Biomechanical comparisons of back and front squats with a straight bar and four squats with a transformer bar

Abstract

The purpose was to quantify trunk and lower extremity biomechanics among back and front squats with a straight bar and four squats with different anterior-posterior load placements imposed by a transformer bar. Ten males and eight females performed six squat conditions: back and front squats with a straight bar, back and front squats with a transformer bar, and squats with more posteriorly or anteriorly placed loads with a transformer bar. A constant load of 70% of the participant’s one-repetition maximum in the straight-bar front squat was used. Kinematic and kinetic data were collected to quantify joint biomechanics at an estimated parallel squat position in the descending and ascending phases. Squats with more anteriorly placed load significantly decreased trunk flexion and pelvis anterior tilt angles with large effect sizes but increased low-back extension moments with medium to large effect sizes. Hip, knee, and ankle extension moments were generally similar among most squat conditions. Participants adjusted their trunk and pelvis to mediate the effects of load placements on low-back and lower extremity moments. While lower extremity loading was similar among different squats, the differences in trunk and pelvis angles and low-back moments should be taken into consideration for people with low-back impairment.

Introduction

Resistance training, when performed appropriately, may improve muscle strength, cardiovascular function, bone mineral densities, and sports performance and lower fall risk (Fragala et al., 2019; Lloyd et al., 2016). One common exercise for lower body strength training is the squat. Squats are convenient and allow an effective transfer of strength gain to activities with similar neuromuscular demands (Wilson et al., 1996; Wirth et al., 2016). Previous studies have extensively examined the effects of load magnitudes, squat depths, and squat techniques on low-back and lower extremity loading to identify optimal training strategies and minimise injury risk (Hartmann et al., 2013). While greater loads and depths generally increase lower extremity joint moments and muscle activities (Bryanton et al., 2012; Cotter et al., 2013), greater trunk flexion could decrease knee moments but increase low-back and hip moments in back squats (Fry et al., 2003).

Two popular forms of squats are the back and front squats with a straight bar. In the back squat, the bar is placed across the shoulders on the upper trapezius with abducted shoulders and flexed elbows for bar stabilisation. In the front squat, the bar sits on the front of the deltoids with flexed shoulders and elbows. Both squats mainly target back, hip, knee, and ankle extensors (Bird & Caswy, 2012), but the different load placements in the anterior-posterior direction have attracted interests from researchers. However, previous studies have observed inconsistent findings in joint moments and muscle activities between the two squats (Braidot et al., 2007; Comfort et al., 2011; Gullett et al., 2009; Korak et al., 2018; Russell & Phillips, 1989; Yavuz et al., 2015). These discrepancies could be due to different testing populations and load magnitudes (relative loads vs. absolute loads). Another possible explanation is the relatively small difference in load placements between the back and front squats, allowing participants to adjust their postures to impose similar external loading to the low back and lower extremities. The effect of a greater difference in anterior-posterior load placements on squat biomechanics was demonstrated by a previous study (Hecker et al., 2019). These investigators quantified lower extremity muscle activities and trunk and hip flexion angles when powerlifters performed back squats with a straight bar or a safety bar at 75% of their respective three-repetition-maximum (3-RM). The safety bar had two forward-facing handles and a more anteriorly placed load. While the straight-bar squat demonstrated a greater 3-RM and increased lower extremity activities, the safety-bar squat decreased trunk and hip flexion angles. However, this study did not analyse low-back or lower extremity joint moments and was limited to one condition of more anteriorly placed loads. The effects of more anteriorly or posteriorly placed loads on squat biomechanics, and how they may differ from the back and front squats with a straight bar need further investigation.

A recently developed transformer bar has been advocated by its manufacturer for mimicking different squat variations, such as the back, front, kettlebell, and cambered squats, due to its adjustments allowing various anterior-posterior load placements (Kabuki Strength, 2020). Additionally, it has two front handles allowing bar stabilisation with shoulders closer to the neutral position, requiring less shoulder mobility and upper extremity involvement. Examining the effect of load placements imposed by a transformer bar could provide additional options for squat exercises that emphasise specific joints while minimising injury risk. In addition, understanding the biomechanics of squats with a transformer bar could reveal potential strategies for rehabilitation and accommodation of individuals with upper extremity limitations.

