1. Introduction
The back squat is a fundamental resistance/strength training exercise, and one of the most frequently utilized movements for the development of lower extremity strength and power [
1]. As a multi-joint exercise, it is uniquely able to activate the quadriceps, hamstrings, gluteus, tibialis anterior, gastrocnemius, soleus and lumbar musculature [
2] and, owing to being a closed chain kinetic exercise, it is also frequently exploited in rehabilitation environments [
3]. Due to its popularity and structural association with performance in a plethora of athletic environments, the squat has garnered considerable research interest within the strength and conditioning literature. There are multiple variants of the squat, and a range of technique manipulations can be made in order to mediate different mechanical outcomes and training stimuli [
4].
The commonly explored and adopted adaptations and manipulations include: the squat depth, stance width and foot placement angle [
5]. The different types of executions may influence the biomechanics of the squat; thus, specific variations in the squat techniques may be adapted and manipulated in order to achieve more favorable training outcomes in distinct athletic groups. However, regarding the specific influence of the stance width, controversy exists regarding the employment of specific muscle groups, with anecdotal and lay texts supporting the notion that a narrow stance width increases the quadriceps muscle recruitment [
6].
Specifically, the stance width has received relatively limited research attention, although recent publications in this area indicate that it is becoming a more prominent aspect of discussion in the strength and conditioning literature. Varying squat stance widths are utilized according to the athlete’s morphology and preferences [
7], although the effects and efficacy of one stance over another is not well established. McCaw & Melrose [
8] examined lower extremity muscle activation when performing the barbell back squat, with low and high barbell loads using stance widths of 75 and 140% shoulder distance. Their observations showed that the muscle activity in the gluteus maximus and adductor longus muscles was significantly larger in the wide stance condition. Escamilla et al. [
5] examined three-dimensional joint kinetics and kinematics in a sample of national level powerlifters separated into narrow, medium and wide groups, based on their self-selected stance width. Their observations revealed that the hip was significantly more flexed, and the thigh was more horizontal, in the wide and medium condition compared to the narrow. In addition, the analysis of the joint moments showed that the knee extensor and ankle plantar flexor moments were significantly larger in the medium and wide conditions compared to the narrow. Escamilla et al. [
7] investigated the effects of the stance width on the electromyographic activity of eight superficial thigh muscles during the leg press and barbell back squat exercises. Their findings show that the hamstring and gastrocnemius muscle activation was significantly greater in the wide squat compared to the narrow position. Similarly, Paoli et al. [
9] examined the influence of the squat width on the lower extremity muscular activation, and their results indicate that the gluteus maximus activation was significantly greater in the wide squat condition. Sogabe et al. [
10] explored the power production measured at the barbell using stance widths of 50, 100, 150 and 200% shoulder distance. Their findings show that peak power was significantly greater in the 150% shoulder distance, in relation to the other conditions. Finally, Lahti et al. [
11] examined the kinematics, ground reaction forces (GRF) and sagittal/frontal plane hip and knee moments during wide (equal to 1.5 * greater trochanter width) and narrow (equal to 1.0 greater trochanter width) barbell back squats. The results show firstly that the knee flexion angle was statistically larger in the narrow stance width and that the hip-to-knee joint extension moment ratio was significantly greater in the wide stance. In addition, the angle of the ground reaction force vector relative to the horizontal axis was shown to be significantly reduced in the wide stance width condition.
1.1. Rationale
Despite the aforementioned scientific outputs concerning the effects of the stance width on the biomechanics of the barbell back squat, there has yet to be any scientific investigation that has concurrently examined the effects of the lifters’ habitually adopted stance width on the kinetics, three-dimensional kinematics and muscle forces, nor has there been any exploration of the effects of manipulating the stance width on these important parameters. Therefore, an exploration of the aforementioned areas may provide further insight regarding the effects of technique manipulations on the biomechanical outcomes during the barbell back squat that may be important for coaches and strength and conditioning practitioners seeking to promote distinct training stimuli.
1.2. Aims
As such, the aims of the current investigation are twofold. Firstly, experiment one aims, using a between-subjects’ design, to comparatively examine the effects of different self-selected stance widths on the kinetics, three-dimensional kinematics and muscle forces during the squat. Secondly, using a repeated measures study design, experiment two aims to explore the effects of manipulating the stance width on the same biomechanical parameters.
1.3. Hypotheses
The current investigation tests the hypotheses that both experiments will observe significantly lower resultant GRF vectors and statistically larger medially directed GRF’s and posterior chain muscle forces in the wide stance condition.
4. Discussion
The aims of the current investigation were twofold. Experiment one comparatively examined the effects of different habitual stance widths on the kinetics, three-dimensional kinematics and muscle forces during the squat and experiment two explored the effects of manipulating the stance width on the same biomechanical parameters. To the best of the authors’ knowledge, this represents the first investigation to explore the aforementioned aims and may therefore provide further insight to coaches and strength and conditioning practitioners seeking to maximize training adaptations.
