Is the combination of interval and resistance training more effective on physical fitness ?

Interval training (HIIT / SIT) combined with resistance training (RT) has been highlighted as a strategy for the improvement of health-related physical fitness markers (HRPF) in adults. Thus, the aim of this meta-analysis was to compare the efficacy of combined training (HIIT / SIT + RT) with other exercise protocols on HRPF markers in adults. A systematic search was performed in MEDLINE via PebMed, CochraneCENTRAL, SPORTDiscus, LILACS, SCIELO and Scopus databases between January and March 2017, using the following keywords in English and Portuguese: physical fitness, high-intensity interval training, sprint interval training, resistance training and adults. The quality of studies was evaluated using the PEDro scale. After applying both inclusion and exclusion criteria, nine articles were selected (n = 231). The extraction of means and standard deviations from studies was performed independently by two authors and the RevMan software was used to perform the meta-analysis. Combined training interventions lasted from 6 to 12 weeks and generated greater increase in maximal oxygen uptake than other forms of exercise. The combination of interval training and strength training may be considered more effective to improve aerobic capacity levels in adults.


INTRODUCTION
Western lifestyle is characterized by changes in dietary intake, increases in dietary energy supply, reductions in levels of physical activity and increases in time spent in sedentary behaviors 1 , and the adult population, due to its social and economic relevance, deserves greater attention and policies aimed at the maintenance and / or recovery of its physical and mental well-being 2 .
The maintenance of minimum performance indexes is necessary to maintain adequate functional (strength / resistance and flexibility), motor (cardiorespiratory fitness) and morphological levels (body composition) of adequate health-related physical fitness (HRPF) 3 .In addition, regular physical activity at all ages is essential to minimize the risk of incubation and early development of chronic-degenerative diseases, thus enabling longevity with higher quality of life, reduction of risk factors, health promotion and performance of daily tasks and leisure activities 4,5 .Thus, different intervention protocols with physical exercises have been used to promote improvements in HRPF indicators, mainly in the reduction of visceral fat and cardiovascular risk factors 6 .
Among the various exercise protocols proposed for obesity management today, interval training has been highlighted as an efficient strategy to achieve innumerable health benefits 7,8 .This method is characterized by intense loads of exercise separated by a period of interval between series 9 , with adaptive benefits superior to other types of training such as continuous aerobic exercise, but with less training volume 10 .In addition, this training protocol has helped to achieve greater adherence to exercise programs, precisely because it presents an efficient dose of time and reduces the health risks associated with obesity 11 .
Among the methodologies observed in the various studies that use the principles of interval training, sprint interval training (SIT) is defined as sprints of short duration (8 to 30 seconds), with intensity greater than 100% of VO 2max and interspersed with recovery periods 8 , while high-intensity interval training (HIIT) is characterized by exercise sessions with intensities between 80% and 100% of maximum heart rate or aerobic capacity, with duration between 60 and 240 seconds, which are within the aerobic capacity of the individual (sub-maximum), but extremely intense 9 .
Another protocol widely used in literature is resistance training, since it is an important component for weight loss by promoting body fat reduction increased lean mass and muscle strength 7,12 .In addition, this protocol produces the EPOC effect (excessive post-exercise oxygen consumption), and through this system, brings long-term benefits to individuals by increasing basal metabolic rate and caloric expenditure and improves lipid oxidation 14 .
Although there is consensus in literature about the benefits of interval and resistance training alone, it is important to analyze the effectiveness of the combination of these two protocols, since the sum of benefits brought by both seems to be an interesting strategy to improve the physical fitness indicators, weight management and health enrichment in adults 11,7 .
Therefore, the aim of this meta-analysis is to compare the effectiveness of combined training (HIIT / SIT + RT) with other exercise protocols on HRPF indicators in adults.