Therefore, the purpose of the current study was to quantify trunk and pelvis angles as well as low-back and lower extremity joint moments during the back and front squats with a straight bar and four squats with different anterior-posterior load placements imposed by a transformer bar. The focus of the current study was the load placement, but the back and front squats with a straight bar were also included as they were commonly used for training and mostly studied in the literature. Straight-bar squats served as comparisons to examine whether a straight bar could provide similar training stimulus as a transformer bar. From a top-down mechanical perspective, with the same joint angles in a squat position, a more anteriorly located load would result in longer moment arms from the load to the low back, hip, and ankle joints, while a more posteriorly located load would increase the moment arm from the load to the knee joint in the sagittal plane. Meanwhile, previous findings suggested that participants were likely to decrease trunk flexion angles to compensate for more anteriorly located loads (Hecker et al., 2019). Therefore, it was hypothesised that the squats with more anteriorly placed loads (front squat with a straight bar, front squat with a transformer bar, and squat with more anteriorly located loads with a transformer bar) would demonstrate greater low-back, hip, and ankle extension moments but less trunk flexion and pelvis anterior tilt angles and knee extension moments compared to the squats with more posteriorly placed loads (back squat with a straight bar, back squat with a transformer bar, and squat with more posteriorly located loads with a transformer bar).

Methods

Participants

Based on a study that quantified knee muscle activities in the back and front squats with a straight bar (Yavuz et al., 2015) and a study that evaluated trunk and hip flexion in squats between a straight bar and a safety bar (Hecker et al., 2019), a large effect size was expected for a pairwise comparison. Assuming a Cohen’s dz of 0.8, a sample size of fifteen was needed to achieve a power of 0.8 at a type-I error rate of 0.05. Twenty-four participants initially completed the study. However, data analyses revealed that six participants did not achieve the estimated parallel squat position, so their data were not included. The final sample consisted of ten males and eight females to give a good representation of the general population (age: 22.1 ± 2.6 years; height: 1.75 ± 0.09 m; mass: 72.6 ± 11.4 kg). Participants had experience performing back and front squats with a straight bar for a minimum of 6 months and had been performing strength training at least two times per week for a total of at least two hours per week at the time of testing. Participants did not have any lower extremity or spinal surgeries or any injury that kept them from participating in physical activity for more than two weeks in the previous six months. Other exclusion criteria included allergies to adhesives and pregnancy. This study was approved by the University of Wyoming Institutional Review Board. Participants signed informed consent forms prior to participation.

Procedure

The current study consisted of two testing sessions that were three to ten days apart. In the first session, participants wore standard athletic shoes (heel height: 3 cm; Ghost 5, Brooks Sports, Inc., Seattle, WA), and performed a standard warm-up protocol, including a 5-minute run at a self-selected pace, toe touches, quadriceps stretch, walking lunges, and side shuffles (Bordelon et al., In press). The front squat one-repetition-maximum (1-RM) with a straight bar was assessed in a progressive manner consistent with the general guidelines (Haff & Triplett, 2016). In the study by Hecker et al. (2019), the relative loads of 3-RMs of the two squats with a straight bar or a safety bar were used. The authors attributed the increased lower extremity activities to the greater 3-RM in the squat with a straight bar, while the isolated effect of the load placement was not clear. As the current study focused on the load placement, we chose to use the relative load of a single 1-RM instead of the 1-RMs of different squats to eliminate the potential effects of different absolute loads on squat biomechanics. The stance width was controlled to be hip-width apart using two tapes on the floor. A previous study has shown that this narrow stance width increased knee flexion angles and decreased hip abduction and internal rotation angles compared to a wide stance width (1.5 times of hip-width) (Lahti et al., 2019). Therefore, we chose a narrow stance width to limit non-sagittal plane movements, as the anterior-posterior load placement was manipulated in the sagittal plane. In addition, the use of 1 time of hip-width helped control the joint angles at the starting position compared to 1.5 times of hip-width due to individual differences in the leg-length to hip-width ratio. The angle of the foot was not controlled.

In the second session, participants wore a spandex top and bottom and the same standard athletic shoes as in the first session. Males could choose not to wear a top. Participants completed the same standard warm-up protocol. Participants also performed eight repetitions of the back and front squats with an empty straight bar and eight repetitions of squats with an empty transformer bar. These warm-up squats continued with additional loads until participants were ready to perform the official trials. Retroreflective markers were then placed on the superior sternum, left and right shoulders, iliac crests, anterior superior iliac spines, posterior superior iliac spines, greater trochanters, anterior mid-thighs, left and right medial and lateral femoral condyles, tibial tuberosities, inferior anterior shanks, medial and lateral malleolus, calcaneus, first toes, and fifth metatarsal heads, as well as on both sides of the bar. The markers for the shoulders were placed slightly lateral to the acromioclavicular joints to avoid being covered by the bar. Kinematic data were recorded using eight Vicon Bonita cameras (Vicon Motion Systems Ltd, Oxford, UK) at a sampling frequency of 160 Hz. Ground reaction forces were captured using two FP4060-05-PT force platforms (Bertec Corp, Columbus, OH, USA) at a sampling frequency of 1,600 Hz.

