Participant characteristics

Seventy-nine children from years four, five and six (aged 10.30.8years) were recruited from two local primary schools (who had not previously implemented The Daily Mile) to participate in this quasi-experimental study, whereby one class from each school would complete either the exercise intervention (The Daily Mile; n=44) or act as the control group, who continued their habitual activities (n=35). A power calculation indicated that for the present study design and an effect size of 0.2 (reflective of the literature e.g. Ref.25), with two groups and two measurement points (=0.05), that a sample size of 68 was required. During familiarisation, participant body mass (Seca 770 digital scale, Hamburg, Germany), stature and sitting stature (Leicester Height Measure, Seca, Hamburg, Germany) were measured and subsequently used to calculate body mass index (BMI).

Ethical approval was received from the Nottingham Trent University Ethical Advisory Committee (application 692), with all methods undertaken thereafter performed in accordance with the relevant guidelines and regulations. Headteacher approval for the study to commence and written informed parental/guardian consent were obtained. Parents/guardians also completed a health screen questionnaire on behalf of their child/dependent, which was checked by a lead investigator to ensure there were no medical conditions (such as a congenital heart condition, or a blood-line relative that had died during or soon after exercise) that would prevent the participant from completing the study. Participant assent (willingness to participate) was also obtained.

Participants completed three data collection visits; including a familiarisation session, a baseline trial, and a follow-up trial (see Fig.1 for protocol schematic). In between baseline and follow-up trials, the intervention group completed 5weeks of daily participation in The Daily Mile, whilst the control group continued with their normal daily school routine. Follow-up data collection was completed within 48h of the final training session completed by the intervention group. Data collection was completed across two primary schools, the first completing the study between MayJuly 2021, and the second between OctoberDecember 2021. Each school had an intervention and a control group for whom baseline and follow-up testing occurred simultaneously, thus controlling for seasonal variations and overcoming limitations of previous work18.

Overview of study design.

During the familiarisation session, participants were introduced to all the procedures involved in the present study and were provided with the opportunity to ask any questions they had in relation to the study protocol. Participants were familiarised with The Daily Mile (to ensure they understood the core components of the physical activity; Ref.9) and the battery of cognitive function tests, which were practised twice to minimise any potential learning effects.

During the 24h prior to baseline and follow-up trials, participants were asked to refrain from strenuous physical activity and to record a food diary so that the same diet could be consumed prior to each assessment (baseline and follow-up). Participants were also asked to arrive to school following an overnight fast from 9pm the previous evening, with only water allowed to be consumed ad libitum during this time. Parents/guardians were contacted the evening prior to baseline and follow-up trials to remind them of the pre-testing requirements. The follow-up trial was scheduled 48h after the final Daily Mile session in the intervention group.

Upon arrival to school, participants were provided with a standardised breakfast (cornflakes, milk, and toast with butter), which provided 1.5g carbohydrate per kg body mass (as previously used, e.g.,26,27). Participants were allowed 15-min to consume breakfast, with water allowed ad libitum throughout. Thereafter, participants completed the battery of cognitive function tests, which commenced 30-min after the start of breakfast (for study timeline, see Fig.2). After the cognitive function test battery was complete, waist circumference, hip circumference and four skinfolds were taken for the assessment of body composition. On a separate day, within 48h, participants completed the multi-stage fitness test (MSFT) as a measure of cardiorespiratory fitness.

Timeline of measurements for baseline and follow-up trials.

The battery of cognitive function tests included the Stroop test (measure of information processing and inhibitory control), Sternberg paradigm (measure of visual working memory) and Flanker task (measure of attention and inhibitory control), which were completed in that order. Full details of each of these cognitive tests and are provided elsewhere16, which have previously been used successfully in a similar study population16 and the reliability reported28. The test battery was completed 30-min after the start of breakfast (Fig.2) and lasted~15-min. All tests were administered via a laptop computer (Lenovo ThinkPad T450, Lenovo, Hong Kong), with participants wearing noise cancelling headphones. During the cognitive function tests, participants also sat separately from one another and in a dimmed room, to minimise distractions. Verbal instructions of each test were provided by a lead investigator, which were followed by written instructions on screen for participants to read. Participants were reminded at the start of each test to answer correctly and as quickly as possible. Questions were encouraged and following confirmation all participants understood the test requirement, the battery of tests were undertaken.

