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European Journal of Sport Science logoLink to European Journal of Sport Science
. 2024 Jan 30;24(1):6–15. doi: 10.1002/ejsc.12042

The effects of squat variations on strength and quadriceps hypertrophy adaptations in recreationally trained females

Alysson Enes 1,✉, Gustavo Oneda 2, Danilo Fonseca Leonel 3, Lucas Lemos 1, Felipe Alves 1, Luis H B Ferreira 1, Guillermo Escalante 4, Brad J Schoenfeld 5, Tácito P Souza‐Junior 1
PMCID: PMC11235860

Abstract

The barbell squat is a multijoint exercise often employed by athletes and fitness enthusiasts due to its beneficial effects on functional and morphological neuromuscular adaptations. This study compared the effects of squat variations on lower limb muscle strength and hypertrophy adaptations. Twenty‐four recreationally trained females were assigned to a 12‐week front squat (FS; n = 12) or back squat (BS; n = 12) resistance training protocol (twice per week). Maximum dynamic strength (1‐RM) on the 45° leg press, a nonspecific strength test, and muscle thickness of the proximal, middle, and distal portions of the lateral thigh were assessed at baseline and post‐training. A significant time versus group interaction was observed for 1‐RM values (F(1,22) = 10.53; p = 0.0004), indicating that BS training elicits greater improvements in muscle strength compared with FS training (p = 0.048). No time versus group interactions were found for muscle thickness (F(1,22) = 0.103; p = 0.752); however, there was a significant main effect of time for the proximal (F(1,22) = 7.794; p = 0.011), middle (F(1,22) = 7.091; p = 0.014), and distal portions (F(1,22) = 7.220; p = 0.013) of the lateral thigh. There were no between‐group differences for any muscle thickness portion (proximal: p = 0.971; middle: p = 0.844; and distal: p = 0.510). Our findings suggest that BS elicits greater improvements in lower limb muscle strength on the 45° leg press than FS, but hypertrophic adaptations are similar regardless of variations during the squat exercise.

Keywords: back squat, front squat, muscle thickness, muscular strength, quadriceps femoris

Highlights

  • Back squat training elicited greater strength‐related improvements in a nonspecific strength test than front squat training.

  • Hypertrophic adaptations of the lateral thigh are similar between both squat variations.

  • Both squat variations elicited similar growth at proximal, middle, and distal regions of the lateral thigh.

1. INTRODUCTION

The squat is a frequently used lower limb exercise in resistance training (RT) programs where the goal is to increase strength, power, conditioning, and/or rehabilitation. The barbell squat is a multijoint exercise often employed by athletes and fitness enthusiasts due to its beneficial effects on functional and morphological neuromuscular adaptations (Ribeiro et al., 2022; B. J. Schoenfeld, 2010). This exercise has variations that allowed previous studies to investigate squat depth (Bloomquist et al., 2013; Kubo et al., 2019; Pallarés et al., 2020), stance width (Sinclair et al., 2022), foot placement (Lorenzetti et al., 2018), movement tempo (Morrissey et al., 1998; Usui et al., 2016), and barbell position (Contreras et al., 2016; Korak et al., 2018). Among these variations, the front squat (FS) and back squat (BS) are common squat variations that alter the placement of the barbell by bracing the barbell either along the clavicle or posteriorly near the level of the acromion, respectively. This difference in barbell position creates biomechanical differences between both squat forms, such as an upright torso and less hip flexion with the FS and a prominent forward torso and more hip flexion during BS (B. J. Schoenfeld, 2010). Due to these kinematic differences, previous studies that compared the FS and BS have observed differences in joint kinetics between the two variations (Contreras et al., 2016; Gullett et al., 2009; Korak et al., 2018; Krzyszkowski & Kipp, 2020; Yavuz & Erdag, 2017). For example, evidence shows that surface electromyographic activity of thigh muscles is generally similar between FS and BS in healthy women (Contreras et al., 2016; Korak et al., 2018) and competitive bodybuilders (Coratella et al., 2021).

