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Original Research

Differences in Muscle Activity and Kinetics Between the Goblet Squat and Landmine Squat in Men and Women

Collins, Kyle S.; Klawitter, Lukus A.; Waldera, Roman W.; Mahoney, Sean J.; Christensen, Bryan K.

Author Information
Journal of Strength and Conditioning Research: October 2021 - Volume 35 - Issue 10 - p 2661-2668
doi: 10.1519/JSC.0000000000004094
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Abstract

Introduction

The squat exercise is commonly used in a variety of settings and is a key component of many strength and conditioning programs for improving lower-body performance (37). It is a closed-chain exercise starting with the lifter standing upright, then flexing at the ankle, knee, and hip joints to a preferred depth before pressing upward, keeping the torso mostly upright and movement pattern stable (17,37). Squats can be unilateral or bilateral and have different foot placement or width, and external load placement can vary but is commonly a barbell in the form of front squats or back squats (14,37,44,46). Additional equipment can be used to further differentiate squat variations. For example, a box squat involves the lifter sitting back onto a box placed behind them and can encourage posterior hip displacement (42).

The goblet squat (GBS) is a squat variation with the weight held in front of the lifter, using a dumbbell or kettlebell, and has been used as a foundational exercise in squat progressions to teach proper form and body position (3). Relatively lighter resistance held in front of the chest can emphasize an upright torso, making the movement less technically demanding than other variations such as the back squat (3). In addition, maintaining upright trunk posture during the squat can decrease shear stress on the lumbar spine making the GBS a good option for some populations (31,37). The GBS is versatile in application and can be programmed as a primary or supplementary exercise in a training routine (30,36). For example, Otto et al. (30) compared 6 weeks of weightlifting and traditional resistance training with kettlebell training methods, and the latter program consisted of kettlebell swings, accelerated swings, and the GBS (30). A 12-week lower-body training study that saw improvements to hamstring and quadriceps strength ratios in men and women used a similar variation to the GBS (with a dumbbell in each hand), in conjunction with hamstring and glute exercises (11). Although some evidence of its practical use exists in the literature, research with the GBS as the main study variable is lacking.

Landmine exercises involve the use of a barbell where the one end is freely manipulated by the lifter and the other pivots, anchored in a hinge apparatus or against the corner of a wall (43). Weight plates can be added to the free end to increase the load, and a multitude of exercise variations exist such as the landmine single-leg Romanian deadlift (43). The landmine squat (LMS) is a variation where the free end of the barbell is gripped with both hands in front of the body (shown in Figure 1), making it potentially easier to hold more load than the GBS. The length of the bar combined with the arced angle forces the lifter to settle backwards during the eccentric portion of movement encouraging good body posture throughout the lift (12). Compared with the conventional GBS, sitting back into the squat may be more advantageous for learning squat progressions by establishing a hip dominant movement pattern and ensuring the lifter keeps heel contact with the ground (8). Anecdotally, the barbell mass appears partially supported by the anchor point during the LMS. Raising the free end of the barbell causes it to rotate around the anchor point, or axis, becoming more vertical and supported by its own base. This means a lesser load at the top and greater load as the barbell is lowered toward the ground.

Figure 1.
Figure 1.:
Illustration of the landmine squat and its arced barpath (dotted line).

Strength and conditioning professionals commonly use the squat and Olympic-style variations to train multidimensional characteristics of strength, power, and speed that athletes require for their sport (30,48). However, these are all vertically oriented movements, and it has been suggested that incorporating horizontally loaded movements may be beneficial, especially if the sport requires horizontal propulsive force for rapid, short accelerations (33,48). Horizontally oriented movements may enhance the transfer of training to field performance by being more specific to the demands and movement patterns of certain sports (48). Performing the LMS involves leaning forward slightly into the bar which may indicate that some horizontal ground reaction force is occurring. This is opposed to the GBS, that like the back squat, it is entirely a vertically oriented movement. Comparing the LMS with the GBS could highlight the difference and help describe the direction and magnitude of ground reaction force occurring during the LMS. This information could better inform application for different training programs.

