Rev Bras Fisiol Exerc 2020;19(6):519-31

doi: 10.33233/rbfex.v19i6.4376

REVIEW

Effects of high-intensity interval training on health-related physical fitness in children and adolescents: a systematic review

Efeitos do treinamento intervalado de alta intensidade na aptidão física relacionada à saúde em crianças e adolescentes: uma revisão sistemática

 

Rafael Luiz Mesquita Souza1,2, Felipe José Aidar1,2,3, Sílvia Schütz1,2, Jymmys Lopes dos Santos2,3, Nara Michelle Moura Soares4, Silvan Silva de Araújo2,5, Anderson Carlos Marçal2,6

 

1Department of Physical Education, Federal University of Sergipe, São Cristóvão, SE, Brazil

2Program of Physical Education, Federal University of Sergipe, São Cristóvão, SE, Brazil

3Group of Studies and Research of Performance, Sport, Health and Paralympic Sports - GEPEPS, Federal University of Sergipe, São Cristóvão, SE, Brazil

4Department of Physical Education, Tiradentes University, Aracaju, SE, Brazil

5Secretary of State for Education, Sport, and Culture (SEED/SE), Aracaju, SE, Brazil

6Department of Morphology, Federal University of Sergipe, São Cristóvão, SE, Brazil

 

Received on: September 18, 2020; Accepted on: November 5, 2020.

Correspondence: Rafael Luiz Mesquita Souza, Rua Quintino de Lacerda, 648, 49500-004 Itabaiana SE, Brazil

 

Rafael Luiz Mesquita Souza: rlms2010@hotmail.com

Felipe José Aidar: fjaidar@gmail.com

Sílvia Schütz: silvia_schutz@hotmail.com

Jymmys Lopes dos Santos, e-mail: jymmyslopes@yahoo.com.br

Nara Michelle Moura Soares: narasoares963@hotmail.com

Silvan Silva de Araújo: silvan.ssa@gmail.com

Anderson Carlos Marçal: acmarcal@yahoo.com

 

Abstract

Introduction: High-intensity interval training (HIIT) can improve health and physical conditioning, and it also contributes to the increase of cardiorespiratory capacity (VO2max), localized muscular resistance and improvement of body composition. However, its effects on health-related physical fitness in children and teenagers are not yet clear. Objective: The objective of this paper was to analyze the effectiveness of HITT on health-related physical fitness with children and teenagers. Methods: This study analyzed papers from the following databases: SportDiscus, Web of Science, Medline by way of PubMed, Scopus, Scielo e Bireme. The PICO strategy was applied in order to select the papers, and CONSORT was applied to assess the quality of randomized clinical trials. 511 papers were found. Out of this number, 101 papers were eligible for abstract analysis. Results: At the end of the selection process, ten papers met the inclusion criteria. The results suggest that among the variables analyzed by the papers that addressed health-related physical fitness, HIIT demonstrated efficacy in VO2max, body mass index and fat percentage in this population. Conclusion: However, it was not possible to state that HIIT promoted adjustments in flexibility, strength and localized muscular resistance, requiring further studies to assess its effects on health-related physical fitness.

Keywords: exercise; adolescent; primary health care

 

Resumo

Introdução: O treinamento intervalado de alta intensidade (HIIT) é capaz de promover melhora da saúde e condicionamento físico e contribuir para o aumento da capacidade cardiorrespiratória (VO2máx), da resistência muscular localizada e melhora da composição corporal. No entanto, seus efeitos sobre a aptidão física relacionada a saúde de crianças e adolescentes ainda não são bem esclarecidos. Objetivo: O objetivo do estudo foi analisar a eficácia do HIIT na aptidão física relacionada a saúde de crianças e adolescentes. Métodos: O presente estudo analisou artigos das bases de dados (SportDiscus, Web of Science, Medline via PubMed, Scopus, Scielo e Bireme). Foi aplicada a estratégia PICO para selecionar os artigos e o CONSORT para avaliar a qualidade dos ensaios clínicos randomizados. Foram encontrados 511 artigos. Desse total, 101 artigos foram elegíveis para a análise do resumo. Resultados: Ao final do processo de seleção resultaram em 10 artigos que compuseram os critérios de inclusão. Os resultados sugerem que entre as variáveis analisadas pelos artigos referentes a aptidão física relacionada a saúde, o HIIT demonstrou eficácia na VO2máx, índice de massa corporal e percentual de gordura nessa população. Conclusão: Contudo, não foi possível afirmar que o HITT promoveu ajustes na flexibilidade, na força e na resistência muscular localizada, sendo necessário maiores estudos para avaliar seus efeitos sobre a aptidão física relacionada a saúde.