The BS and FS are common exercises prescribed in RT programs that aim to develop muscle strength, since the lower limb musculature is used in common sport‐related tasks (i.e., sprinting, jumping, and squatting) (B. J. Schoenfeld, 2010; Stone et al., 2022). Previous studies have investigated the effect of exercise selection, variation, and mode on muscle strength and found that strength‐related improvements were primarily driven by specificity, that is, if the goal is to increase strength in an exercise or task, the specific exercise or task must be preferentially practiced, even though adding accessory exercises may provide advantages or reduce strength adaptations in a specific task (Chaves et al., 2020; Costa et al., 2022; Lee et al., 2018; Remaud et al., 2010; Rossi et al., 2018). There are biomechanical differences between FS and BS that conceivably could influence neural adaptations. Hence, employing the FS or BS for dynamic strength testing could bias results since the strength gains might be specific to “practicing” the test during weekly training sessions (Mattocks et al., 2017). Thus, selecting a neutral testing modality, such as a different multijoint lower limb exercise, could help to ensure that results are related to the effect of the exercise and not a possible effect of specificity.

Despite evidence of similar kinematics and electromyographic activity between the FS and BS in healthy females, these variations appear to produce differences in kinetics. Gullet et al. (2009) found that FS elicits significantly lower compressive forces at the knee joint and reduced lumbar stress compared to the BS; however, there were no statistical differences in electromyographic activity in thigh muscles. Indeed, previous studies showed that varying the barbell position could shift the center of mass forward and alter peak hip extensor net internal joint moment between the FS and BS (Braidot et al., 2007; Korak et al., 2018; Krzyszkowski & Kipp, 2020). These shifts in the center of mass and net internal joint moments might alter torque relationships between the hip/knee joints and muscles, which in turn could increase mechanical tension at different muscle lengths during these squat executions and potentially elicit inhomogeneous hypertrophy along the quadriceps femoris (Earp et al., 2015; Ema et al., 2013; Mangine et al., 2018). Furthermore, training volume could be a confounding variable regarding muscular adaptations (Baz‐Valle et al., 2022; Ralston et al., 2017). Emerging evidence suggests that participants' previous RT volume, quantified by the number of sets performed per week per muscle group, could have an influence on muscular adaptations (Aube et al., 2022; Scarpelli et al., 2020). Thus, employing an individualized approach to training volume based on participants' previous quadriceps sets volume may help to reduce this confounding effect.

Previous research comparing the BS and FS has focused on acute data relating to biomechanical aspects, thereby limiting the ability to draw inferences regarding longitudinal muscular adaptations (Vigotsky et al., 2022). To date, evidence is lacking as to the chronic effects of BS versus FS training on strength and hypertrophy adaptations using an individualized training approach based on the previous volume in healthy females. Considering the gaps in the current literature, the purpose of this study was to investigate the effects of the FS and BS on dynamic strength and regional hypertrophy adaptations of the quadriceps femoris in healthy, recreationally trained females. We hypothesized that varying the barbell position would elicit distinct strength and inhomogeneous hypertrophy responses in healthy females.

2. METHODS

2.1. Study design

We employed a randomized, repeated‐measures parallel‐group design, balanced according to dynamic strength, to investigate the effects of squat variations on strength and hypertrophy adaptations in healthy, recreationally‐trained females. The study began with anthropometric assessments, quadriceps muscle thickness imaging, and familiarization sessions. The anthropometric assessment was performed using a multifrequency bioelectrical impedance (InBody 120) to assess body mass and body fat percentage, and height was assessed with a stadiometer (W200/5). As per the guidelines provided by the manufacturer, participants were instructed to (i) refrain from consuming any food or water for a minimum of 2 h prior to the evaluation; (ii) abstain from consuming beverages containing alcohol or caffeine within a 24‐h period leading up to the evaluation; (iii) avoid engaging in moderate to vigorous physical activity within 12 h before the evaluation; (iv) consume water of at least 2 L on the day preceding the evaluation; and (v) if possible, urinate 30 min before the evaluation. After familiarization sessions, participants underwent 1‐RM tests on the 45° leg press. Afterward, participants undertook a progressive RT program that aimed to compare the effects of the barbell FS versus BS on strength and hypertrophy adaptations.