METHODOLOGICAL PROCEDURES
The systematic review was conducted between January and March 2017, following the recommendations of the Preferred Reporting Items for Systematic Review and Meta-analyses: The PRISMA Statement 15 , developed in a methodical, explicit and reproducible manner with a clear question, search strategy and well-defined inclusion and exclusion criteria, is aimed at guiding future research and systematizing the knowledge produced 16 .
The selection of descriptors used in the review process was done through consultation with DECS (BIREME health sciences subject descriptors) and MESH (Medical Subject Headings -controlled vocabulary used for indexing articles for PubMed).The search was performed in six electronic databases: MEDLINE (Medical Literature Analysis and Retrieval System online) via PubMed, Cochrane-CENTRAL, SPORTDiscus, LILACS, SCIELO (Scientific Electronic Library Online) and Scopus, using the following language descriptors in English and Portuguese, respectively: physical fitness (aptidão física), high-intensity interval training (treinamento intervalado de alta intensidade), sprint interval training (treinamento intervalado de Sprint), strength training (treinamento de força), resistance training (treinamento de resitência) and adults (adultos) using Boolean operators AND and OR to combine descriptors.Searches were carried out by two researchers (F.J.M.J) and (I.C.J), independently, who started reading the titles and abstracts, following for analysis of the articles in full.Disagreements among reviewers were resolved by consensus.
Initially, 685 articles were identified, to which the inclusion and exclusion criteria specified in Figure 1 were applied.The following inclusion criteria were adopted: 1) only original articles; it was decided not to include chapters of books, theses, dissertations and monographs, since the performance of a systematic search of these is logistically infeasible; 2) only studies that presented abstract, which were initially read; 3) studies carried out over the last 10 years (2007-2017); 4) studies using only human samples; 5) studies with minimum intervention of six weeks; 6) studies with combined HIIT / SIT and resistance training intervention.After applying the inclusion criteria, 43 articles were selected.The following exclusion criteria were applied: 7) repeated articles; 8) studies with population of athletes or aiming at sports performance; 9) articles without control group; 10) studies that did not evaluate health-related physical fitness variables; 11) articles that include samples of adolescents and / or elderly, resulting in the selection of nine articles.
Then, the selected articles were analyzed for their methodological quality, following the evaluation protocol based on the adapted PEDro 17 scale, composed of nine criteria judged to be important and susceptible of being scored in the selected articles.The quality score was performed by two investigators independently and the doubts were analyzed and decided by a third evaluator for final decision.
Thus, the following aspects were evaluated: 1) Specified eligibility criteria; 2) Randomization or random designation; 3) Secret allocation; 4) Similar groups at the beginning; 5) Blind assessors; 6) Evaluations made in at least 85% of the sample; 7) All evaluated subjects received intervention according to allocation; 8) Results of intergroup comparisons were described; 9) Study presents precision and variability measures for the results.The better the score, the better the quality of the article.Articles that reached less than 6 points were considered of low methodological quality, between 6 and 7, average quality and above 7, high methodological quality.
Data extraction was performed individually by two independent evaluators, in which means and standard deviations (baseline) and post-interventions of health-related physical fitness indicators were obtained: body composition (fat percentage), cardiorespiratory fitness (Relative VO 2max in kg / ml / min) and force (leg press and bench press, in kg).The RevMan version 5.3 software was used to conduct the meta-analysis, for which the standardized mean difference (SMD) and standard error (SE) of variables to be analyzed between groups were calculated (combined HIIT / SIT + RT training versus other training protocols): interval training (HIIT or SIT), resistance training (RT) and continuous training combined with resistance (CT + RT).In each study, the random effects model (DerSimonian-Approach Laird) 18 was calculated and combined, since samples were taken from populations under various pre-intervention conditions.
To analyze the effects of the combined protocol (HIIT / SIT + RT) versus other protocols, significance level of 5% and 95% confidence interval (CI) were considered.The evaluation of the heterogeneity of the total variations in studies was analyzed by means of the Cochran Q statistic, where I 2 value of <25%, 25-50% and> 50% were considered small, medium and large quantities of inconsistency 19 .
Finally, sensitivity analysis 18 was performed, excluding studies that presented high risk of bias, following the procedure in two moments in order to verify changes in the results of the meta-analysis: 1) including only studies that performed the concurrent training in the same session; 2) excluding studies classified with low methodological quality.
In studies that used interval training, five used the combination of the HIIT protocol with strength training 21,[23][24][25]28 , while four adopted the SIT principles for combined training 20,22,26,27 .
Among the interventions used in the comparison with the combined protocol (HIIT / SIT) + RT, three studies adopted HIIT / SIT 22,27,28 seven RT 20,21,23,24,[26][27][28] and two CT + RT 24,25 .The HIIT / SIT + RT protocols used in the studies showed significant variation among themselves, including combined training interventions in the same week, but not in the same session [26][27][28] , and the training frequency ranged from 2 to 5 times per week.In relation to HIIT / SIT programs, exercises with running in the terrestrial environment 23,26,27 , in treadmill 20,24,25,28 , in cycle ergometer 21 or during aerobic rowing exercises 22 are observed.
The majority of interventions lasted 12 weeks, with the exception of studies with six weeks 21,22 and eight weeks 20,23 .The intensities of the interval exercise among studies ranged from 70 to 100% HR max 25,27,28 , 110 to 120% VO 2max , 9-10 points of the subjective effort perception (Borg scale 0-10) 22 and 120 to 150% of the lactate threshold 24 .Regarding muscle strength training, the load ranged from 9-10 points in the scale of subjective effort perception of Borg 22 and from 40 to 87% of 1RM.Details of groups and training protocols are highlighted in Table 1.
Our findings are similar to those that reported increased maximal oxygen uptake in patients with hypertension 29 , overweight and obese adolescents 10 and in patients with chronic heart failure 30 .The present data suggest that combined training (HIIT / SIT + RT) may promote higher VO 2max benefits when compared to other training methods, specifically when compared with the RT method, although HIIT / SIT training as well as combined CT + RT training may also promote benefits in VO 2max 24,28 .Our results corroborate with the findings of a meta-analysis conducted in 2013 comparing interval training with other exercise protocols, including HIIT / SIT + RT on cardiorespiratory fitness in elderly patients with chronic heart failure, in which the authors pointed out a greater effectiveness in results in protocols that combine interval training and resistance training in cardiorespiratory fitness, provided that with similar energy expenditure 30 .
In this sense, combining these two types of training, adaptations are expected in skeletal muscle mass, peripheral nervous tissue and other tissues directly involved during training 31 .However, significant changes related to muscle strength in the bench press and leg press exercises were not observed, suggesting that the gains in these physical fitness variables are similar between training methods.However, it is important to note that few studies have equated volumes and / or weekly frequency between HIIT / SIT + RT intervention and the other comparison groups, which may underestimate the results of the other control groups, especially those who performed HIIT and RT alone.On the other hand, intervention protocols combining interval training and resistance training have some distinct characteristics in the prescription characteristics, which can contribute to the inconsistency in the improvement of these variables in favor of this exercise method.
In addition, combining strength training with interval training has been recommended for the maintenance of physical fitness and functional capacity throughout adulthood 32 , indicating increases in ribosomal activity, mitochondrial function and gene expression of skeletal muscles, culminating in increases in cardiorespiratory fitness, strength, resistance and muscle mass 33 .It is worth emphasizing the importance of maintaining physical fitness throughout adult life as a relevant factor for the reduction of the development of risk factors, which favor the development of chronic cardiometabolic diseases 34 ; however, our results did not indicate a decrease in the fat percentage in adults who performed the HIIT / SIT + RT training.
However, an important bias risk was the fact that only one study controlled the diet variable 24 .Diet is a factor that has great influence in manipulating morphological factors such as body composition.One study showed that increased protein intake associated with caloric reduction may provide greater fat mass decrease and lean mass increase when compared to low protein intake in trained individuals 35 .
The findings in literature are widely controversial regarding the combination of exercises of distinct metabolic pathways in a single session, suggesting that they exert a metabolic dispute called concurrent training 31 .It is assumed that in this method, residues from one exercise can influence the metabolic adaptations of the other 36 .Although data are still unclear, based on the results presented this assumption may not apply, because both concurrent or combined HIIT / SIT + RT training in the same week can bring significant results in physical fitness, as well as HIIT and RT alone.
The implementation of the combined program (HIIT / SIT + RT) can minimize training time, a factor that has been predominant in the choice of interventions 37 .These results may help in the elaboration of new researches and identify the limitations in the studies to conduct new interventions using this type of protocol in populations of varied ages.In addition, the characteristics of this combined training protocol have reinforced the growing interest in this type of exercise in recent years, becoming an interesting strategy in increasing VO 2max and in maintaining other health-related physical fitness indicators.
In this sense, it was observed that the HIIT / SIT + RT protocol can be used in adults with different nutritional status, reinforcing the applicability and versatility of this type of training.In addition, the HIIT / SIT + RT method has been applied in different age groups, for example in elderly patients with heart failure 38 , in adolescents 34 , as well as in athletes to improve physical fitness indicators related to sports performance 39 .
The present study has limitations that deserve to be pointed out.The small number of individuals in each study selected as well as samples in different health conditions are factors that may interfere with the analyses, because although training with athletes has been excluded, the subjects' initial condition may be a relevant bias.Due to the great variety of weekly frequency, load and duration among HIIT / SIT + RT protocols studied, future studies should focus on analyzing which protocols make better control of total volume and caloric expenditure of training sessions, as well as diet control, so that distortions can be minimized and satisfactory results can be better elucidated and disseminated to the general population.

