Rev Bras Fisiol Exerc 2021;20(1):83-92
doi: 10.33233/rbfex.v20i1.4396
REVIEW
Effects of the back-squat exercise on lower limb
myoelectric activity in trained men: a systematic review
Efeitos
do exercício de agachamento por trás na atividade mioelétrica
de membros inferiores em homens treinados: uma revisão sistemática
Rogério
Santos de Aguiar1,2, Juliana Brandão Pinto de Castro1,2,
Andressa Oliveira Barros dos Santos1,2,3, Giullio
César Pereira Salustiano Mallen da Silva1,2,3,
Fabiana Rodrigues Scartoni4, Rodolfo de Alkmim
Moreira Nunes1,2, Rodrigo Gomes de Souza Vale1,2,5
1Universidade do Estado do Rio de
Janeiro, Rio de Janeiro, Brasil
2Grupo de Pesquisa em Biodinâmica de
Desempenho, Exercícios e Saúde (BIODESA), Universidade Castelo Branco, Rio de
Janeiro, RJ, Brasil
3Universidade Católica de Petrópolis,
Petrópolis, RJ, Brasil
4Universidade Estácio de Sá, Cabo Frio,
RJ, Brasil
Received:
September 26, 2020; accepted:
December 3, 2020.
Correspondence: Rogério Santos de Aguiar, Universidade do Estado do Rio
de Janeiro, Instituto de Educação Física e Esportes, Rua São Francisco Xavier, 524, 9 andar, bloco F, sala 9122 20550-900 Rio de Janeiro,
RJ
Rogério Santos de Aguiar: rogghi@gmail.com
Juliana Brandão Pinto de Castro:
julianabrandaoflp@hotmail.com
Andressa Oliveira Barros dos Santos:
professoraoliveiras.and@gmail.com
Giullio César Pereira
Salustiano Mallen da Silva: giulliocesar.gc@hotmail.com
Rodolfo de Alkmim Moreira Nunes:
rodolfoalkmim@gmail.com
Fabiana Rodrigues Scartoni:
Fabiana.scartoni@ucp.br
Rodrigo Gomes de Souza Vale: rodrigovale@globo.com
Abstract
Aim: The aim of this study was to
describe the effects of the back-squat exercise on the lower limb myoelectric
activity in trained men. Methods: We conducted a systematic review
following the recommendations of PRISMA. Medline (PubMed), Scielo,
Scopus, SPORTDiscus, and Lilacs databases were
searched. The search terms included electromyography, exercise, resistance
training, and squat. We included experimental studies that described the
back-squat exercise using surface electromyography (EMG) in men experienced in
resistance training and back squat exercise at angles from 60º to 90º. Results:
Eight studies met the inclusion criteria. The interventions of the included
studies ranged from 2 to 7 days. The protocols demonstrated to improve the
neuromuscular system and to provide greater acquisition of strength in the
muscles involved in performing the back-squat exercise (p < 0.05).
Thirty-seven muscles were analyzed, with a predominance of the vastus
lateralis, vastus medialis, gluteus maximus, and rectus femoris muscles. Conclusion:
The studies investigated in this review showed that the back-squat exercise at
angles from 60º to 90º increased the lower limb myoelectric activity recorded
in loads of 30% and 100% of 1RM in men experienced in resistance training.
However, more studies with higher methodological quality are needed in the
analysis of the squat exercise to reduce the risk of bias.
Keywords: electromyography; squat exercise;
resistance training; muscle strength.
Resumo
Objetivo: O objetivo deste estudo foi descrever
os efeitos do exercício agachamento por trás sobre a atividade mioelétrica de membros inferiores em homens treinados. Métodos:
Foi realizada uma revisão sistemática seguindo as recomendações do PRISMA.