Palavras-chave: exercício físico; adolescente; atenção primária à saúde

 

Introduction

 

Children and adolescents are affected by diseases resulting from sedentary behavior. Nearly 23% of the world population under the age of 18 perform less than 150 minutes of weekly physical activities [1-3]. Therefore, the prevalence of hypokinetic diseases, such as hypertension and obesity, tends to increase in this age group, which contributes to higher health risk and a low level of physical fitness [4].

Health-related physical fitness (HRPF) is a predictive tool for children's health [5]. For children and adolescents, the principal indicators of HRPF are body composition, cardiorespiratory conditioning, levels of flexibility, and muscle strength [6-8].

For improvement of physical fitness, physical exercise is recommended by physical educators for children, young people, or adolescents, as well as those belonging to other age groups [9]. In this sense, some studies assess the effects of high-intensity exercise on HRPF. Among the different modalities, high-intensity interval training (HIIT) appears as an alternative that requires a short period for its execution when compared to the traditional model of moderate aerobic practice [10].

HIIT is an endurance method characterized by periods of high intensity, interspersed with periods of low intensity or rest for recovery. Most protocols employ 80-100% of the peak aerobic power (VO2peak) [11,12], considering the maximum volume of oxygen consumed by the individual during the assessment and not necessarily the maximum breathing capacity (VO2) [13].

Some studies suggest that HIIT can promote adjustments of biological markers, such as increases in aerobic capacity, ventricular ejection volume fraction, expression of glucose transporter isoform four (GLUT-4) in muscle cells, and VO2max and adjustments of other components, like the reduction in the percentage of abdominal fat, greater capacity for muscle work and changes in body composition in general [9,14,15].

Despite the existence of studies that address the effects of HIIT in children and adolescents, the evidence is not conclusive as to whether this type of training can promote beneficial adjustments on physical fitness in this population. In this sense, the present study aims to assess the impact of HIIT on the components of HRPF in children and adolescents.

 

Methods

 

Systematic literature search strategy

 

The PICO strategy (Patient, Intervention, Comparison, and Outcomes) was used following the Preferred Reporting Items for Systematic Reviews and Meta-Analyzes (PRISMA) methodology. The search for the articles was carried out by two researchers, using the databases Scielo, Pubmed, Scopus, SPORTDiscus, and Medline. At Scielo and Pubmed, keywords were used for a broader range of studies; at SPORTDiscus, Health Sciences Descriptors (DeCS) and synonyms were used; at Medline, Medical Subject Headings (MeSH) were used for indexed articles. The keywords used were: adolescent, children, teenager, physical fitness, high-intensity interval training, and HIIT crossed with the Boolean connectors "AND, OR" or "AND OR". There was no date limit for the starting period in the selection of the articles, which ended in March 2020. The article was submitted on the York University platform (https://www.crd.york.ac.uk/prospero/), which the number of identification on the platform is CRD42020213497.

 

Inclusion and exclusion criteria

 

The inclusion criteria adopted were studies of the randomized clinical trial type involving children and/or adolescents aged 6-19 years [16], having applied HIIT and analyzing some of the related indicators (flexibility, body composition, strength, and muscular resistance) with HRPF. As an exclusion criterion, the training period of fewer than 4 weeks was analyzed.

The interventions of interest were those that showed results after 4 weeks of training with HIIT. This research verified the differences between the intervention and control groups related to the variables: cardiorespiratory fitness (VO2max and VO2peak) and body fat percentage (%F).