Participants performed six different squat variations (Figure 1): back and front squats with a straight bar, back and front squats with a transformer bar, and squats with more posteriorly or anteriorly placed loads with a transformer bar. The settings for the transformer bar were recommended by its manufacturer to mimic the respective bar squat variation, including 2.5 C for the front squat, 1D for the back squat, 3A for the squat with more anteriorly placed loads to simulate kettlebell squats, and 1A for the squat with more posteriorly placed loads to simulate cambered squats (Figure 2) (Kabuki Strength, 2020). Participants started with heels hip-width apart and performed all squats with 70% of their straight-bar front squat 1-RMs. The 70% was consistent with previous studies, which used 70% – 75% of 1-RMs as the load (Gullett et al., 2009; Korak et al., 2018; Russell & Phillips, 1989). An elastic band was placed at the participant’s parallel squat position, and they were instructed to touch the band at the bottom of the squat. Participants were instructed to maintain a neutral spinal position. For squats using the transformer bar, participants were directed to keep their upper arms in line with their torso to make sure they did not significantly shift the weight of the bar with the handles. A metronome was used to control both the descending and ascending phases for two seconds each because the speed of squats was shown to affect joint moments (Manabe et al., 2007). Participants performed one practice trial and two official trails for each squat variation with a minimum of a 1-minute break between trials. A low number of practice trials were performed to minimise fatigue effects. In addition, our pilot study suggested that individuals could consistently perform the squat variations after one practice trial. The order of the six squat conditions was randomised.

Figure 1.

From left to right: back and front squats with a straight bar, back and front squats with a transformer bar, and squats with more posteriorly or anteriorly placed loads with a transformer bar.

Figure 2.

Configuration of the top and bottom parts of the transformer bar. The current setting is ‘2’ for the top part.

Data Reduction

Marker positions and ground reaction forces were filtered via a fourth-order, zero-phase Butterworth filter with a low-pass cut-off of 15 Hz. The same cutoff frequency for both kinematic and force data was recommended by a previous study (Kristianslund et al., 2012). The low-back was defined as the midpoint between the left and right iliac crests. The hip joint was defined as a fixed point in the pelvis reference frame (Bell et al., 1989). The knee joint was defined as the midpoint between the medial and lateral femoral condyles. The ankle joint was defined as the midpoint between the medial and lateral malleolus. The joint centers and markers used to define segment reference frames were calibrated during the static trial and recreated during squat trials using the singular decomposition method (Soderkvist & Wedin, 1993).

The upper trunk reference frame was defined by the two shoulders and the low-back. The pelvis reference frame was defined by the left and right anterior superior iliac spines and the midpoint between the left and right posterior superior iliac spines. The thigh reference frame was defined by the hip and knee centers and the lateral femoral condyle. The shank reference frame was defined by the knee and ankle centers and the lateral malleolus. The foot reference frame was defined by the calcaneus, first toes, and fifth metatarsal head. Cardan angles with an order of rotation of flexion-extension, left-right bending, and left-right rotation were calculated between the upper trunk reference frame and the global reference frame to quantify trunk flexion angles (Figure 3). Cardan angles with an order of rotation of anterior-posterior tilt, left-right tilt, and left-right rotation were calculated between the pelvis reference frame and the global reference frame to quantify pelvis anterior tilt angles. Trunk flexion and pelvis anterior tilt angles were modeled separately because the current study quantified both low-back and hip moments. From a top-down perspective, the pelvis angle would likely affect hip moments more than low-back moments. In addition, the two-dimensional (2D) thigh was defined by the greater trochanter and lateral femoral condyle. 2D thigh flexion angles were calculated as the angle between the 2D thigh and the vertical axis in the sagittal plane to help define the parallel squat position.

Figure 3.

Trunk, pelvis, and thigh angles when the upper thigh is parallel to the ground.

Three-dimensional low-back, hip, knee, and ankle moments were calculated using a bottom-up inverse dynamics approach (Kingma et al., 1996), which were used in previous studies (Critchley et al., 2020; Stephenson et al., 2018). Anthropometric information was based on a previous study (de Leva, 1996). Joint moments were expressed in the distal reference frames as internal moments. Joint moments around the medial-lateral axes, representing extension-flexion moments, were extracted and normalised by the participant’s body mass and height for analyses. The mass of the load was not included in the normalization.

