In recent decades, strength training has gained worldwide prominence, being recommended for both, health and aesthetic programs, and physical preparation of highly trained athletes (Freitas et al., 2018; Loturco et al., 2018; Vinstrup et al., 2017). This fact is related to the various possibilities of applying strength exercises, and their variations. For this, it is essential to understand the patterns of recruitment, neuromuscular activation and multifunctional transfer level of some types of strengthening exercise.
The use of strength exercises varies according to their level of neuromuscular excitation, whose standards can be measured using the electromyography method [EMG] (Contreras et al., 2015, Andersen et al., 2018; Vigotsky et al., 2018). According to Vigotsky et al. (2018), EMG can be defined as an electrophysiological recording technology used for detection of the electric potential resulting from the transmembrane current of the muscle fibers (muscular excitation). Thus, EMG studies enable us to infer which muscles are excitaded in certain movements, being able to compare exercises with different mechanical patterns. Contreras et al. (2015) demonstrated different levels of muscular excitation when comparing barbell hip thrust and traditional squat exercises. In this way, muscles such as the gluteus maximus and quadriceps femoris, being activated differently, have different practical applications regarding their structure (Contreras et al., 2017; Williams et al., 2018). Thus, the interpretation of these data leads us to the possibility of new forms of training prescription, and therefore, more practical results. Nevertheless, kinesiology studies help us to understand, and better interpret, the strategies of exercise choices for specific sports modalities (Hales et al., 2009). Of course, the physical preparation of athletes should use the best tools to improve motor performance, thus making the choice of exercises fundamental to the success of the training program, whether health, aesthetic or athletic. Moreover, it is also suggested that the specificity in the direction of the production of force in certain exercises, causes a more direct transference for certain motor activities. (Loturco et al., 2018; Williams et al., 2018).
It is possible to affirm that despite the mechanical differences between barbell hip thrust (BHT) and more traditional exercises, such as squatting, the inclusion of these in a strength training program can produce summative effects on the performance of international level speed athletes (Loturco et al., 2018). According to Loturco et al. (2018) and Williams et al. (2018), BHT is an exercise whose predominance of hip extension and excitation of its specific muscles is transferred to explosive and short duration exercises as sprints of high speed and short duration, clearly demonstrating the applied principles of specificity and transference.
Recently, BHT has gained considerable attention from the scientific community, and from physical trainers, due to its mechanical nature and the highly neuromuscular demand of the hip extensor muscles (Contreras et al., 2011; Dello Iacono et al., 2018; Dello Iacono and Seitz, 2018; Eckert and Snarr, 2014; Loturco et al., 2018; Williams et al., 2018). This strengthening exercise has muscle activation different from those associated with more traditional exercises such as squatting (front or back barbell), split squats, deadlifts and others (Andersen et al., 2018; Bishop et al., 2017; Williams et al., 2018). As a result, a growing number of researches show that there is a possibility of cute transferring gains from this exercise to horizontal displacement motor activities (Dello Iacono et al., 2018; Dello Iacono and Seitz, 2018; Loturco et al., 2018; Williams et al., 2018). However, it is not clear if BHT training can induce positive chronic effects as seen in the acute type investigations.
The aim of this systematic review was to analyze the activation of the muscles recruited in the BHT and its transfer to sports activities that include horizontal displacement at maximum velocities.
The preferred item declaration guide for systematic review and meta-analysis reports (PRISMA) was used to conduct this systematic review (Moher et al., 2009). In addition, the quality of all eligible studies included was assessed by WKN, TLV, and EFG using the PEDro quality scale. The PEDro scale consists of eleven questions and scores proportional to the number of questions. However, due to the inability to "blind" coaches and practitioners, we excluded 3 questions, determining 8 points as the maximum score. Thus, studies with a score equal to or greater than 5 points were considered of good methodological quality.
A search of the current literature was conducted using the PubMed/Medline, SportDiscuss, Scopus and Google Scholar electronic databases, without restriction of languages and dates until August 6th, 2018. The MeSH descriptors, along with their related terms and keywords included, were used as follows: ((hip thrust OR hip thrusts OR pelvic exercise) AND (resistance training OR resistance exercise OR training, resistance OR strength training OR training, strength OR weight-lifting strengthening program OR strengthening program, weight-lifting OR strengthening programs, weight-lifting OR weight lifting strengthening program OR weight-lifting strengthening programs OR weight-lifting exercise program OR exercise program, weight-lifting OR exercise programs, weight-lifting OR weight lifting exercise program OR weight-lifting exercise programs OR weight-bearing strengthening program OR strengthening program, weight-bearing OR strengthening programs, weight-bearing OR weight bearing strengthening program OR weight-bearing strengthening programs OR weight-bearing exercise program OR exercise program, weight-bearing OR exercise programs, weight-bearing OR weight bearing exercise program OR weight-bearing exercise programs)) AND ((muscle development OR development, muscle OR muscular development OR development, muscular OR myogenesis OR myofibrillogenesis OR muscle hypertrophy OR hypertrophy OR hypertrophies OR electromyography OR electromyographies OR surface electromyography OR electromyographies, surface OR electromyography, surface OR surface electromyographies OR electromyogram OR electromyograms OR muscle strength OR power output OR force OR strength OR horizontal forces OR maximum speed OR fast movement)).
The inclusion criteria were: (a) descriptive studies, (b) studies using physically trained participants, (c) studies that analyzed muscle activation using normalized EMG signals or as a percentage of maximal voluntary isometric contraction (MVIC) during BHT exercise and their respective comparisons and (d) studies that analyzed the acute or chronic transfer of the effects of BHT exercise to horizontal displacement activity. Studies with insufficient data, review, samples formed by ill individuals, poor data presentation, unclear or vague descriptions of protocols applied and with more than one exercise per group were excluded.
Authors WKN, TLV and EFG independently performed the analysis of the data, with a subsequent meeting to decide on the inclusion in the final text of the eligible articles. First, a pre-reading was performed to become familiar with the terminology used in the studies. Then, each article was re-read and the following information was extracted: (1) intervention exercises, (2) sample size, (3) gender, (4) age, (5) experience time, (6) type of study, (7) outcomes and (8) main findings. From this moment, the studies were separated into two types of analysis: (1) neuromuscular recruitment and (2) analysis of the practical transfer of the intervention. Included in this study were articles that analyzed these outcomes separately. Regarding the EMG signal, all articles analyzed reported the MVIC protocol used. These protocols used isometric contraction against a combined resistance for each muscle examined. Likewise, all included articles used surface EMG and reported muscle activation of each muscle separately.
In relation to performance transfer, articles of an acute (potentialization post-activation) and chronic nature (duration between 6 and 8 weeks) were included. The analyzed tests included horizontal jumps and sprints with varying distances (10-150 m).
Three independent reviewers identified a total of 498 articles in the initial survey. Two hundred and thirty-five articles were duplicates, which left 263 articles included for analysis. After sorting the title/abstract and full text, 252 articles were eliminated because they did not meet the inclusion criteria, leaving a total of 11 articles selected for review (Figure 1). However, during the review process of this manuscript, a new article was published and included in the final analysis of the study on November 22nd, totaling 12 articles in this systematic review. Of which, five of these studies compared neuromuscular activity of the BHT with other exercises or variations (Andersen et al., 2018; Collazo Garcia et al., 2018; Contreras et al., 2015; 2017; Williams et al., 2018) and eight studies verified the functional transfer for practical activities (Bishop et al., 2017; Contreras et al., 2017; Della Iacono et...