In comparing similar squat techniques, Lynn & Noffal (22) conducted a study that assessed a normal squat technique to a counter balanced squat (arms out in front) in 31 college-aged men and women. Compared with the regular squat, the counterbalanced squat increased the hip joint moment and gluteus maximus activity while decreasing the knee moment and quadriceps activity. It was surmised that the counterbalanced squat encouraged a more hip dominant movement and reduced torque on the knee, which may be well suited for rehabilitation purposes. Like the effect of the counterbalanced squat, the LMS may act to maintain a certain amount of mass anteriorly while simultaneously pushing the lifter back into a position of greater hip flexion and less knee flexion. The possible result of decreased quadriceps activity may improve the ratio of hamstring to quadriceps recruitment, which would be especially beneficial to women in resistance training programs or rehabilitation settings because of their increased anterior cruciate ligament (ACL) injury risk (11). Females tear their ACL at a disproportionally higher rate than males participating in the same sports or activities, and these injuries are largely a noncontact mechanism during dynamic deceleration movements such as landing, cutting, or pivoting (28,47). Research suggests that one reason is men have greater hamstring activity in relation to the quadriceps, especially at higher speeds of movement in sports activities (11,28). Improvement of neuromuscular control and hamstring strength in relation to quadriceps strength is important for training programs to reduce the risk of knee injury (11,28).

Research on the squat exercise and its variations is abundant (1,14,37,43,45); however, quantitative studies specifically on the GBS and LMS are lacking. Given their common usage, establishing more research on both squat variations is an important addition to the current literature. It would also be advantageous for practitioners to have a better understanding of the practical application as one technique may be more favorable for injury prevention in specific populations, performance related horizontal force production, or squat training progressions. Therefore, the purpose of this study was to compare electromyographic (EMG) activity of the thigh musculature and vertical and anteroposterior kinetics for men and women during the GBS and LMS.

Methods

Experimental Approach to the Problem

This study used a within-subjects counterbalanced design, randomized by a subject, to investigate the muscle activity and kinetic differences between the GBS and LMS. Relative resistance for both squat conditions was set at 30% of body mass. This was performed for practicality purposes, as these squat variations are primarily submaximal accessory exercises and a 1 repetition maximum may be limited by the arms ability to support the anterior load. In addition, relative loading may be more accurate in comparing different exercises, especially if the exercises differ in capability of applying external load (1,23). Similar loads relative to body mass have been used in the literature, for example, 25% of body mass was used for the inexperienced squat group performing 5 repetitions of a randomized squat condition (21). To control for speed given the lighter load, both squat conditions involved a 2-second eccentric phase and a 2-second concentric phase, set to a metronome at 60 beats per second. Subjects were instructed to follow 2 counts down and 2 counts up (down, down, up, up) for the 5 repetitions of each squat condition. Data from repetitions 1 and 5 were disregarded to allow the more stable midset repetitions (2, 3, and 4) to be used for analysis (21). The dependent variables were muscle activity represented by mean EMG amplitude for vastus medialis (VM), vastus lateralis (VL), semitendinosus (ST), and biceps femoris (BF) and ground reaction forces consisting of vertical and anteroposterior (horizontal) directions. The independent variables were the 2 squat conditions and sex.

Subjects

A total of 32 subjects (mean ± SD: age: 22.2 ± 3.2 years; height: 172.9 ± 9.0 cm; and body mass: 75.9 ± 12.4 kg), consisting of 16 men (age: 23.2 ± 3.7 years; height: 179.5 ± 6.7 cm; body mass: 82.4 ± 12.8 kg) and 16 women (age: 21.2 ± 2.3 years; height: 166.4 ± 5.3 cm; body mass: 69.5 ± 8.3 kg), were recruited for this study. Subjects were a convenience sample recruited by university email, word of mouth, and posted flyers. The inclusion criteria included being healthy, recreationally active, and between 18 and 30 years of age. The exclusion criteria included answering “yes” to any of the questions on the PAR-Q+ or a history of lower-body injury in the past 6 months. All subjects were informed of potential risks and signed a written informed consent before data collection. All procedures were approved by the institutional review board of North Dakota State University.