After baseline assessments, participants were allocated to either the FS or the BS group. Initially, participants began the training program with a weekly set volume of 20% more than their previous quadriceps training volume. The weekly quadriceps training volume was increased by 20% every 4 weeks (B. Schoenfeld et al., 2021). Total training volume (TTV) was monitored but not equated to maintain ecological validity as BS training allows the use of greater absolute loads than FS (Yavuz & Erdag, 2017). Seventy‐two hours after the last training session, lateral thigh muscle thickness and 1‐RM tests were conducted in the same manner as baseline.

2.2. Subjects

The sample power was calculated using the software G*Power 3.1 for F family analysis of variance (ANOVA) repeated measures, within‐factors, to determine a sufficient number of participants to meet the study purpose with the following conditions: Power = 0.80, α = 0.05, a moderate effect size of 0.25, and correlation among repeated measures of 0.7. The analysis indicated that 22 participants were required to achieve adequate statistical power. To account for potential dropouts, we recruited 29 healthy females and allocated them to either the FS group (n = 15) or the BS group (n = 14). Randomization was pair‐matched based on the initial maximum dynamic strength (1‐RM) in the 45° leg press.

We employed the following inclusion criteria for the study: (a) age 18–30 years; (b) at least 6 months of RT practice at 4 days per week of RT frequency; (c) negative answers to all items of the Physical Activity Readiness Questionnaire; (d) free from creatine supplementation; and (e) self‐report of the use of at least 3 months of combined oral contraceptive pills with consumption and withdrawal phases according to individual menstrual cycle length. The exclusion criteria were as follows: (a) self‐report of any musculoskeletal injury; (b) self‐report of alcohol abuse; (c) self‐report of anti‐inflammatory or anabolic androgenic steroids use; and (d) self‐report of any menstrual irregularities. Participants were informed of the procedures and details related to training intervention and signed a written informed consent form prior to their participation. All procedures were submitted and approved by the local ethics committee and were in accordance with the Declaration of Helsinki.

2.3. Maximum dynamic strength (1‐RM)

Participants completed two familiarization sessions before the 1‐RM 45° leg press tests at baseline. These familiarization sessions consisted of instructions for technical standards for each exercise that would be used during the training program. Both familiarization sessions were conducted with the same procedures as used during 1‐RM testing (i.e., warm‐up and rest between sets); however, participants used close to but not maximum loads. Each familiarization session was conducted 48h apart. Seventy‐two hours after the final familiarization session, participants performed the first 1‐RM testing session, and the second 1‐RM session was conducted 72 h later. The highest 1RM value between the two testing sessions was considered for the analysis.

Both familiarization sessions and 1‐RM tests were carried out as follows: participants performed a general warm‐up (5 min at 6 km.h−1 at a treadmill and a light full‐body stretching routine) followed by a specific warm‐up of 2 sets of 5 repetitions at an estimated load for 12 and 8 repetitions, respectively, with 2‐min rest interval between sets. Participants received specific instructions regarding the 45° leg press technique (e.g., full range of motion with the knees brought as close as possible to the chest). Three min after the specific warm‐up, 1‐RM attempts began. The load was progressively increased until participants were unable to perform the correct 45° leg press with proper form (e.g., knee flexion less than ∼100°), which was monitored by the research team. The 1‐RM load was determined within 5 attempts, with 3–5 min passive recovery provided between attempts (Brown & Weir, 2001). The postintervention 1‐RM testing was conducted after ultrasound imaging. The coefficient of variation (CV), standard error of measurement (SEM), and intraclass correlation coefficient with 95% confidence interval (ICC) between two 1‐RM tests performed 72 h apart were 3.85%, 5 kg, and 0.95 (0.90–0.98), respectively.