 

Data extraction

 

For data extraction, standardized Excel spreadsheets were used, respecting CONSORT standards as a guide to randomized clinical trials [17]. Information about characteristics, sample, eligibility criteria, intervention methods, and the results found were used by two reviewers. To analyze the level of agreement between the reviewers, the Kappa Test was applied. There was 100% (k = 1.00, P < 0.001, 95% CI) and 81.0% (k = 0.81, P < 0.001 and 95% CI) of agreement for titles and abstracts, respectively. During the search for articles in the databases, the titles, abstracts, complete articles, and their eligibility for this systematic review were analyzed. In case of disagreement, a third reviewer was consulted.

 

Quality assessment of individual studies

 

In this stage, the quality of original works was carefully checked, and that used HIIT physical training. Articles with a percentage of ≥ 80% were selected, following the CONSORT guidelines. Thus, the methodological quality, risk of bias, as well as the inclusion and exclusion criteria, statistical data, and results were analyzed, with 10 articles selected in the last step (Figure 1).

The risk of bias assessment was performed using the Risk of bias tool, following the characteristics: generation of the random sequence; allocation concealment; blinding of participants and professionals; blinding of evaluators and outcome; incomplete outcomes; report of the selective result and other sources of bias (Figure 2).

 

 

HRPF = Health-Related Physical Fitness; CONSORT = Consolidated Standards of Reporting Trials; Source: Elaborated by the authors

Figure 1 - Flowchart of study selection using the inclusion criteria

 

 

Figure 2 - Risk of bias assessment

 

 

Results

 

Inclusion of studies

 

511 articles were found, of which 410 were rejected according to the evaluation of the title, leaving 101 of them eligible for analysis of the abstract. Of these, 38 studies were qualified for a full reading. At the end of the research selection process, 10 articles remained that met the inclusion criteria (Table I).

 

 

Table I - Characteristics of the selected studies. (see PDF annexed)

 

Design and sample

 

The eligible studies presented experimental design and randomization of individuals for their allocation, had intervention and control groups. The works contained 19-197 participants of both sexes, whose physical condition varied between obese, athletes, and schoolchildren.

 

Interventions

 

Some studies showed variation in the duration of interventions. However, 5 studies maintained the same constancy and used 12 weeks [20,22,23,25,26]. The other authors used HIIT for 4 [18,27], 8 [19], 6 [21] and 28 weeks [24].

The works also demonstrated different forms of intervention: long HIIT and Repeated-sprint training (RST) with a ratio between stimulus and rest over 1 minute in duration and intensity of 90% VO2peak, and 30 seconds with an intensity of 170% of peak power [23]; only long HIIT between 90% and 100% of maximum heart rate (HRmax), in the form of running on the treadmill [27].

There were comparisons between HIIT and diet: long HIIT in the running mode on the treadmill between 80% and 90% of HRmax in the intervention group and a reduction of 300 to 400 calories in the control group [26]. Two studies [24,26] applied long HIIT in the intervention group and activities in physical education classes in the control group, with intensities of 80-95% of HRmax and 100% of maximum aerobic speed, in free-running on the soccer field, respectively. HIIT was also used in the form of exercises such as squats, short and long jumps and sprints with maximum efforts in the main group, sports activities in physical education classes, and aerobic exercise and sports activities in the control group [20].

Long HIIT at 140% of the speed of the individual anaerobic threshold was used in Small-Sided Games, with characteristics of interval training with high intensity, in intensity equal to the previous one [26]. Short HIIT at 74% of HRmax was used in resistance training with squats with bodyweight, jumps, push-ups, and skipping, in addition to activities in the physical education class for the control group [19].

Long HIIT between 85-95% of HRmax was also used, with active rest between 50-70% of HRmax in the intervention group, and the control group applied cardiorespiratory training of moderate-intensity, maintaining the same volume of 40 minutes for both and 60-70 % of HRmax of intensity in the running mode on the treadmill [22]. Finally, in another study that used treadmill running on a long HIIT in the main group of 80-95% HRmax, together with motivational messages [21]. In the second group, the same HIIT model was applied, but without motivational messages. Although the training is high intensity, no study has reported severe controversial effects during and after the intervention season.

