Edited by: Amedeo D'Angiulli, Carleton University, Canada
Reviewed by: Ryan Donald Burns, The University of Utah, United States; Cesar Cavinato Cal Abad, NAR–Nucleus of High Performance in Sport, Canada
This article was submitted to Children and Health, a section of the journal Frontiers in Public Health
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The present study assessed the short-term effect of 6 min classroom-based micro-sessions of multi-joint functional high-intensity circuit training (FunctionalHIIT) performed by students during regular classes on parameters related to functional strength and cardiorespiratory fitness. In this randomized controlled 4-week study, 17 students (11 male; 6 female; age: 11.6 ± 0.2 years) performed 6 min of FunctionalHIIT (targeting >17 on the Borg scale) 4 days per week during regular school classes and 18 students (11 male; 7 female; age: 11.7 ± 0.3 years) served as control group (CG) without any additional in-class physical activity. The FunctionalHIIT group completed 86% of all planned sessions (mean duration: 6.0 ± 1.5 min) with a mean RPE of 17.3 ± 2.1. Body height, mass and BMI did not differ between the groups at baseline or between pre- and post-testing (
Youth levels of physical inactivity is increasing in Europe (
Since children spend 40% of their waking hours in the school (
Usually HIIT protocols are running- (
The potential benefits of implementing FunctionalHIIT interventions in the academic classroom include (i) bypassing “lack of time” as one reason for not exercising (
We hypothesized that the FunctionalHIIT group would significantly improve cardiorespiratory fitness and functional strength from baseline (T0) to after the four-week intervention (T1) in comparison to the control group.
In this single-center, two-arm randomized, controlled study,
Anthropometric parameters (means ± SD) for the FunctionalHIIT (
Body height [m] | FunctionalHIIT | 1.57 ± 0.07 | 1.58 ± 0.06 | 0.424 | 0.021 | 0.801 | 0.104 | 0.011 | 0.415 |
Control | 1.58 ± 0.07 | 1.58 ± 0.07 | |||||||
Body mass [kg] | FunctionalHIIT | 40.7 ± 7.0 | 41.3 ± 6.7 | 0.725 | 0.015 | 0.327 | 0.676 | 0.004 | 0.182 |
Control | 46.0 ± 10.0 | 46.2 ± 9.9 | |||||||
BMI [kg·m−2] | FunctionalHIIT | 16.4 ± 2.1 | 16.5 ± 2.0 | 0.119 | 0.218 | 0.436 | 0.184 | 0.028 | 0.143 |
Control | 18.2 ± 2.7 | 18.5 ± 4.1 | |||||||
Age [years] | FunctionalHIIT | 11.6 ± 0.2 | 11.6 ± 0.2 | – | – | – | – | – | – |
Control | 11.7 ± 0.3 | 11.7 ± 0.3 |
During the 4 week intervention,
Details of the 4-week FunctionalHIIT training intervention.
1 | 1 | – 2 series of |
2 | – 2 series of |
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3 | – 45-s jumping jacks + 15-s recovery |
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2 | 1 | – 2 series of |
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3 | – 45-s jumping jacks + 15-s recovery |
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4 | 1 | – 45-s jumping jacks + 15-s recovery |
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All procedures were conducted in accordance with the Declaration of Helsinki (
At baseline (T0) and after the four-week intervention (T1), all students underwent assessment of body height and mass. Functional strength and CRF were assessed using the “German Motor Ability Test” (
The study design, including testing prior to (T0) and following the intervention period (T1).
The school was chosen since the school offered a sufficient number of students and one researcher had contact to the school before the start of the study.
In week 1 participants performed three training sessions, during week 2–4 participants performed four FunctionalHIIT sessions per week (
Height was measured with a portable stadiometer (seca 213, seca, Hamburg, Germany). Body mass was assessed with an electronic scale (seca 799, seca, Hamburg, Germany). Height and body mass were measured while participants wore sports clothes and no shoes. The body-mass-index was calculated as body mass/(body height)−2.
The German Motor Ability Test was performed as described in detail elsewhere (
Participants were familiarized with the Borg 6–20 RPE scale before the start of the intervention (
All data of T0 and T1 were confirmed to be normally distributed by the Kolmogorov-Smirnov test and homogeneity of variance (Levene Test) were tested prior to further statistical analysis, so that no transformation was required. A two way repeated-measures ANOVA [time-point (T0, T1) × group (FunctionalHIIT, CG)] was performed for each outcome variable, with an alpha of
All students of the FunctionalHIIT group achieved the required minimum of 80% of scheduled training with 86% sessions completed during the 4-week intervention. No participant dropped out during the 4-weeks intervention. Two participants were excluded from analyses due to absence during the post-testing. The mean duration of FunctionalHIIT sessions was 6.0 ± 1.5 min, the corresponding mean value for RPE was 17.3 ± 2.1.