2.4. Muscle thickness

A B‐mode ultrasound (ECO3, Chison Medical Imaging Ltd.) with a 5‐MHz linear transducer was used to obtain muscle thickness measurements of the lateral quadriceps as assessed along the proximal (30%), middle (50%), and distal (70%) aspects, considered as the distance from the greater trochanter to the lateral condyle of the femur (Abe et al., 2000). These anatomical points were detected by palpation. We then measured femur length with an anatomical pachymeter, registering the femur length of each participant as well as the respective proximal, middle, and distal aspects of lateral thigh muscles, which were transversally marked to guide imaging (Figure 2). Participants were instructed to refrain from any strenuous exercise or other moderate‐to‐vigorous physical activity 72 h prior to ultrasound imaging to avoid the potential influence of muscle swelling on the primary outcome. Upon arriving at the lab, participants assumed the supine position with all joints relaxed and the knees slightly flexed during image acquisition. Participants were asked which lower limb they would use to kick a ball to determine the dominant lower limb for image acquisition.

FIGURE 2.

FIGURE 2

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Thigh regions and an ultrasound image. Estimation plot of the paired mean difference for hypertrophic responses for proximal (30%; A), middle (50%; B), and distal (70%; C) regions of lateral thigh for within‐subjects (top) and between‐groups (bottom) conditions. The raw data are plotted on the upper axes; each paired set of observations is connected by a line. On the lower axes, each paired mean difference is plotted as a bootstrap sampling distribution. Mean differences are depicted as dots; 95% confidence intervals are indicated by the ends of the vertical error bars. BS, back squat; F, femur; FS, front squat; mm, millimeters; VI, vastus intermedius; VL, vastus lateralis.

A trained ultrasound technician, blinded to group allocation, applied a generous amount of water‐soluble transmission gel to each portion of the lateral thigh. Transverse images were obtained by placing the linear transducer on the skin with caution taken not to depress the skin. When an appropriate image was obtained, it was saved to the hard drive. Muscle thickness was measured as the distance between the internal border of the superficial aponeurosis of the vastus lateralis and external border of the femur, providing a combined measure of muscle thickness of the vastus lateralis and vastus intermedius (Figure 2). Each portion of the lateral thigh had three images captured, and measurements were averaged to obtain a final value. If one of the three images showed a difference greater than 10%, a fourth image was taken and replaced with the closest value. We did not consider the menstrual cycle phase to evaluate the muscle thickness at baseline and post‐training, since neither the menstrual cycle phase nor the use of contraceptive oral pills seems to influence muscle thickness measurement (Kuehne et al., 2021; Sung et al., 2022). The postintervention ultrasound imaging was conducted 72 h after the last training session. The CV, SEM, and ICC with 95% confidence interval among the muscle thickness measurements for each portion of the lateral thigh were 2%, 0.9 mm, and 0.98 (0.96–0.99) for the proximal portion; 0.92%, 0.5 mm, and 0.99 (0.98–0.99) for the middle portion; and 1.45%, 0.6 mm, and 0.99 (0.98–0.99) for the distal portion.

2.5. Training sessions

Training was carried out twice a week for a total of 24 training sessions. The training sessions were performed at the same time of the day. Participants were instructed to maintain their habitual dietary intake during the training program, and their habitual preworkout meal at least 2 h prior to our training sessions. A general and specific warm‐up, similar to that used in the 1RM testing protocol, was performed prior to each training session for both groups. Exercises were performed in the following order for all sessions: barbell squat (front or back according to experimental group), Romanian deadlift, seated knee flexion, and seated hip abduction. We included exercises for the lower body posterior chain to avoid possible dropouts and to maintain the training program as close as possible to the participants' habitual training routine without influencing adaptations in the quadriceps.

The experimental groups performed the training schemes in the same manner; the only difference between groups was the type of squat (front vs. back). The participants were instructed to perform the squat to a parallel depth (e.g., ∼100° knee flexion) and to maintain an external attentional focus during the movement (Coratella, 2022), either in the FS or the BS group, which was monitored by a certified strength and conditioning professional. In addition, participants were instructed to perform the front squat grip in an arm cross grip or clean grip, according to personal preference, to avoid load and technique limitations due to reduced shoulder joint flexibility.