Each test within the battery commenced with 36 practice stimuli, with feedback provided relating to whether the correct answer had been chosen. The practice stimuli allowed re-familiarisation with the test to further remove any potential learning effects. Outcome variables for each of the tests was the percentage of correct responses (%) and the response time (ms) to achieve each correct response. Each of the cognitive function tests included in the battery have previously been used successfully in a similar study population16.

For the assessment of body composition, participants had waist circumference, hip circumference, and four skinfold sites measured. Waist circumference was measured with a tape measure at the narrowest point between the xiphoid process and the iliac crest, whilst hip circumference was measured at the greatest diameter of the hip (both accurate to 0.1cm). The four skinfold sites included the tricep, subscapular, supraspinale and front thigh and were taken in accordance with The International Society for the Advancement of Kinanthropometry guidance by trained kinanthropometrists. Waist circumference, hip circumference and each skinfold were measured twice and the mean value used as the criterion measure. In the event that the initial two measurements were discrepant by>5%, a third measure was taken and the median used as the criterion value. In addition, from body mass and stature BMI (body mass/height2) and BMI percentiles (based on age and sex-specific British 1990 growth reference data29 were calculated.

Distance ran on the multi-stage fitness test (MSFT;30) was the chosen measure of cardiorespiratory fitness, as previously used in a similar study population16. In brief, participants completed 20-m progressive shuttle runs to the point of volitional exhaustion. The test commenced at a speed of 8.0kmh1 and then increased by 0.5kmh1 for each subsequent 1-min stage completed. During the test, participants were paced by an experienced member of the research team and verbal encouragement was provided to ensure participants reached the point of volitional exhaustion.

The Daily Mile requires participants to complete 15-min of informal, self-paced, outdoor physical activity around a pre-determined route9. During the exercise intervention, participants in the intervention group completed The Daily Mile every day (Monday through Friday) for 5weeks. The classroom teacher was responsible for administering The Daily Mile, in line with recommendations from The Daily Mile (to be completed each day at their chosen time, irrespective of the weather). Teacher implementation logs were recorded daily during the intervention. For any days The Daily Mile was not completed, information relating to the reasons for this were recorded. Fidelity (number of days completed/number of days available for implementation100; %) was recorded for each participant, as well as for the experimental group overall. Whilst the intervention group completed five weeks of The Daily Mile, the control group continued with their normal school routine and habitual physical activity.

All cognitive data were initially attended to in the open source software R (www.r-project.org). Minimum (<100ms) and maximum (10004000ms depending on task complexity) response time cut-offs were applied to eliminate any unreasonably fast (anticipatory) or slow (distracted) responses, and the distribution of remaining cognitive data were assessed. This method has previously been used in similar studies16,26,31,32. Statistical analyses were then performed using Statistical Package for the Social Sciences (SPSS; Version 26; SPSS Inc., Chicago, IL., USA). Analysis of covariance (ANCOVA) was used for all outcome variables, to examine the between group (intervention vs. control) differences at follow-up, while controlling for the baseline score (covariate) of that outcome. This approach is considered most appropriate and is recommended for experimental designs such as the one used in the present study33. For all variables, the mean and standard deviation at baseline and follow-up for each group are presented, as well as the adjusted follow-up means and 95% CI; effect sizes are calculated as Cohens d and interpreted as per convention (0.2=small, 0.5=medium, 0.8=large). Statistical significance was accepted as p<0.05.

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Effect of 5-weeks participation in The Daily Mile on cognitive function, physical fitness, and body composition in children | Scientific Reports -...