One of the fundamental exercises for lower body strength, general fitness, and rehabilitation programs is the squat. The majority of previous research on this exercise has been devoted to hip and knee movements (16,32,35), possibly because of the relative ease of measuring the movements of these joints. Research of the lumbar spine and pelvic movements in squats has been limited. Some research methodologies suggest a correct way to perform the squat (8,12), yet the technique described contains limited references to the actual movements of the lumbar and sacrum segments. The appropriate squat technique is controversial, with suggestions that the lumbar curve should be maintained throughout the squat (9,20), whereas other research instructs the subjects to maintain a “flat to arched but not rounded lumbar spine” (28). In the few cases where subjects were instructed on a lumbar position when performing the squat, the lumbar curve, or change in its position, was not actually monitored or measured during the performance. The majority of research on squat technique provides no quantified measure or change in the position for the lumbar spine when squatting (13). To ensure the safety and effectiveness of this common exercise, initially quantifying the movement of the lumbar and sacrum region is necessary.
Most research into lumbar lordosis has been conducted in the field of industrial squat lifting where the weight lifted is in front of the body (3,7,30). In this type of lifting, Burgess-Limerick recommends avoiding extreme lumbar vertebral flexion where flexion increases by >60° (7). For strength training squats, McCaw and Melrose (26) and Liebenson (20) suggest the squat should be performed to full depth as long as the lordotic curve is maintained. In all cases, the recommended posture of the lumbar curve was not substantiated by any quantification of the lumbar spine movements during the exercise. The alignment of the pelvis has also been found to influence the function of squat lifting, with an anterior tilt of the pelvis providing increased trunk muscle activity and therefore more muscular support, or ‘core stability,’ when squat lifting and lowering (10). However, the squat lifting technique is an industrial type of lift, which allows the heels to lift off the floor, and the load is held in front of the body, which is quite different from a strength training deep back squat technique.
Width of stance variations have long been used by the strength coaches and fitness trainers (13,26) as a means to provide a variation of the squat exercise. However, altering the width of stance has produced conflicting results with some studies showing no noticeable change in muscle recruitment (13,31,33) and others finding width of stance did affect muscle recruitment patterns (26), whereas Escamilla et al. (13) showed no significant differences in trunk lean between 3 different widths of stance. In all these studies, there was no evidence or discussion regarding the influence of altered width of stance on the lumbar spine and sacrum movements.
In previous squat research studies, the loads have ranged from body weight squats (1,11,32) to 1 repetition maximum (1RM) (26). Submaximal loads minimize fatigue (8), provide a repetition range similar to those common in training (14), and reduce the likelihood of changes to technique attributable to heavy loads, which allows a consistent performance across repetitions (31). Loads up to 1RM, or equal to 2.5 times a subject's body weight, have been used in studies not specifically researching on movement patterns and limb coordination.
Gender differences in the pelvic dimensions (5,25) lumbar vertebrae sizes (18), and trunk geometry (23) have been reported in the literature. Research suggests that these differences should be considered when developing biomechanical models for the lumbar spine (23), yet the influence of these gender differences on the movements of the lumbar and sacrum segments during the squat have not yet been identified.
The concentric and eccentric contractions associated with squatting break the movement into 2 very distinct phases, the descent and the ascent. Walsh et al. (34) showed differences in lumbar behavior for each phase, and Escamilla et al. (13) found differences in forward trunk lean at similar knee flexion angles in each phase. The combined movement and critical interaction of the lumbar and sacrum region however has not yet been quantified, nor the influence of the descent and ascent phases considered. Based on the previously established differences in concentric and eccentric movement, gender differences in pelvic anatomy, it was hypothesized that gender, phase, and width of stance would influence the lumbar-sacrum profile when performing the back squat. Therefore, the specific aims of this study were to firstly quantify in 3-dimensions, the timing and range of movement in the lumbar-sacrum region when performing the back squat exercise and to secondly to determine the influence of (a) 2 different widths of stance (narrow and wide), (b) gender, and (c) the ascent and descent phases.
Experimental Approach to the Problem
Thirty subjects completed 1 set of 8 back squat repetitions for 2 widths of stance; (a) narrow stance in (NS) which the width of stance was equal to anterior superior iliac spine (ASIS) width and (b) wide stance (WS) with stance twice ASIS width. Each squat was performed with an additional load equal to 50% of the subject's body weight. The time and position of the lumbar spine and sacrum were measured and analyzed.