Foram pesquisadas as bases de dados Medline (Pubmed),
Scielo, Scopus, SPORTDiscus
e Lilacs. Os termos de pesquisa incluíram
eletromiografia, exercício, treinamento de resistência e agachamento. Foram
incluídos estudos experimentais que descreveram o exercício agachamento por
trás por meio da eletromiografia de superfície (EMG) em homens com experiência
em treinamento resistido (TR) e exercício agachamento por trás em ângulos de
60º a 90º. Resultados: Oito estudos preencheram os critérios de
inclusão. As intervenções dos estudos incluídos variaram de 2 a 7 dias. Os
protocolos demonstraram melhorar o sistema neuromuscular e proporcionar maior
aquisição de força nos músculos envolvidos na realização do exercício
agachamento por trás (p < 0,05). Foram analisados 37 músculos, com
predomínio dos músculos vasto lateral, vasto medial, glúteo máximo e reto
femoral. Conclusão: Os estudos investigados nesta revisão mostraram que
o exercício agachamento por trás em ângulos de 60º a 90º aumentou a atividade mioelétrica de membros inferiores registrada em cargas de 30%
e 100% de 1RM em homens experientes em TR. Porém, mais estudos com maior
qualidade metodológica são necessários na análise do exercício agachamento para
reduzir o risco de viés.
Palavras-chave: eletromiografia; exercício de
agachamento; treinamento de resistência; força muscular.
The squat
exercise is one of the most used exercises in the prescription of resistance
training (RT) and physical fitness. This exercise is also applied to
prescriptions intended for clinical treatments. This applicability is due to
the capacity of the squat exercise for strengthening the muscles of the lower
limbs in the treatment of ligament injuries, patellofemoral dysfunction, and
ankle instability [1,2,3].
The squat
exercise has also been part of the sports training programs as it presents
biomechanical and neuromuscular similarities to a wide range of athletic
movements [4]. In this way, it has been included as the central exercise of
many sports routines. Once established the biomechanical model, added to an anatomical
analysis for its execution, the squat exercise is used to improve physical
fitness, with associated benefits that are not limited to the athletic
population [5].
Additionally,
many activities of daily living (ADL) require the coordinated and simultaneous
interaction of various muscle groups. Thus, the squat exercise can be used to
improve muscle strength of the lower limbs, favoring the performance of the
ADL. This stems from the ability to recruit multiple muscle groups in a single
movement [6].
From this
perspective, the study of muscles may be important in providing information on
the control of voluntary movements, in the analysis of reflexes and measurement
of muscle groups involved in the squat exercise [7]. This exercise activates
about 200 muscles [6] and can be performed with a variety of depths, usually
measured by the degree of the knee flexion, such as partial (knee at 40º
angle), half (60, 70 to 90º), and full squat exercise (greater than 90º) [8].
The myoelectric
activity of human muscles can be measured by surface electromyography (EMG).
The EMG allows measuring the change in membrane potential, that is, how the
action potentials are transmitted along with the muscle fiber according to the
exercise stimulus performed [9]. Therefore, this systematic review aimed to
describe the effects of the back-squat exercise on the myoelectric activity of
the lower limbs in trained men.
This systematic
review followed the recommendations of the Preferred Reporting Items for
Systematic Reviews and Meta-Analyses (PRISMA) guidelines [10] and was
registered on the International Prospective Register of Systematic Reviews
(PROSPERO), as number CRD42018082308.
Study eligibility and inclusion criteria
We included
experimental studies using RT with acute intervention that evaluated the back-squat
exercise using EMG in men with experience in RT and squat exercise at 60º to
90º angles. Review studies, studies with individuals who presented some muscle
injury or physical limitations, or written in another language other than
English, Portuguese, or Spanish were excluded from this study review.
Search strategy
A search was
performed without filters in the Medline (via PubMed), SciELO,
Scopus, SPORTDiscus, and Lilacs (via BVS) databases,
in May 2020, using the terms “electromyography”, “exercise”, “resistance
training” and their respective synonyms, and “squat”. These descriptors and
their synonyms were appropriately combined using the logical operators [AND]
between descriptors and [OR] between synonyms (Appendix 1). Although the term
“squat” was not identified on the Health Sciences Descriptors (DeCS) and Medical Subject Headings (MeSH),
it was inserted in the main descriptors as a search strategy because it
appeared in some previous studies on the theme. The reference lists and other
sources were researched to find further studies.
After the
references were extracted using the search terms, they were exported to a
shared Mendeley library. Two authors completed the research, the removal of
duplicates, the analysis of titles and abstracts, and the screening of complete
articles. Any divergences in the analysis were sent to a third author. Then, we
read the full version of the articles that met the eligibility criteria of the
present study.
