A Review of the Biomechanical Differences Between the... : The Journal of Strength & Conditioning Research (original) (raw)
Introduction
The squat is one of the most prevalent exercises in strength and conditioning. The movement is widely regarded as a valid and reliable measure of lower-body/trunk strength and function, and is deemed to be a fundamental process to increase maximal strength of the lower extremities (11–14,23,24,51,70,73,86). Furthermore, the squat is an effective mechanism in injury rehabilitation settings (36,55). The wide-reaching benefits of the squat are acknowledged to originate from the contributions made by the quadriceps, hamstrings, gluteal, erector, and triceps surae muscle groups to complete the movement (23,47,65). Furthermore, it is surmised that more than 200 additional muscles are used in the completion of a single squat repetition (58,77).
Although it involves numerous muscle groups, the squat, in essence, is a simple movement. To complete the squat, an individual starts in an upright position with the knees and hips near full extension; the hips are then lowered toward the ground until a desired depth is reached, and the individual then ascends back to the upright position in one continuous motion (24). In strength and conditioning, squats are typically performed in 2 ways: (a) as a front-squat, where a barbell is placed anteriorly on the shoulder and (b) as a back-squat, where the barbell is placed posteriorly to the shoulder and across the trapezius musculature (32). This review will focus on the back-squat, and more specifically, 2 different barbell positional variations; the traditional “high-bar” back-squat (HBBS) and the alternative “low-bar” back-squat (LBBS). During the traditional HBBS, the bar is placed across the top of the trapezius just below the spinous process of the C7 vertebra. Conversely, during the LBBS, the bar is placed on the lower trapezius just over the posterior deltoid, along the spine of the scapula (87).
Regardless of bar position, the back-squat requires an adequate range of motion at the hip, knee, and ankle joints. Throughout the movement, the mass of the bar applies force to the body in the sagittal, coronal, and transverse planes. To resist perturbation from this force, equal and opposite forces are applied across the 3 planes (42). The back-squat is a closed kinetic chain exercise, as the feet are anchored to the ground throughout the movement. This is in comparison to an open chain kinetic exercise where peripheral segments are allowed to move in free space (i.e., leg extension exercise) (25,74,76). Closed kinetic chain exercises tend to enable a higher degree of joint motion and an increase in muscle recruitment and are therefore thought to replicate athletic tasks better than open kinetic chain exercises (25,64,69,71,75,89). Thus, the muscles used in a variety of different sports can be developed by using the squat in training. For instance, the back-squat is commonly included in the strength and conditioning programs of competitive Olympic weightlifters and powerlifters. In particular, the HBBS is commonly used in Olympic weightlifting training to simulate the catch and recovery stages of the Olympic weightlifting competition lifts; the snatch and clean and jerk (87). The HBBS is defined by an upright torso (4,28,87) and a knee flexion resulting in a “deep” (hips close to the ground, with the crease of the hips well below the level of the knee) squat depth (34,39,44,52,80), such as is displayed at the catch position of both the snatch and clean. The LBBS may have the potential ability to allow for greater loads to be lifted (59), and it is for this reason that the LBBS is commonly used in competitive powerlifting (where the back-squat is one of the 3 competition lifts). The potential to lift greater loads could be due to the maximization of posterior displacement of the hips and increased force through the hip joints in comparison to the knee joints (80). This maximization of the posterior displacement of the hips may manifest as greater engagement and activity of the larger hip muscles (i.e., gluteal muscles). Squat training with maximal effort also enhances powerful movements such as jumping and sprinting (17,48,49,84,90). Endurance-based sports also routinely incorporate the back-squat into their trainings. The back-squat and heavy resistance training in general has been shown to improve aerobic endurance and movement economy in sports such as cycling, running, and cross-country skiing (2,38,60,78,79). Moreover, the back-squat is also commonly used in team sports (e.g., handball, football, and rugby) to facilitate improvements in performance, where a combination of strength, power, endurance, and sprint ability is often required (10,37,66,86).
As alluded to, the LBBS may result in an ability to lift greater loads in comparison to the HBBS. Differences in bar position between the HBBS and LBBS result in an altered center of mass (80). Therefore, different movement strategies are used to ensure that the center of mass remains within the base of support to maintain balance during the execution of these lifts, which will be covered in this review. These movement strategies manifest as differences in joint angles of the lower-body kinetic chain, vertical ground reaction forces (_F_v), and the activity of key muscles.