All subjects freely volunteered to participate in the study. Subjects were informed of the experimental risks, and written informed consent was obtained under the guidelines approved by the University Human Research and Ethics Committee before any experimental testing. The subjects were either in their final year of study to become or already working as an accredited personal trainer. Subjects completed a questionnaire detailing training and medical history. Subjects were eliminated if they had not participated in regular strength training using the back squat exercises in the last 12 months or if they had any history of medical conditions related to the back and lower extremities. Subjects had been performing squats in their strength and conditioning training programs at least twice a week for a minimum of 12 months, and their relative mean (SD) squatting to body weight ratio was 123% (13.9%) for men and 93% (15.6%) for women. Subject data are shown in Table 1.
Standing height was measured to the nearest 1 mm using a portable Stadiometer (Telescopic Metal Height Scale (PE063) Mentone International, Moorabin, Australia). Using calibrated electronic scales (TI BWB-600P-Digital Personal Scale, Wedderburn Pty Ltd, Willawong, Australia) total body mass was measured to the nearest 0.01 kg. Lumbar spine, sacrum, and lower limb motion data were collected in real time at 120 Hz by a 3 Dimensional Magnetic Tracking Device (Motion Monitor, Version 18.104.22.168 Innovative Sports Training, Chicago, IL, USA). Eight magnetic sensors were placed on anatomical segments as described in Table 2. Validation of the system was confirmed against standardized reference measures, and the variation was <0.5° and within 0.0034 m. Three-dimensional magnetic tracking has been previously validated (27).
The subjects were given time to complete their usual warm-up procedure. With the sensors attached, the subjects completed 1 further warm-up set of squats with no additional load. The width of each subject's pelvis was measured between right and left ASIS, using skeletal goniometers (TTM Bone Caliper [PE054] Mentone International, Moorabin, Australia). Based on research by Escamilla et al. (14), narrow stance was defined when the inside distance between the subject's heels equaled the pelvic width, measured from right ASIS to left ASIS. Wide stance squat was defined as twice the pelvic width, measured from right to left ASIS. The width of each stance was established using a steel measuring ruler and marks placed on the floor for each individual stance setup.
To determine if stance influenced movement patterns in a safe and repeatable manner, a submaximal load of body weight + 50% (BW + 50%) was chosen. This is a common load employed by beginners and women and allowed multiple sets of 8 repetitions without fatigue (12). Submaximal loads also reduce the possibility of nonvoluntary technique changes attributed to heavy loads (31) and allowed comparison to previous studies. The capture of 3 blind repetitions for analysis occurred within the set of 8 repetitions, the first and last repetitions were excluded (31).
Foot alignment was not controlled, but in most cases in the narrow stance squat, the feet were aligned parallel to the sagittal plane, that is, toes pointed straight ahead; and in the wide stance squat, the feet were between 20° and 30° away from the midline. The 50% load was achieved by using an aluminum Olympic bar placed across the upper trapezius and the spine of scapula, with the subjects placing their hands in a palm forward grip.
Subjects assumed the squat start position unloaded while an initial reading was taken. Then, with the loaded bar and correct width of stance, descended to the deepest point they felt comfortable with and in control. Subjects were allowed to stop squatting at any time they wished, and spotters were ready to assist the subject should the need arise. The actual technique or depth of the squat were not limited or controlled, but subjects were encouraged to perform the squat in the same fashion they would do in normal training, and all subjects squatted to a depth where thighs level with below parallel. The squat was performed according to the National Strength and Conditioning Association (NSCA) guidelines on squats and monitored by the main researcher who is an NSCA certified Certified Strength and Condiitoning Specialist (CSCS) coach at the elite level. Three repetitions were collected for analysis, and subjects were blind to which 3 of the 8 performed were recorded, although the first and last repetitions were excluded. Subjects were given 120-second recovery between sets. A digital image of the squat technique is shown in Figure 1.
