviewing the reference lists of eligible articles to find additional relevant studies.

2. **Eligibility Criteria (PICOS framework):**

* **Population (P):** Healthy participants aged 4 years or older, without cognitive impairment or neurological conditions. This broad age range allowed for a lifespan perspective.

* **Intervention (I):** Interventions that aimed to reduce sedentary time (ST) or prolonged ST by reallocating that time to physical activity (PA). This included both acute interventions (lasting from 3 hours to 13 days) and chronic interventions (lasting 2 weeks or more).

* **Comparators (C):** Inactive control groups (e.g., usual practice, no intervention, waitlist) or minimally active control groups (e.g., prolonged sitting without PA breaks).

* **Outcomes (O):** Measures of cognitive function, brain function, or brain structure.

* **Study Design (S):** Only Randomized Controlled Trials (RCTs), including crossover and parallel designs, were included. This strict criterion is crucial for robust causal inference.

3. **Study Selection and Data Extraction:** After the initial search, studies were screened for eligibility. For included studies, relevant data on participant characteristics, intervention details, and outcome measures were extracted.

4. **Statistical Approach (Meta-analysis):** The extracted data were then statistically combined using meta-analytic techniques.

* **Effect Sizes:** Hedges' g was used as the standardized mean difference effect size. This allows for combining results from studies that might have used different scales or measures for the same outcome. A positive Hedges' g indicates a beneficial effect of the intervention.

* **Confidence Intervals (CI):** 95% CIs were reported for the pooled effect sizes, indicating the precision of the estimate.

* **P-values:** Used to determine statistical significance.

* **Heterogeneity (I²):** This statistic was used to quantify the percentage of variation across studies that is due to true differences in effects rather than random error. An I² value of 0% indicates no heterogeneity, while higher values (e.g., >50%) suggest substantial heterogeneity, meaning the effects might differ considerably across studies.

**Why this design matters:**

**Randomized Controlled Trials (RCTs):** By only including RCTs, the meta-analysis provides the strongest possible evidence for causality. Randomization helps ensure that groups are comparable at baseline, minimizing the risk that observed effects are due to other factors (confounds) rather than the intervention itself.

**Systematic Review:** The systematic and comprehensive search strategy, combined with predefined eligibility criteria, reduces bias in identifying and selecting studies.

**Meta-analysis:** By statistically pooling results from multiple studies, a meta-analysis increases the statistical power to detect effects that might be missed in individual studies. It also provides a more precise estimate of the overall effect and can explore reasons for differences across studies (e.g., through subgroup analyses).

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

**Can Prove:** This meta-analysis provides strong evidence that *acutely* interrupting sedentary time with physical activity *causes* an improvement in cognitive function.