A comparison of muscle electromyographic activity during different angles of the back and front squat

1. Introduction

Despite its complex execution, the squat is regarded as one of the best exercises in strength training, since it allows the recruitment of various muscle segments in one single motion [1] For this reason, it is one of the most used multi-joint exercises in strength training protocols, both by recreational practitioners and athletes [2, 3, 4] The complexity of the squat, together with its many variables related to performance, demand the understanding of its biomechanics [5] in order to allow ideal muscle development while minimizing the risk of injuries due to poor form [6]. Among the many different squatting techniques, both the back squat (BS, performed with a barbell resting on the shoulders, over the trapezius, slightly above the posterior deltoid), and the front squat (FS, performed with a barbell resting on the anterior deltoids and clavicles, sustained with the help of both arms), have been recommended [4, 7].

Both variations provide important activation of the torso and lower limb muscles. However, the differences in technique also create differences in muscle recruitment, which still need to be investigated [8, 9]. Gullett et al. [8] and Contreras et al. [10] found no significant differences between FS and BS exercises regarding hip and thigh muscle activation. The first study analyzed two sets of three repetitions at 70% of 1 repetition maximum (1RM), while Contreras et al. [10] found no statistical differences in muscle activity between the two variants (both parallel and full range of motion) with a 10RM load in healthy women. On the other hand, Yavuz et al. [9] reported higher muscle activity in the vastus medialis in the FS when compared to the back squat, as well as greater muscle activity in the erector spinae when trained males performed BS using a 1RM load. Korak et al. [11] reported higher muscle activity during the FS (M = 94%, SD = 15%) compared to the deadlift and BS during 3 repetitions at 75% 1RM load.

To better understand the biomechanical aspects of these exercises, a number of studies have aimed to analyze the effects of different squat depths, in order to identify how different muscle groups contribute to perform this motion [12]. For instance, some have reported that the gluteus maximus muscle shows higher activation when squatting with higher degrees of knee flexion [13, 14]. Regarding other muscles, Jaberzadeh et al. [15] showed that the ratio of the EMG activity between the vastus medialis oblique and the vastus lateralis was higher when performing deeper squats. However, since these studies only addressed the BS, we cannot infer that the same would be observed when performing the FS.

The relationship between muscle activation level and the force generated in different types of squats seems to vary as a function of the angular relationships of the joints involved – specially the hip, knee and ankle – as well as the position of the torso [16]. In this sense, some studies reported that higher loads can be lifted during the BS when compared to the FS [8, 9]. This may be due to higher tibiofemoral compressive forces and a bigger moment arm of the extensor muscles [8]. These differences may also be related to variables interfering in the relationship between electromyographic activation and strength, such as angular and contraction velocity [17].

In general, most of the studies that compared EMG activity and the amount of force generated between different types of squats did so during dynamic actions. However, a more concrete association between these measures can be obtained during maximal isometric actions, since there is an increase in the firing rate of motor units in this type of muscle action, even when their recruitment reaches a saturation, causing the muscle to keep producing energy for the EMG signal leading, in turn, to an increase in amplitude [17].

Moreover, when considering the mutual interference between the range of motion of the different types of squat, muscle activation and the force produced in maximal isometric actions, there is a lack of sufficient evidence for clarifying the relationship between these variables. Additionally, different angles of exercise influence the inclination angle of the trunk and, consequently, muscle recruitment during squatting. Such information may play a large role in the prescription of these exercises in strength training programs and in understanding lower body muscle activation patterns between squat variations. These are important factors in strength and conditioning for isolating or creating greater activation of selected lower body muscles for training, performance increases, injury prevention, or rehabilitation techniques.

Therefore, the aim of our study was to compare the EMG activation of the muscles of the thigh, hip and torso during maximal isometric contractions in both BS and FS, performed at different degrees of knee flexion. Our hypothesis was that in both kinds of squats the lower the degree of knee flexion, the higher the level of muscle activation, mainly in the posterior hip and thigh muscles. We also believe that the strength of the triple ankle, knee and hip extension would be inversely proportional to the yours flexion angle and will be higher during the higher degrees of knee flexion, particularly in the BS. Furthermore, we hypothesized, due to previous findings, that the FS would produce greater muscle activity of the GM.

2. Methods

A larger magnitude of force was produced in the BS, in all assessed angles (Fig. 2). We verified that the maximum voluntary isometric force decreased progressively as the degree of knee flexion increased.

Figure 2 shows results related to the isometric force producted.

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The results of the comparison of muscle activity between the FS and the BS are shown in Fig. 3. At 60 ∘ of knee flexion, the GM, BF and VL had a higher activation during the FS. At 120 ∘ , only the RF and the VL had a higher activation during the FS. At 90 ∘ , there was no significant difference between muscle activation during the FS and the BS.

Table 1 outlines the normalized values of the EMG signals between the different angles. The gluteus maximus had a higher activation when knee flexion was at 90 ∘ , particularly during the FS. The erector spinae presented a higher activation at 90 ∘ and 120 ∘ during the FS, while in the BS the highest activation was observed at 90 ∘ . Both the biceps femoris and the rectus femoris were more activated at 90 ∘ and 120 ∘ during the FS. However there were no significant differences between angles during the BS. The vastus lateralis was more activated at 90 ∘ and 120 ∘ , in both squats. Finally, the vastus medialis was the only muscle which did not show any difference in activation between different angles or squat variations.