All parameters obtained at T0 and T1 and statistical analyses are presented in
Parameters of functional strength and endurance (means ± SD) for the FunctionalHIIT and Controle group before (T0) and after (T1) the 4 week intervention.
Push-ups [n] | FunctionalHIIT | 19.4 ± 2.6 | 20.6 ± 3.4 | 1.2 ± 2.8 | 9.9 ± 15.6 | 0.054 | 0.108 | 3.997 | 0.453 | 0.017 | 0.576 |
Control | 19.0 ± 3.8 | 19.6 ± 3.4 | 0.6 ± 2.6 | 5.3 ± 19.4 | |||||||
Sit-ups [n] | FunctionalHIIT | 26.5 ± 2.7 | 27.4 ± 3.7 | 0.9 ± 2.4 | 3.1 ± 8.6 | 0.001 |
0.283 | 13.005 | 0.000 |
0.485 | 31.133 |
Control | 28.4 ± 4.7 | 24.4 ± 4.3 | −4.0 ± 2.6 | −14.0 ± 8.3 | |||||||
Standing long jump [cm] | FunctionalHIIT | 160 ± 1.4 | 165 ± 1.3 | 5.0 ± 1 | 3.4 ± 4.4 | 0.000 |
0.318 | 15.4 | 0.485 | 0.015 | 0.499 |
Control | 157 ± 1.8 | 160 ± 1.8 | 4.0 ± 1 | 2.3 ± 4.2 | |||||||
Lateral jumps [n] | FunctionalHIIT | 41.5 ± 4.3 | 43.5 ± 6.0 | 1.9 ± 3.6 | 4.6 ± 8.6 | 0.141 | 0.065 | 2.277 | 0.000 |
0.382 | 20.400 |
Control | 42.6 ± 5.8 | 38.6 ± 3.1 | −4.0 ± 4.1 | −8.3 ± 10.0 | |||||||
20-m sprint [s] | FunctionalHIIT | 3.93 ± 0.28 | 3.31 ± 0.25 | −0.63 ± 0.23 | −15.8 ± 5.4 | 0.000 |
0.754 | 101,383 | 0.000 |
0.691 | 73.694 |
Control | 4.01 ± 0.25 | 3.96 ± 0.18 | −0.05 ± 0.17 | −1.1 ± 4.3 | |||||||
6 min run [m] | FunctionalHIIT | 1124 ± 123 | 1143 ± 90 | 19 ± 51 | 2.1 ± 4.7 | 0.255 | 0.039 | 1.341 | 0.615 | 0.008 | 0.258 |
Control | 1055 ± 139 | 1063 ± 106 | 8 ± 84 | 1.5 ± 9.1 | |||||||
Flexibilty [cm] |
FunctionalHIIT | −4.6 ± 8.8 | −4.3 ± 8.2 | 0.3 ± 3.1 | −3.3 ± 91.5 | 0.167 | 0.057 | 1.994 | 0.055 | 0.107 | 3.974 |
Control | −3.5 ± 8.4 | −5.1 ± 8.0 | −1.6 ± 2.9 | −33.2 ± 93.0 | |||||||
Balance [steps] | FunctionalHIIT | 2.5 ± 0.6 | 2.4 ± 0.7 | −0.1 ± 0.7 | 4.9 ± 55.5 | 0.715 | 0.004 | 0.135 | 0.992 | 0.000 | 0.000 |
Control | 2.2 ± 0.8 | 2.2 ± 1.0 | −0.1 ± 1.1 | 7.8 ± 69.0 |
The performances in lateral jumping [effect of time × group:
The performance in 6 min run [effect of time × group: F(1, 33) = 0.258;
The main findings of the present study concerning the responses of variables related to functional strength and CRF of untrained 11-year-old students to either 4 weeks of classroom-based FunctionalHIIT or passive control condition with no exercise were as follows:
(i) performance in lateral jumping, sit-ups and 20-m sprint were greater after 4 weeks FunctionalHIIT compared to the control group;
(ii) in both groups no changes occurred after 4 weeks in 6 min run, flexibility, push-ups, balance as well as standing long jump;
(iii) all anthropometric parameters remained unchanged between groups and over time.