Procedures

Subjects were asked to report for one laboratory visit with athletic clothing and shoes. Initially, the informed consent form was read and signed, and then the PAR-Q+ and questionnaire forms were completed. Height was measured to the nearest 0.5 cm using a portable stadiometer (SECA Corporation, Hamburg, Germany) and body mass measured to the nearest 0.1 kg with a digital scale (Health o meter, Sunbeam Products Inc., Boca Raton, L). Subjects then conducted a warm-up of 5 minutes at a self-selected intensity on a cycle ergometer followed by dynamic stretches (walking lunge with trunk rotation, walking quadriceps stretch, walking high knee pull, walking hamstring kicks, balanced gluteal stretch, and body weight squats) led by a member of the research team. Subjects were familiarized with the LMS and allowed several practice repetitions with the 2-second eccentric and 2-second concentric tempo, and the verbal start command of “ready, set, go” in time with the 60 bpm of the metronome. The technique was evaluated by the investigators, and subjects were encouraged to reach a proper depth of thighs parallel to the ground. Subjects then had a 10-minute rest as the EMG electrodes and wireless transmitters attached to their right leg (1,23). Five repetitions of each squat condition were performed with 3 minutes of rest between trials. If the repetition did not follow the tempo consistently or did not go low enough, it was discarded and the subject asked to perform another repetition.

As stated, relative load for the trials was set to (the nearest 2.27 kg) 30% of the subject's body mass for both conditions. The GBS load was a dumbbell held by both hands, palms up, and close to the body at the chest level. The equivalent LMS load was a barbell with one end anchored to the hinge apparatus, and the free end held by both hands, close to the body at the chest level. Weight plates were added to the free end of the barbell, so the total bar load equaled the dumbbell load. Subjects used the same self-selected, approximately shoulder width stance for both trials, and for the LMS had toes set to a mark 1 inch past the free end of the barbell resting on the force plate surface.

Muscle Activity

Surface EMG signals were recorded for the VM, VL, ST, and BF muscles. To improve signal conduction, excess hair was removed with a razor, skin abraded, and cleaned with alcohol. Self-adhesive dual Ag/AgCl snap electrodes (Noraxon, Scottsdale, AZ) with an interelectrode distance of 2 cm were placed close to the muscle belly of VM, VL, ST, and BF and reference electrodes placed on the lateral epicondyle of the femur. Exact placement of the electrodes followed previously established locations and SENIAM guidelines (14,15,45). Specifically, the electrodes were placed at the following locations: VM was four-fifths the distance from the anterior superior iliac spine to the medial knee; VL was two-thirds the distance from the anterior superior iliac spine to the lateral patella; ST was halfway between the ischial tuberosity and medial epicondyle of the tibia; and BF was halfway between the ischial tuberosity and lateral epicondyle of the tibia. The electrodes were attached to a dual channel wireless EMG BioNomadix (BN-EMG2, CMRR >90 dB) matched transmitter and receiver, interfaced with a Biopac MP-150 (Biopac Inc., Goleta, CA) system, and transmitting at 2000 Hz. After connection of electrodes and securing with paper tape, a quality check consisting of seated knee extension and flexion was performed to confirm EMG signal validity (24).

Maximal voluntary isometric contraction (MVIC) trials were performed for the knee extensors and knee flexors to normalize the EMG signal. Subjects performed 2 MVIC trials for each muscle group lasting 5 seconds, each against manual resistance, with one minute of rest between (1,14,25). For the knee extensors, subjects were seated on a treatment table with the knee flexed to 90° and manual resistance applied anteriorly just above the ankle malleoli (14,25). For the knee flexors, subjects lied prone on a treatment table with the knee flexed to 45° and manual resistance applied posteriorly just above the heel (25). All MVIC trials were tested by the same researcher, and a hand-held goniometer was used before each trial to ensure proper joint angle position. All EMG data were recorded at and analyzed with AcqKnowledge 4.4 software (Biopac Inc., Goleta, CA), filtered with a Butterworth bandpass at 10–500 Hz, and rectified using the root mean square method over a 50 ms window (24,38). Peak amplitude from each repetition was recorded, and the mean of the peaks was calculated in Microsoft Excel. These values were normalized to the greatest value of the MVIC trials and expressed as a percentage of MVIC for VM, VL, ST, and BF (9,45,46).