 

Results and intervention measures (primary)

 

From the analysis of the selected articles, variables directly related to HRPF were highlighted, according to the guidelines recommended by FitnessGram, American Alliance for Health, Physical Education, Recreation and Dance, and Projeto Esporte Brasil [28-30]. The cardiorespiratory capacity (VO2max and VO2peak), based on Léger's tests [31], better known as the 20m Shuttle Run, Yo-Yo Intermittent Endurance test or Vaivém [12,19,20,25] and those recommended by German National Teams [18].

The other estimates were performed using ergospirometry using a treadmill ramp protocol [22,27]. Two studies used the lower limb cycle ergometer [23,26]. However, there was an article that applied the 6MWT (6-minute walk test) walk test protocol to assess cardiorespiratory fitness [24].

BMI and %F variables were also analyzed. The methods used were the classification by Cole et al. [32] for BMI and tetrapolar bioimpedance (BIA) [25]. Two other studies used the formula Kg/Height² to measure %F and BMI, respectively [23,26]. Also, the other studies evaluated these same parameters by the differentiated equations regarding sex [21,24].

 

Results and intervention measures (secondary)

 

The selected studies also evaluated waist circumference (WC), waist-to-hip ratio (WHR), and WHtR/WHR ratio [21,24]. WHtR and WHR are related to obesity and overweight, as well as low rates of physical fitness [32]. The WC measurement was measured using the umbilicus line as a reference [21,23], and also as the smallest abdominal circumference, the region located between the 10th rib and the border of the iliac crest considered as the midpoint [20,21,24].

Maximum heart rate was assessed to determine cardiorespiratory fitness [26,27] during the 6-minute run test [21]; from the HR predicted from age [23]; the highest mean HR after performing the exercises in both groups [19] in the 20m Shuttle Run test [20], using the "220-age” equation [26] and the Yo-Yo Intermittent Endurance test [25].

 

Quality of studies

 

According to CONSORT, out of a total of 30 studies, 20 had a classification of 50-79.9% and 10 ≥ 80%. Of the total of 30 articles, 25%, 60%, and 70% indicated the study model in the title, how the sample was calculated; and 70% described the method used to generate randomization, respectively. Also, 85%, 80%, and 45% reported the adverse effects of the interventions, the limitations of the studies, and informed the source of study funding, respectively.

 

Discussion

 

The present review found that HIIT, despite the model applied, promotes beneficial adjustments in the juvenile population. The main ones were: increased cardiorespiratory capacity [20,22,23,35]; decreased BMI [19,21,24-26]; reduction in %F [21,24-26], improvement in maximum heart rate [23,25]; widening of the anaerobic threshold [18]; attenuation of the waist-hip ratio and waist circumference [21,24].

Of the studies evaluated, 4 showed differences between the pre- and post-training in cardiorespiratory fitness [20,22,23,27]. Regarding such parameters, other authors corroborating these findings showed that HIIT improved both anaerobic and aerobic parameters, predictive measures of good cardiovascular health in healthy and obese students [18,36,37].

For HRPF, cardiorespiratory fitness is an important predictor and is inversely related to the development of comorbidities such as obesity, hypertension, and diabetes. Thus, cardiorespiratory conditioning is the best indicator of physical fitness related to longevity [38,39]. Besides, an inverse relationship between BMI and VO2max was demonstrated. These results suggest that physical inactivity contributes to a greater propensity to the development of body adiposity and obesity in this sample population [40]. However, the mechanism involved is still not completely elucidated and requires further research to demonstrate the relationship between VO2max and the genesis of body adiposity in this population.

Regarding HRmax, although not directly involved in HRPF assessment, it is related to VO2max [25,35]. Short HIIT improved both HRmax and VO2max [25]. However, there is still no consensus on the effects of HIIT on body mass and composition. The BMI and/or %F were reduced according to some authors [19,20,24-26], as well as reported an improvement in the level of physical conditioning and less probability of developing diseases related to HRPF. However, in other studies, HIIT has not been able to promote adjustments in these variables [22,23]. Also, other authors suggest that the higher the BMI values, the lower the level of cardiorespiratory and muscle fitness [41].