To control the effect of squat depth on kinematic and kinetic variables, trunk flexion angles, pelvis anterior tilt angles, and low-back, hip, knee, and ankle extension moments were extracted at the parallel squat position in the descending and ascending phases, as participants typically squatted lower than the parallel position. The parallel squat position was defined as a position when the average of bilateral 2D thigh angles was at 70°. A less than 90° of thigh angle was chosen because the greater trochanter markers typically moved above the lateral femoral condyles when the upper thigh was parallel to the ground (Figure 3). A separated testing session with six individuals was conducted when they performed a back squat with an empty straight bar, while their motion was recorded using a camcorder. When the upper thigh reached parallel to the ground, the thigh angle calculated from the greater trochanter and lateral femoral condyle markers was 73.8 ± 2.0 degrees, supporting the use of the 70° of thigh angle to estimate the parallel squat position. None of the lower extremity joint moments demonstrated significant differences between the left and right sides, so the hip, knee, and ankle moments were averaged between the left and right sides. Data reduction was performed using subroutines developed in the MATLAB 2016b software (MathWorks Inc. Natick, MA).

Statistical analyses

The intraclass correlation (ICC [3, k]) values between the two official trials were calculated for each dependent variable. Each dependent variable was analysed among the six squat conditions using a repeated-measures analysis of variance (ANOVA), followed by paired t-tests. The study-wide false discovery rate for all the paired t-tests was controlled at 0.05 (Benjamini & Hochberg, 1995). For each pairwise comparison, Cohen’s dz was calculated to evaluate the effect size, with Cohen’s dz < 0.5 considered ‘small,’ 0.5 < Cohen’s dz < 0.8 considered ‘medium,’ and Cohen’s dz > 0.8 considered ‘large’ (Cohen, 1988). Joint angles and moments were also time-normalized from the initiation of the squat to the parallel position in the descending phase and from the parallel position to the end of the squat in the ascending phase to create time-series graphs. Statistical analyses were performed using the IBM SPSS Statistics 22 software (IBM Corporation, Armonk, NY, USA).

Results

Participants’ front squat 1-RMs were 91.2 ± 31.5 kg (1.2 ± 0.3 body weight). ICC values were greater than 0.9 for all variables except for ankle moments (Table 1). ANOVAs showed significant main effects for all variables except for knee and ankle moments in the ascending phase (Table 2). The largest p-value for a significant paired t-test was 0.019 after the adjustment for the false discovery rate.

Table 1.

Intra-class correlation coefficients (ICC(3,k)) of dependent variables at the parallel squat position in the descending and ascending phases for different squat conditions

		Straight Bar Back Squat	Straight Bar Front Squat	Transformer Bar Back Squat	Transformer Bar Front Squat	Transformer Bar Posterior Load Squat	Transformer Bar Anterior Load Squat
Trunk Flexion Angles	Descending	0.97	0.98	0.96	0.99	0.92	0.97
	Ascending	0.96	0.95	0.96	0.99	0.93	0.98
Pelvis Anterior Tilt Angles	Descending	0.99	0.99	0.99	0.99	0.98	0.98
	Ascending	0.99	0.98	0.99	0.99	0.97	0.97
Low-Back Moments	Descending	0.95	0.98	0.98	0.98	0.97	0.95
	Ascending	0.93	0.95	0.97	0.97	0.96	0.95
Hip Moments	Descending	0.96	0.98	0.97	0.98	0.99	0.95
	Ascending	0.95	0.96	0.98	0.96	0.96	0.96
Knee Moments	Descending	0.98	0.98	0.99	0.98	0.96	0.93
	Ascending	0.98	0.96	0.99	0.98	0.99	0.96
Ankle Moments	Descending	0.81	0.86	0.85	0.91	0.94	0.88
	Ascending	0.63	0.91	0.81	0.94	0.89	0.34

Note: Hip, knee, and ankle moments were the average of the left and right sides.

Table 2.

