Grade-Specific Effects of Recess HIIT and MICT on Classroom Attention-Related Behavior in Primary School Children

This study used a randomized, pretest-posttest, three-arm controlled design to investigate the grade-specific effects of high-intensity interval training and moderate-intensity continuous training conducted during school recess on observable classroom attention-related behavior in primary school children.

Participants were third- and fifth-grade students recruited from a primary school in Shanghai, China. After baseline assessment, 144 eligible children were randomly allocated in a 1:1:1 ratio to the high-intensity interval training group, the moderate-intensity continuous training group, or the routine recess control group, with 48 participants in each group. Randomization was performed at the individual level using a computer-generated sequence and was stratified by grade and sex. The allocation sequence was generated by an independent researcher, and assignments were concealed in sequentially numbered, sealed opaque envelopes.

The study lasted 8 weeks. Week 1 included familiarization and baseline assessments. Weeks 2 through 7 comprised the formal 6-week intervention, and Week 8 included post-intervention assessment. The exercise interventions were delivered three times per week during scheduled school recess, for a total of 18 sessions.

Each structured exercise session lasted 15 minutes and consisted of a 3-minute preparatory activity, a 9-minute main exercise component, and a 3-minute cool-down. Exercise intensity was monitored in real time using heart rate monitors.

The high-intensity interval training group performed interval circuit exercises consisting of jumping jacks, cross jumps, vertical jumps, and burpees. Each exercise was performed for 30 seconds followed by 30 seconds of recovery. The target intensity during the main exercise component was at least 85% of age-predicted maximal heart rate.

The moderate-intensity continuous training group performed continuous aerobic running during the 9-minute main exercise component. The target intensity was maintained at 60% to 69% of age-predicted maximal heart rate.

The control group continued routine school recess activities without receiving additional structured exercise. Routine activities included walking, free play, standing rest, and other usual school-arranged recess behaviors.

The primary outcome was the change in observable classroom attention-related behavior from baseline to post-intervention. Classroom behavior was assessed using an automated video-based analysis system that extracted four observable indicators: head posture, eye gaze, eye opening or closing status, and facial detection persistence. These indicators were standardized and integrated into a composite score, with higher scores indicating greater consistency with classroom-oriented behavior.

The composite score was interpreted as an ecological proxy measure of observable classroom attention-related behavior rather than a direct measure of internal cognitive attention. The primary statistical analysis examined the effects of time, intervention group, grade level, and their interactions, with particular emphasis on the Time × Group × Grade interaction.

Because of the nature of the exercise interventions, participants and exercise instructors were not blinded. However, personnel responsible for video data processing and statistical analysis were not involved in intervention delivery and followed standardized assessment procedures.

Inclusion criteria

• Students enrolled in the third or fifth grade at the participating primary school.
• Aged 8 to 12 years.
• Able to participate in routine physical education classes and school recess activities.
• Free from musculoskeletal injury or any medical condition that contraindicated participation in physical exercise.
• Written informed consent provided by a parent or legal guardian.
• Verbal assent provided by the participating child.
• Willing and able to complete the baseline assessment, exercise intervention, and post-intervention assessment.

Exclusion criteria

• Musculoskeletal injury, cardiovascular disease, respiratory disease, neurological disorder, or other medical condition that made participation in moderate- or high-intensity exercise unsafe.
• A diagnosed attention-deficit/hyperactivity disorder, autism spectrum disorder, anxiety disorder, depressive disorder, intellectual disability, or other psychological or developmental condition that could substantially affect participation or assessment of classroom attention-related behavior.
• Color blindness or another visual impairment that could interfere with the study assessments.
• Participation in a structured high-intensity exercise training program during the 3 months before enrollment.
• Regular participation in systematic extracurricular exercise training that could substantially affect the intervention response.
• Inability to complete the intervention or study assessments.
• Absence from three or more intervention sessions.
• Withdrawal of parental or guardian consent or withdrawal of the child's assent.