In contrast to CRF, the parameters related to functional strength (lateral jumping, sit-ups, 20-m sprint) exhibited significant interaction effects in favor of FunctionalHIIT group, demonstrating the effectiveness of FunctionalHIIT for the improvement of functional strength. Since muscular fitness is inversely associated with cardiovascular disease risk factors and improvements in muscular fitness and sprinting speed seem to demonstrate a positive effect on skeletal health in children and adolescents (
The present results are consistent with other findings demonstrating the positive impact of power and strength training on functional strength as well as on jumping and sprinting performance in children and adolescents in different settings including physical education (
Surprisingly, the control group also demonstrated improvements from T0 to T1 for some test items (e.g., push-ups: 5.3%; standing long jump: 2.3%; flexibility: 33.2%; balance: 7.8%). Since the control group performed no additional physical exercise during the classes, it is possible that improvements occured due to adaptations to regular physical education classes, extracurricular sports engagement or due to developmental effects. However, since all anthropometric parameters remained unchanged between groups and over time it is unlikely that developmental effects caused the improvements. Since the German Motor Ability Test demonstrates a high mean correlation coefficient for all eight test items of
From a practical perspective and in contrast to traditional apparatus-based strength training the present FunctionalHIIT did not require any barbells, devices or machinery. All exercises were performed with the children's own bodyweight and rather high velocities and repetitions. The changes in functional strength after 4 weeks represent a good cost-benefit ratio since no expensive equipment was necessary—a prerequisite for school-based sports activities. Since the space requirement for FunctionalHIIT is very low (all exercises comprised mainly vertical movements in an area of about 1 m2 per student) the chosen FunctionalHIIT in connection with the low space requirements favor this type of exercise as classroom based PA.
Despite the current HIIT-related research in children and adolescents (
In contrast to other studies dealing with HIIT and young and adolescent students in a school-based environment (
In the light of the present results, we cannot recommend 6 min FunctionalHIIT in order to improve CRF in students. Running-based protocols with high or maximum intensities similar to recent studies (
The FunctionalHIIT sessions of the present study took place during regular classes and therefore replaced 6 min of sedentary behavior with 6 min of intensive physical exercise. Consequently, daily FunctionalHIIT sessions during regular classes may have the potential to reduce the amount of sedentary time and may contribute to increase the general level of physical activity in children.
The adherence to the current intervention of FunctionalHIIT was quite high (>86% of all planned sessions), which is not surprising since the exercise program took place during regular classes. Nevertheless, the classroom teacher had no concerns that FunctionalHIIT session would be disruptive in any form for achieving the curricular aims of the class and therefore approved the intervention beforehand. This high degree of adherence in students and teachers may reflect the fact that the intervention was relatively short and detailed, as well as the fact that the individual sessions did not take much time. In this regards the present study is in line with a very similar study of Ekström et al. (
Considering the intensity of FunctionalHIIT, students were instructed to perform the FunctionalHIIT between 17 and 20 on the Borg scale, whereas 20 represents “all-out” intensity. Since the mean intensity of FunctionalHIIT sessions was 17 ± 2, the RPE scale did reflected the lower range and not the higher range of the intended high-intensity effort.
A greater number of participants would have given more statistical power to the data interpretation with less risk of calculating type 2 errors. At the same time, the degree to which these results can be generalized is limited due to the small number of participants. The study shows an imbalanced ratio of male and female participants, which represents a limitation since effects of PA interventions may depend on sex in children (
The inclusion of a physical activity enjoyment scale could have given information on the enjoyment of FunctionalHIIT and may had identified if enjoyment of FunctionalHIIT had mediated the relationship between the exercise program and the outcome.
In order to describe the study participants precisely, some studies applying HIIT with children and/or adolescents (
Initially we aimed to record heart rate during the FunctionalHIIT sessions and we aimed to record FunctionalHIIT sessions as well as leisure time with accelerometers. However, we refrained of employing this methodology because of the frequent (un)dressing with heart rate straps and the accelerometers.
Four weeks with 3–4 session per week of 6 min FunctionalHIIT performed during regular classes do not enhance CRF (assessed by 6 min test) but improve certain dimensions of functional strength (lateral jumping, sit-ups, 20-m sprint) in 11-year-old students. The FunctionalHIIT intervention improves 20-m sprint, lateral jumps and sit-ups to a higher extent compared to a passive in-class control condition. Consequently, the present intervention of FunctionalHIIT represents a feasible, sufficient, and effective exercise intervention, which enhances health relevant muscular fitness among young students and could be added as a complement to regular physical education and curricular learning activities during regular classes. As time is limited in the educational system of schools, FunctionalHIIT during regular school classes may offer a novel and feasible intervention for increasing functional strength in young students. Since FunctionalHIIT did not improve CRF in children, we recommend performing this type of exercise with greater intensity, longer session duration as well as interventional period.
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
All procedures were conducted in accordance with the Declaration of Helsinki (
All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The authors thank the students for their enthusiastic participation as well as the teachers and the undergraduate students for their assistance and support.