Although a variety of sports use the back-squat in training, little is known as to why the LBBS may enable greater loads to be lifted. The purposes of this review are to (a) provide a summary of prior kinematic, kinetic, and muscle activity research on the HBBS and LBBS; (b) examine whether or not the LBBS enables greater loads to be lifted; and (c) hypothesize why this might be the case. This review will present current literature in each of these categories for both the HBBS and LBBS to allow educated decisions to be made by practitioners concerning exercise prescription and the optimal style of back-squat for different sport specific applications.
Methods
Definition of Terms
Many authors examining the squat use different terminology when describing their study's experimental procedures. Therefore, definitions of these terms are vital to the clarity of this review. Where authors did not use the same definitions for variables such as the specific terms HBBS and LBBS, their raw (unprocessed) data were used to derive the variables as defined in our review. Thus, for the purposes of this article, a “high-bar” squat is synonymous with the “traditional” squat and “Olympic” squat, whereas a “low-bar” squat is synonymous with a “powerlifting” squat. A “squat” is synonymous with a back-squat and is not to be confused or compared with other squat variations that use different bar positions or loading modalities. An analysis of squat styles besides HBBS and LBBS is outside of the scope of this review, but for more information on other squat styles, the reader is referred to texts by Delavier (16) and Newton (56). Furthermore, all results that are presented in this review are based on “un-equipped” or “raw” lifters. That is to say, those lifters that do not perform the squat with external assistance such as squat suits or elastic knee wraps (5,22,35).
Search Parameters and Criteria
PubMed, SPORTDiscus, CHINAHL, MEDLINE (EBSCO), and Scopus electronic databases were searched online up to March 2017 (Figure 1). The following strings of keywords were arranged and searched in each database: (a) squat AND kine* OR exercise OR biomechanics OR weight, (b) squat AND kine* AND knee OR barbell, (c) squat AND force OR load, and (d) squat AND emg OR activ* OR electro* OR muscle. The search strategy used limited database results to academic journals, reviews, dissertations, and human subjects when applicable.
Study exclusion and inclusion process.
Inclusion criteria for this review comprised articles that included (a) healthy (showing no symptoms of sickness, able to take part fully in the study); (b) resistance-trained (≥6-months experience); (c) adults (≥18 years); and (d) provided one of the following variables: hip, knee, or ankle joint angles, _F_v or lower extremity electromyography (EMG) during a squat.
Articles were excluded if (a) they were not available in English; (b) the full text was not available; (c) male and female subjects were not separated; or (d) comprised a case study, a poorly designed cohort/case-control study, anecdotal evidence, animal research, laboratory-based research, or unpublished clinical observations (i.e., levels of clinical evidence and study design consisting of a score of 4 or 5 as adapted from the Oxford Center for Evidence-Based Medicine) (54). Only full text sources were included so that methodology could be assessed. Finally, a comprehensive hand search of article reference lists and citation tracking on Google Scholar were used to identify any additional relevant articles. In total, 41 studies were included in this review.
Study Characteristics
Physically active and healthy (i.e., showing no symptoms of sickness, able to take part fully in the study) individuals, from a mixture of sports including both individual and team-based sports, and from different levels of competition, comprised the study participants that were included in this review (Tables 1–7). The total mean age, body-height, and body-mass in the included studies were 28.6 ± 27.1 years, 177.7 ± 6.1 cm, and 80.1 ± 12.6 kg, respectively.