Bias analysis
The ROBINS-I
(Risk Of Bias In Non-randomised
Studies - of Interventions) tool was used to assess the risk of bias in the
studies included in this systematic review [11]. The studies were classified as
“selection bias”, “performance bias”, “detection bias”, “monitoring bias”,
“report bias”, “lack of data bias”, and “bias” in result selection reported,
with the answers “yes”, “probably yes”, “probably not”, and “no”. Two
independent and experienced evaluators analyzed the risk of bias in the
included studies. Disagreements were assessed by a third researcher.
Data collection process
The following
data were extracted from the selected studies: country, number of participants
in each group, age, body mass, height (Table I), intervention protocol, muscles
tested, methodologies, tests used for data analysis, and main results (Table
II).
In total, 350
studies were found following the proposed research methodology. After using the
selection criteria, eight articles were included in this review (Figure 1).
Figure 1 - Flow chart of the selected
articles
Table I presents
the descriptive characteristics of the studies included in this review. When
analyzing the eight studies [12,13,14,15,16,17,18,19] in Table I, it was observed a population of
107 trained men (mean age: 25.13 ± 1.93 years; body mass: 82.62 ± 2.05 kg; height:
1.74 ± 0.03 m).
Table I - Descriptive characteristics
of the studies included in the review
USA =
United States of America; BM = Body mass
Table II
presents the methodological characteristics and the results regarding the
resting conditions in the imposition of the maximum load in all studies,
including the muscle strength test used, the muscle group evaluated and the EMG
results before and after the intervention. Thirty-seven muscles were analyzed,
with a predominance in the analysis of the rectus femoris (RF), vastus
lateralis (VL), vastus medialis (VM), gluteus maximus (GM), and rectus femoris
muscle.
Table II - Methods and outcomes of the
studies included in this review (see PDF annexed)
Table III shows
the studies’ risk of bias through the ROBINS-I tool. Regarding the studies
analyzed using the ROBINS-I tool, 70% [13,15,16,17,19] were considered with
critical risk of bias, while only 30% [12,14,18] were considered with moderate
risk of bias.
Table III - Analysis of risk of bias
using the ROBINS-I tool
P =
Probably; 1 = Selection bias; 2 = Performance bias; 3 = Detection bias; 4 =
Monitoring bias; 5 = Reporting bias; 6 = Lack of data bias; 7 = Bias in the
selection of the reported result
The purpose of
this systematic review was to describe the effects of the back-squat exercise
on the lower limb myoelectric activity in trained men. The analysis of the
eight cross-sectional studies [12,13,14,15,16,17,19] showed greater muscle myoelectric
activity during different squat protocols, but these do not represent a greater
strength gain promoted by the type of exercise (p < 0.05).
The mean EMG of
the RMS signals (20 Hz to 392 Hz) varied for the muscles analyzed (VM, VL, and
GM) during the rising phase of the lift with each load during the repetition
maximum (RM) tests of the back squat exercise. However, the findings of these
experimental studies should be interpreted with caution, as they were
classified as uncertain risk of bias (Table III).
As for the interventions,
five studies used the RT through free weights with bars and washers [12,13,14,15,19],
two [16,17] used barbell and the Smith Machine, and one study [18] did not describe
the device used. Furthermore, two of these studies [12,14] performed the RM tests
in the back-squat exercise, adding the knee pad. One of these studies [18]
performed the 1RM test with loads of 80%, 90%, and 100%, using the EMG in the
back-squat exercise. However, general muscle myoelectric activities amplified
with increasing loads, but significant increases in EMG signals were observed
only in the vastus medialis (VM) and gluteus maximus (GM) muscles with 90% and
100% of 1RM loads. Likewise, Silva et al. [18] when submitted the sample
to the squat exercise, observed an increase in the EMG activity of the GM and
VM with increasing loads of 60% to 90% of 1RM.
McBride et al.
[20] reported the use of heavy loads of 70% to 90% of 1RM to analyze the effect
of instability and stability of the back-squat exercise. The results showed a
significant increase in the level of the EMG signal in the muscular activity of
the vastus lateralis (VL), biceps femoris (BF), and erector of the spine, with
the load of 90% of 1RM in the stable back squat exercise. On the other hand,
Contreras et al. [21] and Aagaard et al.