The training sessions were performed twice a week, and the weekly volume for the squat was equally divided between these two training sessions. The first and second weekly training sessions were established in a loading zone of 6–‐8 repetitions and 10–12 repetitions, respectively. The posterior chain exercises comprised 2 sets per exercise using the same repetition and load schemes as in the squat protocol. Each training session was performed at least 72 h apart. Table 1 provides an overview of the training routine.

TABLE 1.

Training routine throughout 12 weeks.

Exercises Session A Session B
Front squat or back squat ITV × 6–8 ITV × 10–12
Romanian deadlift 2 × 6–8 2 × 10–12
Seated knee flexion 2 × 6–8 2 × 10–12
Seated hip abduction 2 × 6–8 2 × 10–12
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Note: The sets scheme in the front squat or back squat were calculated according to participants' previous weekly set volume for the quadriceps and increased by 20% every 4 weeks.

Abbreviation: ITV, Individualized training volume.

Participants were instructed to perform each set at ∼2 repetitions in reserve, except for the last set which was performed to momentary failure. If a participant could not achieve the prescribed repetition range on a given set, the research supervisor provided slight assistance on the final 1–‐2 repetitions to achieve the target zone. The load was adjusted in each set according to ∼2 repetitions in reserve and momentary failure to the specific repetition zone. If the participant could not achieve the minimum number of repetitions for the target loading zone or completed the set with ease (e.g., >2 repetitions in reserve), the load was reduced or increased for the next set, respectively. A 3‐min rest interval was provided between sets and exercises. Participants were instructed to maintain a repetition tempo of 1s for the concentric action and 2 s for the eccentric action of each repetition without pausing in transition phases during the repetitions. All participants were instructed to avoid any additional moderate‐to‐vigorous lower body exercise during the training intervention.

The total training volume (TTV) was calculated as follows: TTV = sets * repetitions * load. We analyzed TTV only for squat training; the additional exercises (i.e., posterior chain) were not included in TTV analysis. Importantly, TTV was monitored but not equated between‐conditions, given that set volume is generally regarded as the most appropriate gauge for hypertrophy training (Baz‐Valle et al., 2021; B. Schoenfeld & Grgic, 2017). Moreover, we employed an individualized approach that considered participants' previous quadriceps RT volume quantified by the number of sets per week based on emerging evidence that suggests such a strategy may optimize muscular adaptations (Aube et al., 2022; Nóbrega et al., 2022; Scarpelli et al., 2020).

2.6. Statistical analysis

Descriptive statistics are expressed as mean ± standard deviation (SD). Data normality and homogeneity of variances were assessed using Shapiro–Wilk and Levene's test, respectively. After confirming data normality, an independent t‐test was used to detect possible differences on each variable at baseline, and a Mann–Whitney test was used to detect possible differences on TTV and absolute load values between groups. A 2‐way repeated measures ANOVA was used to identify possible interactions for dependent variables (muscle strength and muscle thickness). Weekly set volume was analyzed using an independent t‐test. An additional interpretation of data was made from 95% confidence intervals (95% CIs) of the mean difference (Meandiff) within‐ and between‐conditions. Partial eta squared ( p n 2) effect sizes were calculated and classified as follows: 0.02 small, 0.13 medium, and 0.26 large effect (Bakeman, 2005). The significance level was established a priori at p ≤ 0.05. All analyses were carried out in SPSS 25.0 software (IBM SPSS Statistics, IBM Corp, version 25.0).

3. RESULTS

During the study period, 3 subjects dropped out of the FS group (loss of interest: n = 2; excessive shoulder joint pain due to barbell position: n = 1) and 2 subjects dropped out of the BS group (injuries not related to current study: n = 1; personal reasons: n = 1). Thus, 24 participants (FS = 12; BS = 12) completed the training intervention. Table 2 shows general characteristics of participants at baseline and the accumulated TTV between groups. Over the duration of the training program, the accumulated TTV and the absolute load of BS were higher than in FS (p = 0.033 for both variables) as shown in Table 2. The average number of weekly sets for squats was statistically similar between conditions (19.1 ± 4.1 sets and 21.9 ± 5.4 sets for FS and BS, respectively; p = 0.171).