Kinetics

All subjects performed the squat trials with both feet on a 76 × 102 cm AMTI AccuPower force plate (Advanced Mechanical Technologies Inc., Watertown, MA) recorded at 1,200 Hz. Kinetic data from the squat trials were exported to and processed with a custom MATLAB 2017a (MathWorks, Natick, MA) program. Force plate data were filtered using a zero lag, fourth order Butterworth low pass filter with cutoff at 10 Hz (16,19,32). Velocity of system mass (subject + load) was obtained by the trapezoidal integration of acceleration, which was derived from the force-time data by dividing the vertical ground reaction force by system mass (18,20,29,40). Initiation of the eccentric phase was defined as the point where system mass began downward movement and velocity became negative or crossed from positive to negative (6,39). Onset of the concentric phase was defined as the point where system mass velocity crossed zero, changing from a negative to a positive (20,39,41).

Due to the arced bar path of the LMS, the lifter has some forward inclination at the top while in full lower-body extension, compared with the more vertical nature of the GBS. To assess the distribution of vertical and horizontal force in the LMS, and differences with the GBS, vertical and anteroposterior horizontal ground reaction forces were recorded at the start of the eccentric (eccVGRF and eccHGRF, respectively) and concentric (conVGRF and conHGRF, respectively) movement phases. Peak vertical ground reaction force (peakVGRF) has been shown to occur at or near the transition from eccentric to concentric (39) and was obtained for each repetition in the trials for additional comparison. To negate the influence of the different body masses of men and women, both vertical and horizontal ground reaction forces were normalized (N·kg−1) to body mass (6,26,29). Kinetic data variables were then exported to Microsoft Excel sheets for statistical analysis.

Statistical Analyses

All data were analyzed to compare differences between squat conditions and sexes with statistical significance set at p ≤ 0.05, using Statistical Package for the Social Sciences (SPSS 27, IBM Corp., Armonk, NY). A 2-way mixed repeated measures analysis of variance (ANOVA) was used to investigate the differences between squat condition and sex on the dependent variables. For further analysis, separate 1-way repeated measures ANOVAs assessed the differences between the GBS and LMS for men and women, and simple ANOVAs assessed the differences between men and women in each squat condition. Partial eta squared (ηp2) was used to estimate explained variance and effect size, and a value of 0.01 considered a small effect, 0.06 a medium effect, and 0.14 a large effect (34). Significance of pairwise comparisons and 95% confidence intervals of the difference in means were based on the Bonferroni adjustment for multiple comparisons. Levene's test was used to assess homogeneity of variance, and results are displayed as mean and standard deviation.

Results

Results of the 2-way mixed repeated measures ANOVA indicated significant main effects for squat condition (F = 257.20, p < 0.001, ηp2 = 0.99) and sex (F = 5.02, p = 0.001, ηp2 = 0.67), but the interaction (squat × sex) was not significant. Results of muscle activity in the quadriceps and hamstring muscles between men and women and between the LMS and GBS are displayed in Table 1. For overall sex differences (GBS and LMS combined), women showed significantly greater mean VM (F = 7.58, p = 0.01) activity compared with men. For the total sample (men and women combined), the LMS showed significantly less activity for the VM (F = 16.53, p < 0.001), VL (F = 39.76, p < 0.001), and ST (F = 11.40, p = 0.002) compared with the GBS.

Table 1 - Mean muscle activity data (mean ± SD) normalized as a percent of maximal voluntary isometric contraction (%MVIC), effect sizes (partial eta squared [
ηp2
]), mean difference (MD), and 95% confidence intervals for mean difference (95% CI) between men and women, and within squat conditions.*
Muscle Sex differences Squat condition differences
Men Women MD (95% CI) GBS LMS MD (95% CI)
VM 61.6 ± 32.0 92.7 ± 32.0 0.20 −31.1 (−54.1 to −8.0) 84.3 ± 41.3 70.0 ± 31.1 0.36 14.3 (7.1 to 21.6)
VL 67.9 ± 36.0 92.0 ± 36.0 0.11 −24.1 (−50.0 to 1.7) 88.6 ± 42.1 71.3 ± 33.6 0.57 17.3 (11.7 to 22.9)
ST 12.0 ± 6.0 14.9 ± 6.0 0.06 −2.9 (−7.2 to 1.4) 15.4 ± 7.4 11.5 ± 6.4 0.28 3.9 (1.6 to 6.3)
BF 18.7 ± 19.2 14.4 ± 19.2 0.01 4.3 (−9.5 to 18.1) 18.1 ± 14.9 14.9 ± 24.4 0.05 3.2 (−2.0 to 8.4)
*GBS = goblet squat; LMS = landmine squat; VM = vastus medialis; VL = vastus lateralis; ST = semitendinosus; BF = biceps femoris.
Significant (p < 0.05) difference between men and women.
Significant (p < 0.05) difference between squat conditions.