Findings
Kinematics
Hip
The back-squat is performed by the simultaneous flexion or extension of 3 key joints (e.g., hip, knee, and ankle) known as the lower-body kinetic chain (61). The resultant angle between the trunk and the thigh is synonymous with the names hip, trunk, and torso angle. A difference in trunk angle manifests as either a greater forward lean (i.e., a reduced trunk angle) or a more upright orientation of the torso, relative to the thigh (i.e., an increased trunk angle). Authors of previous research specifically comparing the HBBS to the LBBS have shown that the LBBS is defined by a smaller absolute trunk angle, and therefore, greater forward lean (4,28,87). This forward lean effectively maximizes the posterior displacement of the hips, and therefore, increases the force placed on the hips in comparison to the knee joints. Thus, there may also be a decreased moment arm when placing the bar lower on the back, which may attribute to the ability to lift larger loads. There may also be an increase in stability and potential decrease in stress placed on the lumbar region and ankle compared with the HBBS (67,80). These factors may contribute to understanding why the LBBS might allow for greater loads to be lifted. However, these joint angle results are not definitive, and there are mixed results in the literature for the size of HBBS and LBBS trunk angles at peak hip flexion (19,26,27,33,39,44,46,52,53,80) (Tables 1 and 2). These discrepancies in joint angles may result from differences in participant age, training experience, strength, anthropometry, or the presented joint angle (i.e., presenting unprocessed segment angles or specific joint angles known as “absolute” or “relative” angles respectively) (Figure 2).
HBBS peak hip flexion.*†
HBBS peak hip flexion.*†
LBBS peak hip flexion.*†
Relative and absolute joint angles of the hip, knee, and ankle.
Last, it is common for Olympic weightlifters and powerlifters to wear special weightlifting/squat shoes/boots when performing the back-squat (41,67,68,72). These shoes are characterized by designs incorporating a raised heel, usually of ∼2.5 cm in height, and stiff noncompressible soles with a reinforced outer sole. The raised heel present in weightlifting shoes has been shown to reduce overall trunk lean during the back-squat compared with barefoot or running shoes (46,67,68,72). This may be attributed to increased stability, as the added heel height allows for a lifter to reach depth while requiring less dorsiflexion range of motion and thus a more vertical alignment of all segments during the lift, attributing to greater balance and resistance to tipping forward (68).
Knee
In competitive powerlifting, there are regulations that each lifter must comply with in order for each lift to count toward their total (40). One such regulation pertaining to the back-squat is that sufficient “depth” must be reached. That is, there must be sufficient flexion of the knees and lowering of the hips toward the ground, so that “the top surface of the legs at the hip joint is lower than the top of the knees” (40). As a result, it is common for powerlifters to replicate this required depth in training. In Olympic weightlifting, the back-squat is not a competition lift, and therefore, in training back-squat depth is commonly modeled after the catch position of the snatch and clean and jerk. This often manifests as a deeper back-squat, with greater knee and ankle flexion.
There are apparent differences in knee joint angle between the HBBS and LBBS, resulting from differences in required depths. The HBBS can be defined as a “deeper squat,” with greater knee flexion at maximum depth (70–90°), in comparison to the LBBS (100–120°) (19,26,27,33,34,39,44,52,80,83) (Tables 3 and 4). However, there are some studies which have reported the opposite (33,44,80). This may have resulted from the experience of the participants in the case of Kobayashi et al. (44) and Hales et al. (33) and the fact that Swinton et al. (80) studied powerlifters (who typically perform the LBBS) complete the HBBS and LBBS. Legg et al. (46) also showed that wearing weightlifting shoes, in comparison to running shoes, results in a deeper squat. This may result from the increased stability provided from the noncompressible soles.
HBBS peak knee flexion.*†
HBBS peak knee flexion.*†
LBBS peak knee flexion.*†
Ankle
Currently, only 9 studies have recorded ankle joint angle data, 7 from the HBBS only (27,44,46,67,72,85,86), one from the LBBS only (33), and one from both the HBBS and LBBS (80). One study also looked at the ankle segment angle in Olympic weightlifters and powerlifters (28). These studies show similar results for the HBBS ankle joint angle across studies; however, there are mixed results for the LBBS (Tables 5 and 6). Whitting et al. (85) showed that wearing weightlifting shoes while performing the HBBS results in a significantly lower peak dorsiflexion angle than when wearing running shoes. Further research is warranted to provide definitive differences between the HBBS and LBBS.