CONCLUSION
In conclusion, the studies found suggest that the use of interval training (HIIT / SIT) combined with strength training in adults may be more efficient for improving cardiorespiratory fitness when compared to other exercise protocols.However, our results are insufficient and new analyses should be conducted for a better understanding of the optimization of effects and greater knowledge of metabolic adaptations of this protocol in adults.

Figure 1 .
Figure 1.Application of the inclusion and exclusion criteria.

Figure 2 .
Figure 2. Meta-analysis of the results of combined protocol (HIIT / SIT + RT) versus other exercise protocols in parameters fat mass (A) and cardiorespiratory fitness (B), respectively.CI -confidence interval, HIIT / SIT + RT -high intensity interval training or sprint interval training combined with resistance training, IV -inverse variance, SD -standard deviation, total -number of participants in each study group.

Figure 3 .
Figure 3. Meta-analysis of the results of combined protocol (HIIT / SIT + RT) versus other exercise protocols in parameter strength, leg press, in kg (A) and bench press, in kg (B), respectively.CI -confidence interval, HIIT / SIT + RT -high intensity interval training or sprint interval training combined with resistance training, IV -inverse variance, SD -standard deviation, total -number of participants in each study group

Table 1 .
Main features of selected studies * -not specified by the authors; AB = two training divisions.624 DISCUSSION The aim of this meta-analysis was to analyze the effectiveness of combined training (HIIT / SIT + RT) compared to other exercise protocols on the HRPF indicators of adults.Our search indicates that interventions using this combined training demonstrate more effective adaptations related to