The post hoc results of muscle activity are displayed in Table 2. Men showed significantly less activity for the VM (F = 16.41, p = 0.001), VL (F = 30.09, p < 0.001), and ST (F = 13.41, p = 0.002) muscles in the LMS compared with the GBS. Women showed significantly less activity for the VM (F = 6.56, p = 0.022), VL (F = 15.01, p = 0.001), and BF (F = 12.45, p = 0.003) muscles in the LMS compared with the GBS. For the hamstring musculature between squat conditions, men showed no difference in the BF muscle, whereas women showed no difference in the ST muscle. For the GBS, women showed significantly greater mean VM (F = 5.94, p = 0.021) activity compared with men. For the LMS, women showed significantly greater mean VM (F = 8.82, p = 0.006), VL (F = 4.51, p = 0.042), and ST (F = 5.08, p = 0.032) activity compared with men.

Table 2 - Mean muscle activity data (mean ± SD) normalized as a percent of maximal voluntary isometric contraction (%MVIC), effect sizes (partial eta squared [
ηp2
]), mean difference (MD), and 95% confidence intervals for mean difference (95% CI) between and within sex and squat conditions.*
Muscle GBS LMS GBS, men vs. women LMS, men vs. women Men, GBS vs. LMS Women, GBS vs. LMS
Men Women Men Women MD (95% CI) MD (95% CI) MD (95% CI) MD (95% CI)
VM 67.8 ± 19.2 100.8 ± 50.7 55.4 ± 17.8 84.6 ± 34.9 0.17 −33.0 (−60.7 to −5.3) 0.23 −29.1 (−49.2 to −9.1) 0.52 12.4 (5.9 to 18.9) 0.30 16.3 (2.7 to 29.8)
VL 76.4 ± 30.1 100.7 ± 49.3 59.3 ± 24.3 83.3 ± 38.0 0.08 −24.3 (−53.8 to 5.2) 0.13 −23.9 (−47.0 to −0.9) 0.67 17.1 (10.5 to 23.7) 0.50 17.5 (7.9 to 27.1)
ST 15.0 ± 6.6 15.9 ± 8.3 9.1 ± 4.0 13.9 ± 7.5 0.004 −1.0 (−6.4 to 4.4) 0.15 −4.8 (−9.1 to −0.5) 0.47 5.9 (2.5 to 9.3) 0.09 2.0 (−1.6 to 5.6)
BF 19.0 ± 18.7 17.3 ± 10.3 18.4 ± 34.3 11.5 ± 5.6 0.003 1.6 (−9.3 to 12.5) 0.02 7.0 (−10.8 to 24.7) 0.001 0.6 (−9.7 to 10.8) 0.45 5.9 (2.3 to 9.4)
*GBS = goblet squat; LMS = landmine squat; VM = vastus medialis; VL = vastus lateralis; ST = semitendinosus; BF = biceps femoris.
Significant (p < 0.05) difference between men and women.
Significant (p < 0.05) difference between squat conditions.

Results of the force variables between men and women and between the LMS and GBS are displayed in Table 3. For overall sex differences (GBS and LMS combined), women showed significantly greater eccVGRF (F = 7.20, p = 0.012), and significantly less horizontal forces, eccHGRF (F = 4.33, p = 0.046) and conHGRF (F = 7.62, p = 0.01), when compared with men. For the total sample (men and women combined), the LMS showed significantly less peakVGRF (F = 452.97, p < 0.001), eccVGRF (F = 635.53, p < 0.001), conVGRF (F = 241.29, p < 0.001), and significantly more eccHGRF (F = 1,363.87, p < 0.001), and conHGRF (F = 16.55, p = 0.002) compared with the GBS.