Our results indicate that, in both the FS and BS, the lowest electrical activity of the assessed muscles was observed when the knee was flexed at 60 ∘ . However, the highest levels of isometric force were obtained at this range, as we had previously hypothesized. Despite this, our hypothesis as a whole was only partially confirmed since the increase in the range of knee flexion to 120 ∘ did not result in a larger activation of the assessed musculature. Only in the 90 ∘ range no significant difference in muscle activation between BS and FS were observed.

Mechanical conditions, as opposed to neural ones, have a much bigger influence on the reduction electrical activity of the assessed muscles observed during knee flexion at 60 ∘ . At this range, the rotatory components show a possible decrease, as well as an increase in the participation of translatory components of compression in triple knee extension (knee, ankle and hip) when compared to other ranges of motion of this exercise. On the other hand, the increase in maximum isometric strength, also observed at 60 ∘ of knee flexion may be explained by the increase in moment. In this angle, the resistance moment is lower in the triple knee extension, when compared to the 90 ∘ and 120 ∘ angles, due to a shorter resistance arm – a lower perpendicular distance between the weight (fixed bar) and the rotation axis of the involved joints [19].

Despite the lack of a kinematic analysis in our study, it is possible to affirm that squatting with knees flexed at 60 ∘ , in both variations, demanded lower degrees of hip and ankle flexion, thus reducing moment over the requested segments, even during an increase in the maximum force generated in this range. In these conditions, we observed a higher capacity to support axial load at the expense of a lower activation of the knee, hip and torso extensors, similar to Yavuz et al. [9]. This relationship may make it undesirable to perform squats in ranges of motion equivalent to 60 ∘ of knee flexion, since the increase in load may promote a linear increase in the compression of the body of the vertebrae as well as intradiscal compression [19].

The similarity in activation of the assessed muscles during MVIC at 90 ∘ of knee flexion, in both squat variations, supports previous findings obtained through EMG analysis during dynamic exercise [8, 9]. Lower loads can be used in the front squat in order to promote equivalent muscle activation levels compared to those obtained during the BS. When considering this premise, we speculate that the front squat may be a safe and efficient alternative for the recruitment of thigh, hip and torso muscles, resulting in lower overloads for the joints involved in this motion. During the back squat, considering all three ranges of motion, the gluteus maximus muscle showed higher EMG activation when the knee was flexed at 90 ∘ . This can be justified by the optimal length-tension relationship obtained for this muscle during this circumstance [10].

Our findings do not match those obtained in a previous study [13] which had indicated a direct relationship between gluteus maximus activation and the depth of the dynamic squat. We suggest that this finding was due to the utilization of submaximal loads, a fact that may have altered the recruitment pattern of high threshold motor units, especially in more experienced individuals [9]. In lower load conditions, stronger muscles may compensate the activation of weaker ones, promoting variations in their respective EMG activities, which can compromise the results found. Furthermore, in this study the loads were not balanced for comparison of deep and partial squats, which might have compromised the results.

The rectus femoris has manifested lower EMG activation in all variations and ranges of motion when compared to the other muscles of the quadriceps – the vastus lateralis and vastus medialis. This may be due to a possible stabilization of its length, since its mutual function as a knee extensor and hip flexor promotes a simultaneous shortening of one of its ends while stretching the other [5, 7, 9]. Regarding the low activation of the biceps femoris in all assessed ranges and variations, we suggest that the squat may not be the best exercise option for the work of the hamstring muscles, as proposed by Contreras et al. [10] and Gullett et al. [8]. In this context, Ebben et al. [20] indicated that knee flexion exercises and the stiff-legged deadlift (performed with both knees fully extended) might be better options.

One of the more significant clinical implications of the current results lies in the relationship between lifted load and muscle activation. Considering that the use of lower loads when performing the front squat did not generate a decrease in the activation of the targeted muscles this variation may be safer and better suited for practitioners who want to prioritize the preservation of joints that have a potential to be vulnerable to external load.

The main limitation of our study is due to the manner through which we performed the analysis. All squat exercises were performed as maximal isometric contractions, which reduces the similarity of the tests we analyzed with the practices observed in gyms and training centers, where squats are commonly performed as dynamic exercise. Additionally, the normalization of the EMG signals from MVIC obtained in various angles and variations may have interfered with our results, despite the existing precedents in literature [4]. The normalization obtained through own maximal isometric contractions, performed during the execution of the assessed squats, might have provided more reliable parameters for the assessment of the proposed variables. Lastly, anthropometric and kinematic differences between sexes exist, so future research is needed to fill in this gap [21].

5. Conclusion

Our results indicate that in both the front and back squats, muscle activation was lower at 60 ∘ of knee flexion, despite the highest force production being obtained during this range. Furthermore, an increase in range to 120 ∘ did not result in a higher EMG activity of the assessed muscles. Regarding comparisons between the front and back squat, we observed that the front squat demands the use of lower loads in order to promote levels of similar muscle activation to those obtained during the back squat. This is a practical conclusion as far as strength and conditioning professionals, trainers, and therapists who want to increase performance without increasing the load during squatting, are concerned.