Importantly, the findings from experiment two revealed that the peak power output was significantly larger in the NARROW stance width condition in relation to the MID and WIDE conditions. This observation does not concur with that of Sogabe et al. [
10], who found that the peak power output was significantly greater at the 150% shoulder distance. It is possible that the distinction between the studies relates to the approach that was adopted to quantify the peak power output during the squat, as Sogabe et al. [
10] utilized an external dynamometer attached to the barbell. It is proposed that this observation is the cumulative product of the increased peak vertical GRFs and increased bar velocity that were also observed in the NARROW stance width condition. Notably, recent evidence has shown that, in order to optimally enhance power and performance in explosive athletic tasks, it is most effective to train with the resistance and exercise modality that maximizes the mechanical power output [
19]. The findings from experiment two therefore suggest that the manipulation of an athlete’s set-up during the squat exercise to a NARROW stance width may improve the training stimuli necessary to mediate improvements in sports movements necessitating high mechanical power outputs.
In agreement with our hypotheses, the findings from experiments one and two show that both angle of the resultant GRF vector, as well as the impulse of the medial GRF in the ascent and descent phases of the squat, were significantly influenced as a function of the stance width. Specifically, the angle of the GRF vector was larger in the NARROW stance and the medial GRF impulse the greatest with a wider stance. These observations concur specifically with those of Lahti et al. [
11], who also found increased medial GRFs and a reduced angle of the resultant force vector to the horizontal in the wide stance condition. It is proposed that these observations were mediated as a function simply of the different stance widths being examined across both experiments. As the resultant GRF vector initiates at the center of pressure and orients towards the body centee of mass, this finding relates to the altered position of the center of pressure relative to both the center of mass of the body in the WIDE stance, with conditions which served to enhance the magnitude of the medially directed GRFs. In relation to the distinct mechanical stimuli mediated through the increased medial GRFs, Lahti et al. [
11] proposed that greater medial forces during the squat are able to improve proficiency in athletic disciplines and movements necessitating rapid changes of direction. Nagahara et al. [
20] markedly showed that the impulse of the medial GRF was associated with enhanced sprint performance and plays a key role in enhancing propulsive performance. Therefore, this study indicates that individuals habitually adopting a WIDE stance width, and those manipulating their stance width to a wider position during the squat, may improve their performance in sprinting and rapid change-of-direction activities.
It is noteworthy that both experiments one and two also found the lower extremity muscle forces to be significantly influenced by the stance widths that were examined in this study. Specifically, it was revealed in experiment two that the NARROW squat condition mediated statistically increased quadriceps muscle forces, whereas, in support of our hypotheses, both experiments showed that the WIDE stance width condition was associated with significantly greater gluteus maximus, hamstring gastrocnemius and soleus forces. When also taking into account the observations from both experiments relating them to the joint energy production, this study indicates that the NARROW condition appears to arbitrate a knee-dominant squat strategy that targets the anterior chain musculature, whereas the WIDE condition produced a hip-dominant strategy [
16] and targets the posterior chain musculature more effectively. This observation concurs with the electromyographic investigations of Escamilla et al. [
7], McCaw & Melrose [
8] and Paoli et al. [
9], who found that wide-stance squat positions recruited the posterior chain musculature to a significantly greater extent. Furthermore, our observations in relation to the joint dominance are also supported by the examination of the joint moments by Lahti et al. [
11], who showed that the hip-to-knee joint extension moment ratio was significantly greater when adopting a wide stance. Physiological analyses have demonstrated that the muscle forces are the primary mechanism responsible for the initiation of hypertrophic adaptations within the skeletal muscles [
2], and also that the total muscle cross-sectional area is the central determiner of peak muscle force production [
21]. Therefore, as the stimuli imposed by resistance training regulates the extent to which the skeletal muscles’ adaptive responses are made [
22], this indicates that the utilization of a NARROW stance width may be advisable in athletes/coaches seeking to maximize quadriceps development, but that the utilization/manipulation of the stance width to a WIDE position appears to be the most effective mechanism to stimulate gluteus maximus, hamstring, gastrocnemius and soleus muscular development.
A limitation to the current study is that it represents an acute exploration of the effects of habitual and manipulated stance widths on the biomechanics of the squat. Whilst this investigation does represent an extension of the current literature base as concurrent kinetic, three-dimensional kinematic and muscle force indices are examined, it remains unknown as to whether alterations in stance width are able to mediate alterations in athletic performance indices. As such, a randomized and controlled intervention study is clearly warranted in order to explore the effects of manipulating the stance width of the squat on the performance-based outcomes in sport.
5. Conclusions
To conclude, although the influence of the stance width during the squat has been investigated previously, this study provides new information to the scientific literature by providing a two-experiment evaluation concerning the effects of habitual and manipulated stance widths on the kinetics, kinematics and muscle forces of the squat. Importantly, experiment two showed that the NARROW stance width significantly enhanced the peak power production during the squat and also enhanced the quadriceps forces. Both experiments also showed that the WIDE stance width significantly increased medial GRF impulses and mediated significant increases in the gluteus maximus, hamstring, gastrocnemius and soleus muscle kinetics. The NARROW condition may be able to mediate improvements in the high mechanical power movements and to promote the quadriceps muscle development. The increased medial GRFs suggest that greater stance widths may improve sprint and rapid change-of-direction performance and promote gluteus maximus, hamstring, gastrocnemius and soleus hypertrophy. As such, whilst it appears that there is not an optimal stance width, the observations from this study can be utilized by both strength and conditioning practitioners seeking to utilize and maximize training adaptations.