TABLE 2.

Baseline characteristics, resistance training schemes information, and accumulated total training volume data after intervention.

Variables FS (n = 12) BS (n = 12) p
Age (years) 22.2 ± 3.3 23.8 ± 1.6 0.152
Height (cm) 165.0 ± 4.3 165.1 ± 9.2 0.955
Body mass (kg) 62.4 ± 8.5 66.5 ± 13.6 0.386
Body fat (%) 19.8 ± 3.4 19.4 ± 3.0 0.765
Experience (years) 1.8 ± 1.1 2.3 ± 1.5 0.418
1‐RM 45° leg press (kg) 219.5 ± 93.5 247.5 ± 84.2 0.450
1‐RM:Body mass ratio (a.u.) 3.4 ± 1.0 3.7 ± 1.0 0.511
Previous QTV (sets.week‐1) 13.1 ± 2.8 15.0 ± 3.7 0.171
Absolute load (kg) 48.6 ± 23.7 63.4 ± 16.9 0.033*
Training volume (sets.week‐1) 19.1 ± 4.1 21.9 ± 5.4 0.171
Total training volume (kg) 140087.1 ± 84358.2 188731.5 ± 68984.4 0.033*
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Note: Data are expressed in mean ± SD.

Abbreviations: 1‐RM, one repetition maximum; BS, back squat; cm, centimeters; FS, ront squat; kg, kilograms; QTV, quadriceps training volume.

*significantly difference between‐groups (p ≤ 0.05).

Both groups increased their maximum dynamic strength compared to baseline (F(1,22) = 78.47; p = 0.0001; pn2 = 0.78, large effect); Figure 1 (Ho et al., 2019)). A time versus group interaction was found for the 1‐RM 45° leg press test (F(1,22) = 10.53; p = 0.0004; pn2 = 0.32, large effect) with the BS eliciting greater muscle strength adaptations compared to the FS (p = 0.048; between‐group Meandiff (95% CI) = 67.9 kg (0.55–135.2); pn2 = 0.16, medium effect). Within‐group Meandiff (95% CI) data are depicted at Figure 1.

FIGURE 1.

FIGURE 1

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Estimation plot of the paired mean difference 1‐RM changes in the 45° leg press for within‐subjects and between‐groups conditions. The raw data are plotted on the upper axes; each paired set of observations is connected by a line. On the lower axes, each paired mean difference is plotted as a bootstrap sampling distribution. Mean differences are depicted as dots; 95% confidence intervals are indicated by the ends of the vertical error bars. * = significantly differences between groups (p < 0.05); BS, back squat; FS, front squat; kg, kilograms.

Both groups increased their muscle thickness at the proximal, middle, and distal regions of the lateral thigh in a similar fashion (Figure 2). Analysis revealed no time versus group interactions (F(1,22) = 0.103; p = 0.752; pn2 = 0.005, small effect). However, there was a significant main effect of time for proximal (F(1,22) = 7.794; p = 0.011; pn2 = 0.26, large effect), middle (F(1,22) = 7.091; p = 0.014; pn2 = 0.24, medium effect), and distal portions (F(1,22) = 7.220; p = 0.013; 0.24, medium effect). No between‐group differences were observed for any muscle thickness region: proximal (p = 0.971; Figure 2A; Meandiff (95%CI) = 0.10 mm (−5.77 – 5.98); pn2 = 0.000, small effect); middle (p = 0.844; Figure 2B; Meandiff (95%CI) = 0.56 mm (−5.29 – 6.42); pn2 = 0.002, small effect); and distal (p = 0.510; Figure 2C; Meandiff (95%CI) = 1.39 mm (−2.92 – 5.72); pn2 = 0.02, small effect). Within‐group Meandiff (95%CI) data are depicted at Figure 2.

