Hip extension is a fundamental movement in daily life and athletic activities. Previous research has proposed an increasing role of hip extensor musculature with heavier lower body exercises (e.g., squats, lunges, and deadlifts) and explosive sports actions (e.g., jumping, sprinting and change of direction) (Beardsley and Contreras, 2014). The primary muscles responsible for this movement are gluteus maximus (GMax), long head of biceps femoris, semimembranosus, semitendinosus, and the ischiocondylar portion of the adductor magnus (Broski et al., 2015; Neumann, 2010; Youdas et al., 2017). Despite the involvement of all these muscles, GMax has been identified as the primary muscle responsible for hip extension, specifically on loaded exercises that typically do not sufficiently activate the hamstrings in tasks involving simultaneous hip and knee extension, such as the squat and the leg press (Krause Neto et al., 2019, McCurdy et al., 2018; Williams et al., 2018; Sugisaki et al., 2014). There is a significant number of studies comparing GMax activation levels between several loaded and bodyweight exercises (Bishop et al., 2018; Boren et al., 2011; Macadam et al., 2015; Macadam and Feser, 2019; Selkowitz et al., 2016).
Electromyography (EMG) is a technique for measuring the electric potential field generated by the depolarization of the sarcolemma (Merletti and Parker, 2004). Despite limitations and common misinterpretations (Vigostky et al., 2015; 2016), under controlled conditions, the EMG signal comprises the summation of motor unit action potentials and provides an index of muscle activation (Enoka and Duchateau, 2015). Therefore, EMG has been widely used to compare the muscle activation between exercises, which can assist the strength and conditioning coach on selecting and systematically progressing exercise intensity (Vigostky et al., 2015, Macadam and Feser, 2019).
Previous studies have systematically reviewed the gluteal muscle activity, measured by EMG, in a variety of lower body exercises (Macadam et al., 2015; Macadam and Feser, 2019). The systematic review conducted by Macadam et al. (2015) showed that exercises with dynamic hip abduction and external rotation elicited high levels of GMax activation (ranging from 79% to 113% of a maximal voluntary isometric contraction [MVIC]). Recently, Macadam and Feser (2019) have found that it is still possible to achieve high levels of GMax activation (>60% of MVIC) by performing exercises with bodyweight as resistance. However, due to the inclusion/exclusion criteria chosen by the authors to answer their research questions, both studies eventually excluded more ecologically valid studies for strength and conditioning coaches that investigated exercises with higher intensity (external load) and neuromuscular demand. As external load may affect exercise mechanics and the resultant muscular activation (Bryanton et al., 2012; Da Silva et al., 2008; Riemann et al., 2012; Swinton et al., 2011), currently there is ambiguity on which exercises that incorporate hip extension and use of external load achieve the most significant Gmax activation.
Several factors, including relative external load, movement velocity, level of fatigue, the mechanical complexity of the exercise (open or closed kinetic chain, weight bearing or non-weight bearing), and the need for joint stabilization, may directly influence GMax activation. The purpose of this systematic review was to describe the GMax activation levels during dynamic exercises that incorporate hip extension and use of external load. To assist strength and conditioning coaches in selecting exercises for the GMax, we categorized the exercises as low level of activation (0 to 20% of MVIC), moderate level of activation (21 to 40% of MVIC), high level of activation (41 to 60% of MVIC), and very high level of activation (greater than 60% of MVIC) accordingly to the recommendations of Macadam and Feser (2019).
Literature research strategies
The preferred item declaration guide for systematic review and meta-analysis reports (PRISMA) was used to conduct this systematic review (Liberati et al., 2009).
On February 15th, 2019, a systematic review was conducted using the PubMed/Medline, SportDiscuss, Scopus, Google Scholar, and Science Direct electronic databases. The MeSH descriptors, along with the related terms and keywords, were used as follows: ((((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 OR isometric OR exercise OR rehab OR physical therapy OR load OR training))) 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 muscular excitation OR excitation OR EMG OR muscle activation OR activation))) AND ((gluteus maximus OR gluteus OR hip extensor OR hip extensors)).
After reading the titles and abstracts, all eligible full text was assessed for methodological quality using the PEDro methodological quality scale. This scale is composed of eleven questions and scores proportional to the number of items. However, due to the inability to "blind" coaches and practitioners, we excluded three questions, setting the eight as the maximum score. Thus, studies with scores equal to or higher than five were considered of good methodological quality, excluding those with scores equal to or less than 4 (Krause Neto et al., 2019).
Inclusion and exclusion criteria
The inclusion criteria were: (a) original articles; (b) descriptive studies (in case of no raw description of the data, an e-mail was sent to the authors requesting the raw data); (c) studies with physically trained participants; (d) studies that measured surface EMG and reported muscle activation as a percentage of maximal voluntary isometric contraction (MVIC); (e) studies which analyzed the muscle activation of the GMax using strength exercises with external load and (f) English language. Studies with insufficient data, review articles, conference papers, student thesis, samples from metabolic patients, patients with musculoskeletal trauma and older people, poor presentation of data, unclear or vague descriptions of the protocols applied, and articles evaluating isometric, plyometrics, and calisthenics exercises were excluded.
Authors W KN, RA, and TAC in dependently performed the data analysis with two subsequent meetings to decide on the inclusion of eligible articles in the final text. After each article was read, the following information was extracted: (1) exercise performed, (2) EMG normalization procedure, (3) electrode placement, (4) external load used in the exercise, (5) main findings and (6) mean %MVIC values achieved in each exercise. If two or more studies evaluated the same exercises, the data were pooled as an average of the mean % MVIC of each exercise. Only the mean %MVIC data from each study was used here.
To classify the Gmax activation measured, we used the following levels: 0-20% MVIC, low muscle activation; 21-40% MVIC, moderate muscle activation; 41-60% MVIC, high muscle activation; >60% MVIC, very high muscle activation (Escamilla et al., 2010; Youdas et al., 2014, Cacchio et al., 2008). According to Macadam and Feser (p. 17, 2019), "this classification scheme provides a means by which the practitioner can select exercises, that match the capabilities of their client/athlete thus targeting neuromuscular, endurance, or strength type training, and provides a means by which the GMax can be progressively overloaded in a systematic fashion."
A total of 1963 articles were identified in the initial survey. After the analysis of the titles/abstracts, 1853 articles were eliminated, leaving 110 articles selected for full-text examination. After two meetings and discussion of the data, 61 items were included and evaluated by the methodological quality scale and inclusion/exclusion criteria, of which 16 articles were eligible for this systematic review (Figure 1).
In total, 231 participants (90 women and 141 men) underwent 24 strength exercises variations. Table 1 describes the exercises investigated, methods of EMG normalization, testing load, and the main findings. Of these, ten studies investigated the back squat exercise and its variations [partial, parallel and full] (Aspe and Swinton, 2014; Contreras et al., 2015b; 2016a; Da Silva et al., 2017; Evans et al., 2019; Gomes et al., 2015; McCurdy et al., 2018; Williams et al., 2018; Yavuz et al., 2015; Yavuz and Erdag, 2017), five studies investigated the barbell hip thrust and its variations [American and traditional styles and different feet positions] (Andersen et al., 2018; Collazo Garcia et al., 2018; Contreras et al., 2015b; 2016b; Williams et al., 2018), three studies investigated the deadlift, and its variations [traditional and hex bar]...