The magnetic tracking device measures the orthopedic axes of the lumbar and sacrum. This instrument uses a single transmitter strapped to the sacrum over L5/S1 and another transmitter strapped to the lumbar-thoracic junction over the spinous processes of T12/L1 as shown in Figure 2. These 2 sensors feed back information regarding their relative position to the world axis (lumbar and sacrum angles), and relative to each other (lumbar flexion). Lumbar flexion was measured comparing the change in relative angle between these 2 sensors during the squat. An initial positive measurement would indicate a kyphotic lumbar curve and an initial negative measurement would indicate a lordotic lumbar curve. A reduction in this angle reflects a move toward a more lordotic or increased curve in the lumbar spine and an increase in the lumbar flexion angle would indicate a move toward a flat or more kyphotic curve of the lumbar spine. The results reported show the absolute values for the starting angle and maximum angle of each variable and the timing of when those angles were achieved.
The time for each phase (descent and ascent) was normalized, with the starting point at the top of the squat (highest vertical displacement of the sacrum) represented as 0%, whereas the deepest part of the squat (lowest vertical displacement of the sacrum) represented as 100%, regardless of the phase of the movement.
A total of 6 different responses were analyzed (maximum lumbar angle; the time at which the maximum lumbar angle occurred; maximum sacrum angle; the time at which the maximum sacrum angle occurred; the maximum lumbar flexion angle; and the time at which the maximum lumbar flexion angle occurred). Each of these 6 responses were analyzed separately for differences between gender (men or women), phase (ascent or decent), and stance (WS or NS). The results (Tables 4-7) are presented as mean and standard error analyzed for gender (men or women), stance (WS or NS), and phase (ascent or decent). Intraclass correlation coefficients (ICCs) were calculated to assess the reliability of the repeated measures using the Bland and Altman method (4). Intraclass correlation coefficient values <0.4 represented poor reliability, 0.4-0.7 fair, 0.70-0.90 good, and >0.9 excellent reliability (15). One hundred percent of ICC values were in the good category (>0.7) and 68.8% in the excellent category (>0.9), showing good reliability of the data. Statistical interpretation focused on the main effects and the threshold for statistical significance was set to p ≤ 0.05. Linear mixed models were used to model the data using subjects as random effects. Models were fitted individually with 1 response and 1 explanatory variable (gender, phase, or stance) as a fixed effect. Differences were then detected using the p values from the analysis of variance F-tests on the corresponding fixed effect.
The ICC for the dependent variables is included in Table 3.
The initial angles for the lumbar, sacrum, and lumbar flexion angles for unloaded and loaded setups are presented in Table 4.
These results show the adjustment made by the subjects to the lumbar, sacrum, and lumbar flexion angles in response to the addition of the loaded bar. Men adjusted lumbar and sacrum angles more in the wide stance squat in response to the load, whereas women adjusted these angles more in the narrow stance squat. For unloaded and loaded, wide and narrow stance setup positions, men and women adjusted lumbar flexion angles so as to create a more kyphotic lumbar spine as a means of coping with the additional load before squatting.
The maximum lumbar angle and normalized time when the maximum occurred during the descent, ascent for both men and women are presented in Table 5.
The maximum sacrum angle and normalized time when the maximum occurred during the descent, ascent for both men and women are presented in Table 6.
The maximum lumbar flexion angle and normalized time when the maximum occurred during the descent, ascent for both men and women are presented in Table 7.
Lumbar and sacrum movements and the timing of when the maximums occur in these movements differ significantly across gender and width of stance. Lumbar starting angles and ranges of movement were similar across gender for each stance, but the maximum lumbar angle achieved was different intragender with both groups achieving a significantly reduced maximum angle and range of movement with the wide stance squat compared to the narrow squat. Women achieved a greater range of movement of the sacrum angle in the descent phase and the timing of this maximum also occurred significantly earlier than for men.
There were significant differences in techniques when comparing the effect of width of stance. The wider stance allowed all subjects to achieve reduced lumbar, sacrum, and lumbar flexion angles, whereas the narrow stance squat caused increased lumbar and sacrum angles and increased lumbar flexion. Men reached maximum sacrum angles sooner in the descent and later in the ascent for both widths of stance. In lumbar flexion timing, women reached the maximum position sooner in descent and later in ascent for both widths of stance. The lumbar-sacrum ratios showing contribution toward total trunk inclination in the squat are presented in Table 8.
Men showed a higher lumbar-sacrum ratio for both widths of stance. Men also increased the lumbar-sacrum ratio substantially for the wide squat compared to the narrow squat. Women achieved similar ratios regardless of width of stance. A sample of the typical movements of lumbar angle, sacrum angle, and lumbar flexion for both phases of a narrow squat for a man is shown in Figure 3.