4. DISCUSSION

To our knowledge, this is the first paper to compare the effects of the BS and FS on lower limb strength and hypertrophy adaptations in healthy females. Our main findings were: (a) BS training promotes greater dynamic strength‐related improvements in a nonspecific strength test (i.e., 45° leg press) compared to FS training and (b) hypertrophic adaptations were similar between conditions among lateral thigh regions.

In agreement with our initial hypothesis, strength‐related improvements varied between conditions with the BS eliciting greater adaptations (∼37.2%) when compared to FS (∼19.6%) after a 12‐week period. Importantly, these differences were assessed in the 45° leg press, thus indicating that strength adaptations transferred to a nonspecific strength task. Previous studies that aimed to compare dynamic strength gains induced by squatting techniques focused on different depths (full vs. partial range of motion) and movement tempo (Kubo et al., 2019; Morrissey et al., 1998; Pallarés et al., 2020; Usui et al., 2016). Collectively, the findings of these studies, which investigated squat variations in healthy untrained or recreationally trained individuals over 7‐ to 12‐week periods were similar to our findings related to within‐group dynamic strength gains after our 12‐week intervention. Given that our study is the first to investigate the strength‐related changes between the FS versus BS, direct comparisons cannot be made between investigations.

Moreover, we employed a nonspecific dynamic strength test in an exercise that was not employed in the training program. This decision is consistent with the principle of specificity, since regularly training with the same exercise used in the strength assessment can influence the outcome (Mattocks et al., 2017). However, to date, there are no longitudinal data regarding the effects of the FS versus BS on a neutral strength test; thus, our study adds novel findings to the literature related to nonspecific strength testing.

Evidence suggests that biomechanical differences exist between BS and FS training (B. J. Schoenfeld, 2010). Yavuz and Erdag (2017) compared to kinematic activities between the BS and FS and found that the BS has greater hip flexion angles than the FS. Krzyszkowski and Kipp (2020) found that peak hip extensor net internal joint moments were higher in BS compared to FS training. These biomechanical differences might uniquely influence the activation of hip extensor muscles during each respective squatting technique, resulting in differential dynamic strength adaptations. Nevertheless, it is important to point out that neither discrepancies in TTV between groups nor individualized volume approaches seem to influence strength adaptations (Aube et al., 2022; Nóbrega et al., 2022). Similar to our data, Yavuz and Erdag (2017) showed that participants could lift greater absolute loads in the BS than the FS, perhaps due to kinematic and kinetic differences (i.e., hip angle) between them. In addition, it is well‐documented that high loads are a main driver of muscle strength adaptations (Lopez et al., 2021). Since the BS allows the use of higher absolute loads compared to the FS, this may have elicited higher neural adaptations, which may partially explain our findings. These explanations remain speculative and warrant further research to provide insight into these hypotheses.

Although both squatting techniques elicited hypertrophic adaptations, the changes were similar among lateral thigh regions (pooled mean increases ∼4.4% for back squat and ∼5.1% for front squat training). In contrast to our findings, Usui et al. (2016) compared fast versus slow movement tempos in the BS at low‐load conditions in untrained males. After an 8‐week training period, poststudy results showed an inhomogeneous muscle growth only for the slow movement tempo, with increases observed in the middle and distal vastus intermedius sites (∼6%‐9%) but not in the proximal site (50%, 70%, and 30% of the femur length, respectively). Alternatively, the fast movement tempo group showed no significant increases for the vastus intermedius at any site. Moreover, the vastus lateralis muscle thickness (analyzed only at 50% of the femur length) showed no poststudy statistical change for both squat variations. The relative increases found by Usui et al. (2016) were slightly higher to our relative increases, which may be attributed to differences in training status and movement tempos between studies. In addition, it is well‐established that training volume plays a key role in muscle plasticity (B. J. Schoenfeld et al., 2017), and although TTV was not equated between conditions, we ensured an individualized progressive RT volume. The muscle thickness responses in all portions of the lateral thigh were similar between conditions, which is consistent with previous studies despite not equalizing the TTV (Barcelos et al., 2018; B. J. Schoenfeld et al., 2015).