Means ± standard deviations of dependent variables at the parallel squat position in the descending and ascending phases for different squat conditions and p values of analyses of variance

		Straight Bar Back Squat	Straight Bar Front Squat	Transformer Bar Back Squat	Transformer Bar Front Squat	Transformer Bar Posterior Load Squat	Transformer Bar Anterior Load Squat	p-values of ANOVA
Trunk Flexion Angles (°)	Descending	35.9 ± 6.0 BC	37.4 ± 5.7 B	46.5 ± 7.1 A	36.9 ± 7.8 B	46.0 ± 6.3 A	33.7 ± 6.2 C	<0.001
	Ascending	37.9 ± 6.4 BC	40.6 ± 5.7 B	48.3 ± 7.3 A	39.2 ± 8.0 B	48.0 ± 6.2 A	36.4 ± 6.7 C	<0.001
Pelvis Anterior Tilt Angles (°)	Descending	28.1 ± 8.1 A	20.1 ± 6.1 C	28.2 ± 7.3 A	22.8 ± 6.2 B	28.7 ± 7.3 A	20 ± 7.1 C	<0.001
	Ascending	29.4 ± 8.2 A	21 ± 6.8 C	29.1 ± 7.7 A	23.4 ± 6.8 B	29.6 ± 7.3 A	20.4 ± 7.3 C	<0.001
Low-Back Extension Moments (Nm/(kg*m))	Descending	1.47 ± 0.30 C	1.58 ± 0.33 B	1.51 ± 0.32 C	1.59 ± 0.33 AB	1.49 ± 0.32 C	1.66 ± 0.34 A	<0.001
	Ascending	1.53 ± 0.25 D	1.64 ± 0.27 BC	1.57 ± 0.30 CD	1.66 ± 0.31 B	1.57 ± 0.32 CD	1.75 ± 0.33 A	<0.001
Hip Extension Moments (Nm/(kg*m))	Descending	0.90 ± 0.18 A	0.86 ± 0.16 B	0.91 ± 0.18 A	0.90 ± 0.16 A	0.92 ± 0.18 A	0.90 ± 0.17 A	0.001
	Ascending	0.93 ± 0.15 A	0.89 ± 0.14 B	0.94 ± 0.18 A	0.92 ± 0.15 A	0.95 ± 0.17 A	0.93 ± 0.17 A	0.002
Knee Extension (−) Moments (Nm/(kg*m))	Descending	−0.71 ± 0.17 B	−0.75 ± 0.17 A	−0.71 ± 0.15 B	−0.72 ± 0.16 AB	−0.72 ± 0.15 B	−0.71 ± 0.14 B	0.006
	Ascending	−0.67 ± 0.15	−0.70 ± 0.15	−0.69 ± 0.16	−0.68 ± 0.14	−0.68 ± 0.15	−0.68 ± 0.16	0.272
Ankle Plantarflexion Moments (Nm/(kg*m))	Descending	0.26 ± 0.13 A	0.19 ± 0.11 B	0.26 ± 0.11 A	0.28 ± 0.12 A	0.25 ± 0.11 A	0.25 ± 0.14 A	<0.001
	Ascending	0.28 ± 0.11	0.25 ± 0.11	0.27 ± 0.09	0.27 ± 0.10	0.28 ± 0.09	0.23 ± 0.09	0.059

Note: Moments were normalized to body mass (kg) and body height (m). Hip, knee, and ankle moments were the average of the left and right sides. Significant analysis of variance (ANOVA) effects were followed by paired t-tests. The statistical significance of paired t-tests for each dependent variable was grouped, where A>B>C>D. Squat conditions with the same letters indicate non-significant differences among them. Significance level set at 0.05 after the adjustment for the false discovery rate.

Trunk flexion angles at the parallel squat position were significantly greatest for the transformer-bar back squat and transformer-bar posterior-load squat and least for the transformer-bar anterior-load squat in both the descending and ascending phases. The effect sizes between the transformer-bar back squat or transformer-bar posterior-load squat and the other squats were large (Table 3). Similar differences were observed throughout the entire descending and ascending phases based on the time-series graphs (Figure 4). Pelvis anterior tilt angles at the parallel squat position were significantly greater for the straight-bar back squat, transformer-bar back squat, and transformer-bar posterior-load squat compared to the other three squats with large effect sizes in both the descending and ascending phases. Similar differences were observed throughout the middle and later phases of the descending phase and the entire phase of the ascending phase (Figure 5). Low-back moments at the parallel squat position were significantly greater for the straight-bar front squat, transformer-bar front squat, and transformer-bar anterior-load squat compared to the other three squats with mostly medium to large effect sizes in both the descending and ascending phases. Similar differences were observed throughout the later phase of the descending phase and the early phase of the ascending phase (Figure 6). Hip, knee, and ankle moments at the parallel squat position were not significantly different among most squat conditions (Supplements 1, 2, 3), except that the straight-bar front squat had significantly decreased hip moments in the descending and ascending phases, greater knee moments in the descending phase, and decreased ankle moments in the descending phase compared to other squats with medium to large effect sizes.

Table 3.