HBBS peak ankle flexion.*†
HBBS peak ankle flexion.*†
LBBS peak ankle flexion.*†
Kinetics
In the back-squat, the resultant ground reaction force, and load on the lower extremity is influenced largely by the position of the upper body because of its larger mass (8). As the load of the back-squat is increased, the resulting _F_v also increases in a proportional fashion (27,43,91) during both the concentric and eccentric phases of the movement (20,21). In addition, the cadence at which the back-squat load is lifted may also affect the magnitude of _F_v produced. A faster cadence will result in a shorter repetition duration in comparison to a slow cadence. Completing a back-squat repetition with an intentionally fast cadence has been shown to result in a larger _F_v, than when performing a repetition of the same weight at a slower cadence (3). However, there is also evidence to support no significant difference (45).
In 2 of the 5 studies specifically comparing the HBBS with the LBBS, differences were recorded and reported in _F_v; all of which used force platforms to record _F_v data. Several other studies also used linear position transducers; however, these were to measure bar velocities and not _F_v. Swinton et al. (80) reported that both the HBBS and LBBS produced similar _F_v profiles (no significant differences) across all loads (percentages of LBBS 1 repetition maximum [1RM]). In addition, as load increased, the _F_v time curve showed a drop off, followed by a second peak in the concentric phase. This second peak is expected with an increase in load and represents the force produced overcoming the “sticking” point or region after the initial “drive” out of the bottom of the squat (82,83). Goodin (29) compared the HBBS with the LBBS with loads up to 90% of each participants HBBS 1RM. The HBBS produced larger peak force with loads of 20–80% 1RM, larger peak power with loads of 20–60% 1RM and 80–90% 1RM, greater total work with loads of 20%, 40%, and 60–90% 1RM, as well as greater peak velocity and vertical displacement at all loads. Although all participants in this study were experienced in both the HBBS and LBBS, up to 90% of each participants' HBBS 1RM was used as the comparison load for both squat styles. Therefore, variation between each participants' HBBS and LBBS 1RMs may have resulted in the HBBS results being better, as the effort required at each percentage of 1RM may have differed compared with if it was the same percentage of their LBBS 1RM. However, the LBBS produced greater impulse at 30–90% 1RM than the HBBS.
As shown in the knee joint angle section of this review, the HBBS is typically defined as a “deeper squat,” with greater knee flexion at maximum depth. A study by Dali et al. (15) showed that as depth increases from a semi-squat (40° knee flexion) to a half-squat (70° knee flexion) to a deep-squat (110° knee flexion), there is significant increases in peak _F_v as depth increases between each level of squat. In addition to differences in squat depth, the HBBS and LBBS are typically characterized by different stance widths. Although there are no limits on the stance width of either back-squat variation, the LBBS is typically performed with a stance wider than shoulder width (97%–183% of shoulder width) (24), and the HBBS is typically performed at shoulder width (7). Swinton et al. (80) analyzed the back-squat at different stance widths, and showed that both a typical HBBS stance (shoulder width) and a wider powerlifting style stance produced a larger peak _F_v when compared with a back-squat performed to a box at the same load, without significant differences between the stances. Swinton et al. (80) recorded _F_v over the movement as a whole, without distinctly measuring peak _F_v for each phase. In the back-squat, however, the largest active forces are often found during the eccentric, lowering phase because of the force required to break the momentum of the mass, and come to a stop before transitioning into the concentric phase. It may then be fair to assume that the results of Swinton et al. (80) show a lower peak _F_v for the box-squat than the HBBS and wider powerlifting style back-squat because of the participants intentionally lowering themselves to the box more slowly, to avoid a heavy “landing” on the box before trying to ascend back to an upright position. The external stimulus of the box makes the box-squat a very different movement, and difficult to directly compare with the HBBS and wider powerlifting style back-squat. However, in contrast to these results, McBride et al. (50) showed a back-squat to produce a significantly smaller peak _F_v than a box-squat at 70% back-squat 1RM (p ≤ 0.05). Comparable peak _F_v results (no significant differences) were observed between the back-squat and box-squat at 60% and 80% 1RM also in this study.