[22] compared the mean and peak of the EMG amplitude in the back squat
exercises in an estimate of 10RM and found no significant differences in the EMG
signal in the GM, BF, and VL muscles between the squats. The discrepancy
between the findings of these studies may be due to the samples having
experience in RT (<3 years). This may suggest a better strategy of muscle
recruitment in the frequency of myoelectric activity during the execution of
the back-squat exercise used in different percentages of loads and RM [23].
This way, RT
with different percentages of repetition maximum (% of 1RM) is used to improve
different muscle properties, such as increased maximum strength, explosive
strength, and hypertrophy [24]. Additionally, heavy loads (> 80% of 1RM)
were used to recruit high threshold fast contraction motor units, according to
the size of the muscle fiber, while smaller loads (60% of 1RM) are used to
maintain the specificity of the training speed and improve the mechanical power
[25]. Loads with different % of 1RM result in different neuromuscular and
kinematic adaptations in the eccentric phase of the squat exercise [26].
Hence, the squat
exercise is one of the most used exercises in many training protocols due to
its applicability and functionality in sports and daily activities. Squat
variations (example: back, front, Bulgarian, Sumo, and sink) are applied to
physical conditioning, strength training, and physiotherapeutic rehabilitation
[27]. Due to biomechanics and neuromuscular similarities to a range of athletic
movements, squat is an essential exercise in many sports routines [28].
Thus, the squat
aims to train the thigh muscles, the knee extensors (example: rectus femoris,
vastus lateralis and vastus medialis), and strengthen the hip extensors
(example: gluteus maximus, biceps femoris, and semitendinosus). Moreover, this
exercise can develop muscle strength in the lower back, to perform basic skills
required in sports and daily activities [29].
It is worth
noting that, as the locomotor system adapts to an RT program, the individual
must continue to undergo new percentages of load to continue to increase
strength and muscle mass by gradually increasing the load and the number of
sets or training frequency [30]. These variables are used to maintain the
specificity of the speed execution of the squat exercise and improve mechanical
power, strength and muscle hypertrophy [31].
All
interventions with RT from the experimental studies analyzed in this systematic
review are in accordance with the American College of Sports Medicine (ACSM)
guidelines [32] for individuals experienced in RT, which includes changes in
training loads (<80% of 1RM) to induce acute metabolic, hormonal, neural
changes and cardiovascular responses to RT. Also, the 1RM test has been used as
a gold standard in determining maximum dynamic strength and uses percentage
values of maximum strength to determine training zones [33].
This systematic
review has some limitations that should be highlighted. First, the samples were
composed only of trained men and did not include women experienced in RT.
Therefore, the results cannot be generalized to other populations. However, we
opted for the exclusivity of men in the sample, as they tend to have greater
body mass and muscle strength than women, due to the higher levels of anabolic
hormones. In addition, there is a risk of interference of the muscles close to
those analyzed using EMG, which would generate inaccurate edits. Hence, none of
the studies was an analysis of the different moments of the myoelectric signal
presented in the concentric and eccentric phases of the back-squat exercise
with different loads.
The studies
analyzed in the present systematic review showed that the back-squat exercise
at angles from 60º to 90º increased the lower limb myoelectric activity
recorded in loads of 30% and 100% of 1RM in men experienced in RT. Nonetheless,
it is suggested more studies with higher methodological quality in the analysis
of the squat exercise to reduce the risk of bias.
Potential conflict of interest
No conflicts of interest with potential for this
article have been reported.
Financing source
There were no external sources of funding for this
study.
Academic link
This study is linked to the thesis of doctoral student
Aguiar RS, from the Postgraduate Program in Exercise and Sports Sciences at the
Rio de Janeiro State University.
Authors’ contributions
Conception and design of the research:
Aguiar RS, Castro JBP, Nunes RAM, Vale RGS, Scartoni
FR; Analysis and interpretation of data: Aguiar RS, Castro JBP, Santos
AOB, Silva GCPSM; Statistical analysis: Not applicable; Obtaining
financing: Not applicable; Writing of the manuscript: Aguiar RS,
Castro JBP; Critical review of the manuscript for important intellectual content:
Aguiar RS, Castro JBP, Vale RGS.
Appendix 1 - Searches adopted in the present study