The first aim of this study was to quantify lumbar and sacrum movements when performing the back squat exercise and subsequently determine if width of stance influenced these movements with regard to gender and phase.
The unloaded spine positions of the sacrum were similar across gender and stance width. However, the lumbar starting angles differed significantly between gender for both widths of stance and lumbar flexion angles differed between genders in the wide stance unloaded start position. Regardless of stance and starting position, the adjustment of the lumbar, sacrum and lumbar flexion angles from the unloaded to loaded start positions suggest that the human body has a means of determining the ideal posture of the spine, thus allowing it to achieve the optimal position to manage the additional load. This premise is supported by Martin and Nelson (24) who suggests that the trunk moves forward to align the combined subject and bar load vertically over the center of gravity causing the spinal curve to decrease the lordosis.
Once loaded with the 50% additional weight, men and women had similar lumbar starting angles before the descent phase, with a difference of <1.8° for the narrow and wide stance squats. Sacrum starting angles were also similar between genders in the narrow squat, but men significantly altered their starting sacrum angle for wide stance squats, whereas women maintained similar sacrum angles regardless of width of stance. Men appear to alter their sacrum angle to accommodate either structural or mechanical differences in the pelvic girdle. This significantly different sacrum starting angle between the 2 widths of stance for men results in a more posterior tilt to the sacrum for narrow squats as shown by the increased sacrum starting angle. Width of stance also had a significant difference in the starting lumbar flexion angle for both genders, with a more kyphotic lumbar curve in the narrow squat setup position than for the start of the wide squat position. This feature may be explained by the reported increase size of lumbar vertebral bodies and distance between lumbar vertebrae (18), and the narrower taller pelvis found in men (25) which may cause men to stand more upright through the trunk than women.
In performing the squats, there were significant differences in the maximum lumbar angle and resultant lumbar range of movement. Both men and women achieved a significantly smaller lumbar angle in the wide stance squat compared to the narrow squat in both phases. Although these angles are very small, this significant difference suggests that the wide squat allows the lumbar position to remain slightly more upright and not lean as far forwards as with the narrow squat. In addition, there is also a significant difference for gender in the range of lumbar motion between the 2 squats stances in the descent phase, showing that men perform the narrow stance by allowing more forward lean of the lumbar spine than in the wide stance squat. The difference in timing for the maximum lumbar angle reached by men in the ascent phase of the narrow squat may also be because of the gender differences in lumbar structure mentioned previously.
Women subjects reached the maximum forward position at a later stage of the squat descent than men and significantly later in the ascent for wide stance squats, suggesting the wider stance provides women with a greater range of flexibility in the deepest part of the squat. Furthermore, this interesting result indicates sacrum movement timing in the ascent can occur much later, creating 2 different pathways for descent and ascent in wide stance squats. This feature may be attributed to the proportionally wider more oval pelvis and shorter lumbar vertebral length of women (18,25) and the accepted increased level of flexibility of women (2,17).
Lumbar flexion ranges have been shown to change with torso inclination (22), and the greater the trunk inclination, the more likely the lumbar curvature will become kyphotic. Maximum lumbar flexion range of movement of 60° has been reported (29), whereas in the current study for both widths of stance, the start position for the lumbar spine was already in a kyphotic rather than lordotic position, with the lumbar flexion angle ranging from 5.9° to 8.5°. During the squat descent, this kyphosis increased for both genders and in both widths of stance. This is supported by Walsh et al. (34), who found that “weightlifting using a squat bar causes athletes to significantly hyperextend their lumbar spines.” In the current study, men significantly increased their lumbar flexion and nearly doubled the lumbar flexion range of movement achieved by women, a finding that is also supported by Walsh et al. To squat, it appears that men have a limited range of movement at the sacrum, and this is compensated by an increased range of movement in lumbar flexion. This decreased movement of the sacrum requires men to achieve an increased kyphosis as part of their movement pattern. This may relate to the increased level of inherent stiffness men have in the lumbar spine, compared to women (6). Women appear to maintain a greater stiffness in the lumbar flexion angle and make up for this by increased range of movement in the sacrum. This lower level of inherent stiffness in the lumbar spine may necessitate women to establish more muscular control and stability of the lumbar spine and thus keep a stiffer lumbar curve in movements such as squatting.