Effect sizes (Cohen’s dz) of dependent variables at the parallel squat position in the descending and ascending phases between each pair of squat conditions

		SB vs. SF	SB vs. TB	SB vs. TF	SB vs. TPL	SB vs. TAL	SF vs. TB	SF vs. TF	SF vs. TPL	SF vs. TAL	TB vs. TF	TB vs. TPL	TB vs. TAL	TF vs. TPL	TF vs. TAL	TPL vs. TAL
Trunk Flexion Angles	Descending	−0.3	−2.5	−0.2	−2.1	0.5	−2.1	0.1	−2.0	1.0	2.5	0.2	3.3	−1.9	0.9	3.1
	Ascending	−0.6	−2.2	−0.3	−2.2	0.3	−1.5	0.3	−1.7	1.2	2.6	0.1	2.9	−2.1	1.0	2.8
Pelvis Anterior Tilt Angles	Descending	2.3	0.0	1.5	−0.2	2.3	−2.8	−1.2	−2.4	0.0	2.2	−0.3	2.7	−2.2	0.9	2.1
	Ascending	2.4	0.1	1.7	−0.1	2.6	−3.0	−1.0	−2.5	0.3	2.5	−0.3	2.9	−2.6	1.0	2.3
Low-Back Moments	Descending	−0.9	−0.5	−0.9	−0.3	−1.3	0.6	−0.1	0.6	−0.7	−0.7	0.3	−1.2	0.8	−0.5	−1.1
	Ascending	−1.1	−0.4	−1.0	−0.3	−1.7	0.5	−0.1	0.5	−0.8	−0.8	0.0	−1.7	0.7	−0.9	−1.3
Hip Moments	Descending	0.8	−0.2	0.1	−0.4	0.1	−0.9	−0.7	−0.9	−0.6	0.4	−0.1	0.2	−0.4	−0.1	0.3
	Ascending	0.9	−0.2	0.2	−0.3	0.0	−0.7	−0.5	−0.8	−0.7	0.4	−0.2	0.2	−0.4	−0.3	0.3
Knee Moments	Descending	0.6	−0.1	0.2	0.1	0.0	−0.6	−0.5	−0.6	−0.7	0.3	0.1	0.1	−0.2	−0.3	−0.1
	Ascending	0.5	0.6	0.4	0.3	0.5	−0.2	−0.3	−0.2	−0.2	0.0	−0.1	0.0	0.0	0.0	0.1
Ankle Moments	Descending	0.7	−0.1	−0.3	0.0	0.0	−0.8	−1.5	−1.0	−0.7	−0.3	0.1	0.1	0.4	0.3	0.0
	Ascending	0.4	0.2	0.1	0.0	0.5	−0.3	−0.4	−0.6	0.1	−0.1	−0.2	0.4	−0.1	0.5	0.5

Note: SB: straight bar back squat; SF: straight bar front squat; TB: transformer bar back squat; TF: transformer bar front squat; TPL: transformer bar posterior load squat; TAL: transformer bar anterior load squat

Figure 4.

Time-normalised graphs for the trunk flexion angles (°) in the descending (left) and ascending phases (right). The pooled standard errors of the mean of the difference at the parallel position were 1.05 and 1.04 for the descending and ascending phases, respectively.

Figure 5.

Time-normalised graphs for the pelvis anterior tilt angles (°) in the descending (left) and ascending phases (right). The pooled standard errors of the mean of the difference at the parallel position were 0.73 and 0.72 for the descending and ascending phases, respectively.

Figure 6.

Time-normalised graphs for the low-back extension moments (Nm/(kg*m)) in the descending (left) and ascending phases (right). The pooled standard errors of the mean of the difference at the parallel position were 0.03 and 0.03 for the descending and ascending phases, respectively.

Discussion and implications

The purpose of the current study was to quantify trunk and pelvis angles as well as low-back and lower extremity joint moments during the back and front squats with a straight bar and four squats with a transformer bar. It was hypothesised that squats with more anteriorly placed loads would demonstrate greater low-back, hip, and ankle extension moments but less trunk and pelvis anterior tilt angles and knee extension moments compared to squats with more posteriorly placed loads.