In summary, currently, there are few known differences between the HBBS and LBBS in ground reaction forces. The potential for the LBBS to enable greater loads to be lifted may be attributed to the joint angles specific to the movement, and a shortened moment arm. These factors may result in a mechanical advantage. That is, greater loads may be lifted through a more effective application of force through the bar, and increased torque at the hip. The results above show limited differences between the HBBS and LBBS in _F_v, and therefore, indicate that _F_v alone may not portray these mechanical differences effectively. Mechanical advantage by the LBBS will be characterized by a greater load lifted, while producing a similar force to the HBBS. Therefore, the analysis of joint angles or muscle activity may be more appropriate to determine whether and why an LBBS enables heavier loads to be lifted. Further research should compare the HBBS with the LBBS across a full range of loads, to create a full profile and understanding of _F_v differences between the 2 back-squat variations. Strength and conditioning coaches for a range of sports may then make informed decisions as to the most appropriate style of squats to produce a stimulus applicable to their athletes (Table 7).
HBBS and LBBS kinetic results.*†
HBBS and LBBS kinetic results.*†
Muscle Activity
Study of the back-squat muscle activity typically focuses on the HBBS (1,6,26,31,57,62,63), with only 2 studies specifically analyzing the activity of the LBBS (51,87). The LBBS is characterized by a greater forward lean at the trunk in comparison to the HBBS (59). As forward lean increases, it has been shown that the lumbar erector spinae muscle activity also increases (81). In addition, because of the wider stance width that is common for the LBBS (7,24), a different EMG profile arises from this squat variation. Escamilla et al. (26) observed a significantly larger EMG amplitude in the gastrocnemius during narrow stance squatting compared with wide stance squatting. Furthermore, McCaw and Melrose (51) compared the activity of the rectus femoris, vastus medialis, vastus lateralis, adductor longus, biceps femoris, and gluteus maximus during the parallel LBBS at different stance widths and bar loads. They observed no change in quadriceps activity with a different stance width, but muscle activity was higher in the adductors and gluteus maximus with a wider stance. Anderson et al. (1) observed similar outcomes as McCaw and Melrose (51), finding no significant differences in the EMG of the vastus medialis and vastus lateralis with a change in stance width. However, Anderson et al. (1), did not assess gluteus maximus activity. The resultant increase in gluteus maximus activity from a wider stance width was also shown by Paoli et al. (62) during the performance of the HBBS, and in a review by Clark, Lambert, and Hunter (9). The authors propose that the lack of change in quadriceps muscle activity during different stance widths, results from similar muscle lengths in both stances. On the other hand, longer muscle lengths, and anthropometric influences may explain the increase in adductor and gluteus maximus activity as stance widens. However, the higher EMG in the gluteus maximus with increased stance width was only observed with high loads, which draws into question whether an increase in muscle activity results from increases in load or changes in muscle length. Contrastingly, Wretenberg et al. (87) found a significant difference in the EMG of the rectus femoris (p ≤ 0.05) during the performance of the LBBS by 6 powerlifters who typically use a wider stance, when compared with the HBBS as performed by 8 Olympic weightlifters. Two other muscles in this study, the vastus lateralis and rectus femoris, also presented larger EMG magnitudes; however, these results were nonsignificant. The EMG amplitudes of this study for both groups were normalized to a 3-second static contraction at 65% 1RM (LBBS for powerlifters or HBBS for Olympic weightlifters).
As described in the joint angles section of this review, the differences in squat depth between the HBBS and LBBS can influence the corresponding muscle activity. Wretenberg et al. (88) showed that in the vastus lateralis, rectus femoris, and long head of biceps femoris, muscle activity generally increases with depth, from 45° to 90° knee flexion and to “parallel” where the posterior borders of the hamstrings muscles are parallel to the floor. These results are further supported by Ninos et al. (57) who showed significant increases in the activity of the vastus medialis and vastus lateralis with increased depth in the squat (10–60°), but also no significant increases in muscle activity in the biceps femoris, semimembranosus or semitendinosus muscles. Wretenberg et al. (88) observed no further increase in muscle activity with a deep-squat past parallel (knees maximally flexed). This may imply that for the specific purpose of training the quadriceps, parallel squats could be sufficient, without a benefit from performing deeper squats. Similarly to Wretenberg et al. (88), Gorsuch et al. (31) showed that squatting to a deeper knee angle of 90° increased the activation of the rectus femoris and lumbar erector spinae muscles more so than when squatting to a depth of 45° (absolute angle, Figure 2). Increasing muscular activity of the thigh with greater depth and standardized neutral hip position (external rotation at 30° or 50° external rotation) is also supported by Pereira et al. (63) who showed that the thigh musculature was most active in the bottom 30° of the back-squat, in both the eccentric and concentric phases. Furthermore, a review by Clark et al. (9) showed no variation in muscle activity to different depths at moderate loads, only at high loads.