The timing of the different spine segments demonstrates the different coordination methods between men and women. In the descent phase, for both stance widths, men reach maximum angles of the 3 measures in the order, sacrum angle, lumbar angle, and lumbar flexion angle. In the ascent phase, men simply reversed the order. For women in the narrow stance squat, the sequence in the descent phase was lumbar flexion angle, sacrum angle, and lumbar angle. With a different pattern in the wide stance squat of sacrum angle, lumbar angle, and lumbar flexion angle. A different pattern emerged in the narrow stance ascent phase with the order lumbar angle, sacrum angle, and lumbar flexion angle. Finally, there was a different pattern again in the wide stance, with the order of lumbar angle, lumbar flexion angle, and sacrum angle. The reason for this variation found among the women is unknown and is a topic for future studies.
Lumbar-sacrum ratios were also calculated. This interaction of the lumbar and sacrum described by the lumbar-sacrum ratio shows the differences between genders for the manner in which they achieve trunk inclination during squatting. Men show a significant difference in lumbar-sacrum ratios with more lumbar movement in the descent phase and less lumbar movement in the ascent than do women. Women achieved a more similar lumbar-sacrum ratio across width of stance in the descent, whereas men decreased sacrum movement in the wide stance squat, thus increasing the lumbar-sacrum ratio. This is supported by other research studies (19,21,30) that show differences in the patterns of movement and coordination between genders. As demonstrated in the current study, the segmental coordination within the squat exercise is a complex issue, predominately as there are many joints, levers, and structural aspects that contribute to the overall movement patterns. Because of these many aspects and the differences in physique, men and women differ in the manner in which they coordinate the lumbar sacral region to perform a squat.
Previous research has not reported on squat movement pattern differences between men and women. The current research has shown that there are significant differences in the squatting behavior for men and women when compared across both WS and NS setups. In the squat exercise environment, professionals need to understand the influence that the width of stance plays on these behaviors and the different movement patterns men and women achieve under loads equal to 50% of BW. This would be most evident when viewing women in the sagittal plane where health professionals would see differences in the timing of the sequences between descent and ascent phases when considering the movements of the sacrum and lumbar spines. Further, the use of the same screening protocols for squatting movements between men and women must be questioned. The squat is commonly used as a screening tool in the practical setting and coaches and trainers should avoid comparing male squat patterns to that of women especially when considering scarum and lumbar movements and timing.
The common squat technique presented in previous research suggests maintaining a flat to semicurved lumbar spine when squatting. The current research has shown that as soon as the 50% BW load is placed across the shoulders, the lumbar curve flattens and becomes slightly kyphotic before the subject commences the descent. This change of lumbar curve is marginal, and we suggest it would be difficult to view with the naked eye. Coaches using the change in lumbar curve, or lumbar flexion angle, as a determining factor, or teaching cue for good squat technique may in fact be interfering with normal lumbar movement behavior. Further, the point at which the subject loses the lumbar curve cannot be used as a cue to determine when a person should cease the descent. We suggest that kyphosis of the lumbar spine in deep squatting is a natural part of the squat movement when using loads equal to 50% BW and coaches should not prevent experienced squatters from allowing this to happen to the small extent shown in this research.
The current research also shows that the movement pattern used by women when squatting involves a much stiffer lumbar spine position or less change in the lumbar flexion. Previous research suggests men have a stiffer lumbar spine because of the additional soft tissue support than women, and the authors suggest that women may benefit from enhanced strength training of the surrounding musculature of the lumbar spine and trunk region to provide additional stiffness to support the lumbar spine region during squat type activities. Coaches may see improved squatting abilities in women who develop supporting muscles of the trunk during the preparation phases or before each progressive increase in training loads.
Finally width of stance created several changes in squatting movement patterns across both men and women. Modifying stance width is a common exercise variation in the strength training environment, and this research has shown that the movement patterns so alter significantly with regards to the different spinal segments. Coaches need to appreciate this when altering stance width and not expect a person to perform the squat in an identical pattern of movement. This also suggests that the motor pattern developed by people who squat is different between stances and both widths of stance should be taught progressively to ensure the movement pattern is equally learned across both widths of stance.
No funding was received for this research project.
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