The hypothesis related to the effects of load placements on low-back moments, trunk, and pelvis anterior tilt angles were supported. On average, the descending and ascending phases demonstrated comparable dependent kinematic and kinetic variables, and their changes as a function of squat conditions were similar. These similarities were likely due to the controlled thigh position in both phases and slow movement speed near the bottom of the squat. From a top-down perspective, low-back moments were mainly affected by two factors when the load was constant: the initial anterior location of the load and trunk flexion. Increasing either factor would increase the external moment arms from the load and upper body to the low back. The trunk flexion angles decreased by approximately 12 degrees (25%) for the transformer-bar anterior-load squat compared to the transformer-bar posterior-load squat. On the other hand, the low-back moments increased by approximately 0.18 Nm/(kg*m) (11%) between these two squats. Low-back moments for the two front squats with a straight or transformer bar were also significantly greater than the other squats with more posteriorly placed loads. Participants compensated for the anterior location of the load by decreasing their trunk flexion, but this decrease in trunk flexion did not completely offset the increase in the initial load placement in the anterior direction, resulting in increased low-back moments. The decreased trunk flexion with a more anteriorly placed load was consistent with previous observations of less trunk or hip flexion in back squats with a safety bar compared to back squats with a straight bar (Hecker et al., 2019) as well as in front squats with a straight bar compared to back squats with a straight bar (Braidot et al., 2007; Yavuz et al., 2015). However, the current study observed increased low-back moments but similar trunk flexion angles between the back and front squats with a straight bar. This was likely due to the separation of the trunk and pelvis angles. If the trunk and pelvis were modeled as one segment, the summed flexion angles would decrease because of the decreased pelvis anterior tilt angles for the front squat with a straight bar. In addition, the literature has documented similar (Gullett et al., 2009; Yavuz et al., 2015) or increased low-back muscle activities (Comfort et al., 2011) for the straight-bar front squat compared to the straight-bar back squat. Another study demonstrated a 7% increase in low-back moments for the front squat with a straight bar compared to the back squat with a straight bar (Russell & Phillips, 1989). This percentage of increase was similar to the findings of the current study, suggesting that the front squat was likely to slightly increase low-back moments. Overall, participants compensated for the anteriorly placed load with decreased trunk flexion, but the low-back moment still increased.

The hypotheses related to load placements and lower extremity moments were not supported. Non-significantly different hip, knee, and ankle moments were found among all squat conditions, except for the straight-bar front squat. Interestingly, the transformer-bar front squat demonstrated more similar lower extremity moments as the straight-bar back squat instead of the straight-bar front squat. As the thigh angle and load was controlled in the analysis, lower extremity moments were mainly affected by three factors: the initial anterior placement of the load, trunk flexion, and pelvis anterior tilt. Theoretically, with the same joint angles, the more anteriorly located load will increase the external moment arms from the load to the hip and ankle joints but decrease them to the knee joint. However, for the four load placements with the transformer bar, it was postulated that participants adjusted their trunk and pelvis angles so that the weight vector of the load and the participant’s upper body passed through a similar point among the hip, knee, and ankle joints as the straight-bar back squat. When the load was placed anteriorly with a transformer bar, participants compensated with decreased trunk flexion as well as decreased pelvis anterior tilt. For example, the pelvis anterior tilt angles decreased by an average of 9 degrees (30%) for the transformer-bar anterior-load squat compared to the transformer-bar posterior-load squat. The decreased trunk flexion and pelvis anterior tilt were enough to offset the anterior placement of the load and resulted in similar lower extremity moments. These findings again highlight the importance of modeling the upper trunk and pelvis as two segments to understand changes in joint moments as a function of joint angles, as trunk flexion would affect low-back moments and both trunk and pelvis anterior tilt would affect lower extremity moments. In summary, for all four squats with a transformer bar was used, it appeared that participants utilised a generalised motor control pattern, which was to keep lower extremity moments demands similar to a straight-bar back squat through modifying the trunk and pelvis anterior tilt angles. As the straight-bar back squat was the most used for training and the transformer bar had similar contact areas with the shoulder as the straight-bar back squat, participants likely transferred the lower extremity loading patterns of the straight-bar back squat to the lower extremity loading patterns of all four squats with a transformer bar.

While most conditions demonstrated similar lower extremity moments, the straight-bar front squat was the only condition that showed increased knee moments but decreased hip and ankle moments, mostly in the descending phase. When the straight-bar back and front squats were compared, the front squat showed decreased pelvis anterior tilt compared to the back squat, while both demonstrated similar trunk flexion. The decreased pelvis anterior tilt could be the strategy participants utilised to move the load in the posterior direction to maintain stability. As the bar was held in front, excessive pelvis and trunk flexion could result in dropping the bar. As a result of the posterior movement of the load, hip and ankle moments decreased, and knee moments increased. These increased knee moments were consistent with a previous study (Braidot et al., 2007), demonstrating a 7.5% increase in knee energy for the straight-bar front squat compared to the straight-bar back squat. Yavuz et al. (2015) also observed increased vastus medialis activities and decreased semitendinosus activities for the straight-bar front squat compared to the straight-bar back squat. Based on the increased knee extensor activities and decreased trunk flexion, these researchers recommended the front squat as an exercise to develop knee strength and prevent spinal injuries. On the other hand, some studies have observed similar knee extensor activities (Gullett et al., 2009; Korak et al., 2018) and moments (Russell & Phillips, 1989) or even increased knee moments (Gullett et al., 2009) between the straight-bar front and back squats, which could result from different testing populations, relative or absolute loads, and the number of participants to detect statistical significance. In general, the most significant difference between the straight-bar front and back squats was the pelvis anterior tilt angle, and the straight-bar front squat might be more challenging for the knee joint in the descending phase.