Therefore, these results suggest a need for further investigation of the influence of these variables on physical fitness. It is relevant to note that 60% of the studies evaluated were composed of overweight or obese children and/or adolescents, indicating that other pathologies, as well as an increase in cardiovascular risk, are associated with a decrease in the level of physical activity and/or an increase in behavior sedentary; conditions antagonistic to HRPF [42,43].

HIIT was effective in improving VO2max and/or BMI parameters in eutrophic children and adolescents [19]. In another study, in obese/overweight individuals, HIIT was more effective in decreasing BMI and %F [44]. We confirm the hypothesis demonstrated by these same authors, who suggest that this effect is due to the higher release of catecholamines, which could induce an exacerbated lipolytic effect in this population after the practice of HIIT by mechanisms still unknown.

Sex is a factor that can influence the parameters evaluated. Some authors suggest that the physiological responses, fat percentage, and muscle mass levels are different [45] in the maturation stages of girls and boys. A male group obtained better adjustments in VO2max, flexibility, muscle strength, and endurance, BMI, and %F than the female group [46]. In this same article, the authors suggest that due to the lower level of physical activity, girls show a more pronounced response when submitted to some form of exercise, which may have contributed to the improvement of BMI and VO2max parameters.

However, some evidence is contradictory. In another study that assessed the HRPF level of adolescents, boys had better HRPF scores, except for flexibility [47]. In this study, it was also proposed that due to the high variability in the age group of children and adolescents, the different maturation levels may have influenced the interpretation of the results.

In this sense, although both sexes show changes in BMI and %F after HIIT, girls obtained better results in these variables [24]. The mixed control group, which performed moderate activities in physical education classes, showed an increase in BMI. However, moderate physical activity was not able to change the BMI and %F in children and adolescents, respectively [19,25].

It is important to note that the influence of training time and duration of interventions with HIIT on the variables BMI, %F, and VO2max still need further investigation. The training period capable of inducing adjustments in the body was 4 to 28 weeks, as well as the session time between 4 to 60 minutes in duration. Even with these variations, it is noted that there was an improvement in BMI, %F or VO2max, for HRPF. However, the study that used the supra HIIT (RST) lasting 4 minutes per session, only promoted an improvement in VO2max.

It is noteworthy that most protocols that maintained intensities between 85 to 100% of heart rate or MAS (Maximal Aerobic Speed), with an average duration of 40 minutes, had positive effects both on body composition and on cardiorespiratory capacity. For the latter, significant adjustments occurred after 4 weeks of training [21-26].

It should be noted that among the different types of existing HIIT, the most used were: long HIIT, greater than 1 minute of stimulus/rest; Short HIIT, up to 1 minute of stimulus/rest; Sprint Interval Training (SIT), up to 30sec of stimulus/4min of rest; and RST, 10sec of stimulus/20sec of rest. Among them, the one that most provided positive changes for HRPF was the long HIIT that promoted beneficial adjustments in cardiorespiratory capacity, BMI, and %F [20-24,26,38-40]. Short HIIT and SIT, in turn, were also able to promote adjustments in BMI, while RST modulated only VO2max [48]. Regarding the comparisons made in the study mentioned above, it is relevant to note that the type of exercise used in the form of HIIT (running, cycling or rowing) has been shown to influence the magnitude of the results, thus limiting generalizations and more accurate comparisons.

 

Conclusion

 

HIIT was effective in improving BMI, %F, and VO2max, indicating that this training methodology promotes positive adjustments in these HRPF parameters. However, it was not possible to state that HIIT was effective in improving local muscle strength and endurance, as well as flexibility. Also, long HIIT was more effective in promoting beneficial adjustments in some components of HRPF (body mass index, cardiorespiratory capacity, and fat percentage) and secondary variables (heart rate, hip circumference, and waist-to-hip ratio). Therefore, it is recommended a minimum time between 4 and 8 weeks with minimum intensities of 85% of HRmax and an average time of 40 minutes of session duration to be able to modulate BMI, %F, and/or VO2max. Besides, it is crucial to standardize and/or normalize the groups according to the age groups with the smallest interval, according to the maturation level of each participant.

 

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