However, Caterisano et al. (6) demonstrated no change in activity in the vastus medalis, vastus lateralis, or biceps femoris with increased squat depth; instead, they observed EMG activity changes only in the gluteus maximus. The premise that performing squats past parallel only serves to increase the EMG activity in the gluteus maximus rather than the quadriceps is also supported in a review by Schoenfeld (69). A greater gluteal muscle activity in the HBBS may also be possible when compared with the LBBS because of the impact of hamstring active insufficiency. Throughout the HBBS, the hamstrings act to extend the hip and maintain an upright torso position. However, with increasing depth, the hamstrings reach a point of active insufficiency where they are unable to produce force because of their shortened length, with respect to the length-tension relationship (30). As the ability of the hamstrings to apply force decreases, there is a possible increase in gluteal muscle activity as a compensation strategy. For an in-depth analysis of the key muscle activity strategies throughout a bodyweight squat, refer to Dionisio et al. (18).
Drawing on the present literature, it is difficult to conclude what differences exist in muscular activity of the lower-body between the HBBS and LBBS. However, the differences that do exist seem interrelated with the associated depths typically displayed in each squat variation. Further research is required to provide an authoritative text on the matter. It is acknowledged, however, that this research may be difficult to produce because of the great variation in athletes even within a study. We also acknowledge differences in the muscle activity signal processing used in the data presented in this review. Refer to Table 8 for a summary of the different processing methods used in this review.
HBBS and LBBS muscle activity processing and units.*
In summary, previous research has demonstrated that in comparison to the HBBS, the LBBS results in increased erector spinae muscle activity because of the increased forward lean, increased activity of the adductors and gluteal muscles, and reduced gastrocnemius activity from a wider stance width. Larger loads can be lifted during the LBBS, which results in a greater overall muscle activity, and potentially larger increases in strength and muscle size. However, those looking to induce anterior kinetic chain, and quadriceps specific adaptations, may be advised to perform the HBBS.
Practical Applications
The purpose of this review was to examine the literature relating to the HBBS and LBBS to improve understanding of why the LBBS enables greater loads to be lifted. Three distinct categories were explored to further this understanding; joint angles, _F_v, and muscle activity. This review showed that in the present literature, _F_v studies yielded few known differences between the HBBS and LBBS. However, several potential differences can be observed in joint angle and muscle activity literature on the topic. In fact, the answer to why the LBBS may allow for greater loads to be lifted can be found in the joint angle differences between each squat. Specifically, the LBBS is presented with a greater forward lean and reduced knee flexion (i.e., reduced depth). This results in greater posterior displacement of the hip, and a maximization of the associated force-producing ability. Such displacement of the hip engages the stronger posterior hip musculature (i.e., gluteal, hamstring and spinal erector muscle groups), as supported in this review though analysis of muscle activity studies on each back-squat variation. By contrast, the HBBS presents with greater activation of the anterior thigh musculature (i.e., quadriceps). With knowledge of this, practitioners seeking to specifically develop either the anterior or posterior musculature can make an informed decision regarding the type of back-squat prescribed. Furthermore, this review showed that because of its joint angles, the HBBS appears to be more suited to those athletes who are required to strengthen and replicate movements that exhibit a more upright torso, such as the Olympic weightlifting competition lifts.
At present, a comparison of the kinematics, kinetics, or the muscle activity of both back-squat variations has not been completed above 90% 1RM. To create a full profile of the differences between each squat style, future research should be performed up to and including 100% 1RM. This will allow for a full range of loads to be assessed, and the effects of maximal effort on the joint angles, _F_v, and muscle activities associated with the HBBS and LBBS. Moreover, factors such as anthropometry, sex, age, strength, and experience should be taken into consideration. Future research should look to include analysis of joint angles, _F_v, and muscle activity together. Each variable alone is unable to provide a complete picture of each back-squat variation. Instead, the combination of kinematic, kinetic, and muscle activity is necessary to create a full profile.
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Keywords:
joint angles; ground reaction forces; EMG; powerlifting; Olympic weightlifting
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