The current study had several limitations. First, a load of 70% of the 1-RM in the straight-bar front squat was used for all squat conditions to emphasise the effect of load placements on squat mechanics. Different squat variations are not likely to have the same 1-RM, and a relative load may result in different changes in joint motion and loading. Second, the findings were limited to a narrow stance width, which could limit several participants’ ability to squat to a parallel position and resulted in the exclusion of these participants’ data for analyses. In addition, the orientation of the foot was not controlled. Future studies should assess the interactions among load placements, stance width, and foot orientation. Third, the current participants were mainly college students participating in recreational strength and conditioning activities. These participants’ front squat 1-RMs were slightly greater than several previous studies (Gullett et al., 2009; Korak et al., 2018; Russell & Phillips, 1989; Yavuz et al., 2015), but future studies should also recruit elite athletes. Fourth, participants performed the squat variations with preferred techniques with minimal instructions. Also, most participants did not have experience in squatting with a transformer bar. Additional instructions, feedback, and training may change participants’ responses to different load placements. Fifth, makers were placed on participants’ skin, which could affect joint kinematic and kinetic data. To limit the artifact associated with skin motion, all markers placed on one segment were used to form a cluster to track the motion of the segment (Soderkvist & Wedin, 1993). In addition, a repeated-measure design was implemented to minimise the variation associated marker placements. Furthermore, the current study focused on sagittal-plane kinematics and kinetics, and previous studies have supported better reliability and validity for kinematic variables in the sagittal plane compared to the frontal and transverse planes (Leardini et al., 2005). The ICCs also supported the excellent reliability for most of the dependent variables in the current study. Sixth, the analyses were limited to kinematics and kinetics without assessing muscle activities. Muscle co-activation could increase muscle activities and joint loading without changing joint moments and should be evaluated in the future. Last, internal spinal loading was not measured. An anteriorly placed load resulted in decreased trunk flexion and pelvis anterior tilt but also increased low-back moments. It was unknown how these two changes affected the internal spinal loading and its associated injury risk.

Conclusion

Squats with more anteriorly placed loads resulted in decreased trunk flexion and pelvis anterior tilt angles, increased low-back moments, and similar lower extremity moments compared to squats with more posteriorly placed loads. First, the transformer bar imposes similar lower extremity loading as a straight-bar back squat and can be considered as an alternative training strategy for people with upper extremity limitations. Second, changing anterior-posterior load placement may not induce as many changes in low-back and lower extremity moments as theoretical values based on the assumption of similar joint angles because individuals can adjust their trunk and pelvis angles to mediate the effects of load placements on joint moments. Additional instruction and feedback may be needed to achieve greater effects of load placement manipulation on low-back and lower extremity joint moments. Third, the straight-bar front squat has more knee loading, but less ankle and hip loading in the descending phase compared to the other squats and may be used for those who want to focus on knee training. Last, anteriorly placed loads may result in greater low-back moments while posteriorly placed loads may demonstrate greater trunk flexion and pelvis anterior tilt, which should be taken into consideration for people with low-back impairment.

Supplementary Material

Supp 1

Supplement 1. Time-normalised graphs for the hip extension moments (Nm/(kg*m)) in the descending (left) and ascending phases (right). The pooled standard errors of the mean of the difference at the parallel position were 0.01 and 0.02 for the descending and ascending phases, respectively.

Supp 2

Supplement 2. Time-normalised graphs for the knee extension (−) moments (Nm/(kg*m)) in the descending (left) and ascending phases (right). The pooled standard errors of the mean of the difference at the parallel position were 0.01 and 0.01 for the descending and ascending phases, respectively.

Supp 3

Supplement 3. Time-normalised graphs for the ankle plantarflexion moments (Nm/(kg*m)) in the descending (left) and ascending phases (right). The pooled standard errors of the mean of the difference at the parallel position were 0.02 and 0.02 for the descending and ascending phases, respectively.