Emerging evidence has shown that variation in exercise selection can influence nonuniform skeletal muscle adaptations (Kassiano et al., 2022; Zabaleta‐Korta et al., 2020). Previous studies that investigated interventions with squat training on regional hypertrophy among quadriceps muscles found similar increases between proximal to distal portions, similar to our findings (Kojic et al., 2022; Merrigan et al., 2019). It should also be noted that the BS may not optimize hypertrophy of all the quadriceps heads. Fonseca et al. (2014) found that a combination of different lower body exercises promoted more uniform development of the quadriceps compared to the BS alone, which showed inferior hypertrophy in the vastus medialis and rectus femoris in a cohort of untrained young men over a 12‐week period. Similarly, Kubo et al. (2019) found that the BS preferentially hypertrophied the vasti muscles, with no appreciable effect on the rectus femoris in a cohort of untrained young men over a 10‐week interventional period. Our study only assessed the lateral quadriceps, and thus we cannot draw conclusions on this topic. However, our data indicate that both FS and BS elicited a sufficient training stimulus to promote growth along the length of the lateral thigh, specifically to the vastus lateralis and intermedius.

The present study is not without limitations. First, our findings are specific to recreationally trained females and should not be extrapolated to other populations, such as strength‐oriented athletes, males, or older individuals. Second, although we attempted to verbally encourage participants to reach momentary failure in the last set, some participants volitionally terminated the set prior to failure due to discomfort with the barbell position. Additionally, failure in the barbell squat can arise from the fatigue of other muscle groups, not necessarily the quadriceps femoris, due to biomechanical characteristics of the exercise. Although we cannot rule out that this occurrence may have influenced results, the literature indicates that training to failure is not obligatory for muscle adaptations (Grgic et al., 2021), and thus confounding from this variable appears unlikely. Third, in addition to reports of shoulder discomfort due to the FS positioning, we did not consider previous experiences with the FS as a requirement for eligibility in our study, which may have confounded results. Fourth, we used a dynamic strength test and only assessed muscle thickness, a one‐dimensional imaging modality, of the vastus lateralis and vastus intermedius. Thus, results may be different when assessing strength with isometric testing or employing two‐ or three‐dimensional imaging measures, such as cross‐sectional area or muscle volume in other lower limb muscles, such as the vastus medialis and recuts femoris. Fifth, although the participants were instructed to maintain their habitual dietary intake during the training program, we did not directly control this variable and thus cannot rule out the possibility that differences in nutritional consumption may have influenced results. Sixth, we cannot extrapolate conclusions to programs that equalize TTV between conditions. Finally, we did not account for the dosage of ethynil‐estradiol and other associated factors of contraceptive pills pharmacokinetics, which conceivably may have influenced the response to resistance training.

5. CONCLUSIONS

Our findings suggest that BS training elicits greater lower limb dynamic strength on a nonspecific strength test (1‐RM 45° leg press) than FS training in recreationally trained females; however, both variations elicit similar hypertrophy adaptations among proximal, middle, and distal portions of the lateral thigh. Accordingly, coaches and practitioners who seek to maximize lower limb strength gains on a nonspecific strength test may benefit from including the BS in their training routine. In addition, squat variations can be used in a training block designed to promote homogeneous lateral thigh hypertrophy. Additionally, if a specific training block does not allow for a high training volume or the practitioner is unable to use higher loads, the FS promotes similar hypertrophy in the lateral thigh as in the BS, even with a lower total training volume and lower absolute loads.

CONFLICT OF INTEREST STATEMENT

BJS serves on the scientific advisory board of Tonal Corporation, a manufacturer of exercise equipment. The other authors declare that they have no potential conflict of interest.

ACKNOWLEDGMENTS

The authors thank the volunteers' effort during the intervention and to Coordination for the Improvement of Higher Education Personnel (CAPES – Finance Code 001) for their respective scholarships.

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