Table 3 - Kinetic data (mean ± SD) normalized to body mass (N·kg−1), effect sizes (partial eta squared [
ηp2
]), mean difference (MD), and 95% confidence intervals for mean difference (95% CI) between men and women, and within squat conditions.*
Force Sex differences Squat condition differences
Men Women MD (95% CI) GS LS MD (95% CI)
PeakVGRF 14.20 ± 0.73 13.89 ± 0.73 0.05 0.31 (−0.21 to 0.84) 14.91 ± 0.81 13.18 ± 0.74§ 0.94 1.73 (1.56 to 1.90)
EccVGRF 10.81 ± 0.58 11.37 ± 0.58 0.19 −0.55 (−0.98 to −0.13) 12.08 ± 0.67 10.11 ± 0.68§ 0.96 1.97 (1.81 to 2.13)
EccHGRF 0.90 ± 0.13 0.80 ± 0.13 0.13 0.10 (0.002 to 0.20) 0.16 ± 0.12 1.54 ± 0.22§ 0.98 −1.38 (−1.45 to −1.30)
ConVGRF 13.59 ± 0.85 13.27 ± 0.85 0.04 0.32 (−0.29 to 0.94) 14.25 ± 0.96 12.61 ± 0.85§ 0.89 1.64 (1.42 to 1.85)
ConHGRF 0.39 ± 0.19 0.20 ± 0.19 0.20 0.19 (0.05 to 0.32) 0.19 ± 0.15 0.40 ± 0.35§ 0.36 −0.20 (−0.31 to −0.10)
*Horizontal forces are all posteriorly directed.
GBS = goblet squat; LMS = landmine squat; PeakVGRF = peak vertical force; EccVGRF = eccentric vertical force; EccHGRF = eccentric horizontal force; ConVGRF = concentric vertical force; ConHGRF = concentric horizontal force.
Significant (p < 0.05) difference between men and women.
§Significant (p < 0.05) difference between squat conditions.

The post hoc results of the force variables are displayed in Table 4. For men, the LMS compared with the GBS showed significantly less peakVGRF (F = 171.16, p < 0.001), eccVGRF (F = 214.08, p < 0.001), conVGRF (F = 70.81, p < 0.001), and significantly more eccHGRF (F = 887.45, p < 0.001) and conHGRF (F = 18.57, p = 0.001). For women, the LMS compared with the GBS showed significantly less peakVGRF (F = 350.46, p < 0.001), eccVGRF (F = 751.59, p < 0.001), conVGRF (F = 295.32, p < 0.001), and significantly more eccHGRF (F = 532.94, p < 0.001). The only force variable that was not significant between squat conditions was the conHGRF for women. For the GBS, women showed significantly greater eccVGRF (F = 4.40, p = 0.045) compared with men. For the LMS, women showed significantly greater eccVGRF (F = 8.62, p = 0.006) and significantly less eccHGRF (F = 5.50, p = 0.026) and conHGRF (F = 12.08, p = 0.002).

Table 4 - Kinetic data (mean ± SD) normalized to body mass (N·kg−1), effect sizes (partial eta squared [
ηp2
]), mean difference (MD), and 95% confidence intervals for mean difference (95% CI) between and within sex and squat conditions.*
Force GBS LMS GBS, men vs. women LMS, men vs. women Men, GBS vs. LMS Women, GBS vs. LMS
Men Women Men Women MD (95% CI) MD (95% CI) MD (95% CI) MD (95% CI)
PeakVGRF 15.09 ± 0.58 14.73 ± 0.97 13.32 ± 0.68§ 13.04 ± 0.79§ 0.05 0.35 (−0.22 to 0.93) 0.04 0.28 (−0.25 to 0.81) 0.92 1.77 (1.48 to 2.06) 0.96 1.69 (1.50 to 1.88)
EccVGRF 11.84 ± 0.45 12.31 ± 0.78 9.79 ± 0.38§ 10.42 ± 0.78§ 0.13 −0.47 (−0.93 to −0.01) 0.22 −0.64 (−1.08 to −0.19) 0.94 2.05 (1.75 to 2.35) 0.98 1.89 (1.74 to 2.03)
EccHGRF 0.18 ± 0.11 0.15 ± 0.14 1.62 ± 0.19§ 1.45 ± 0.22§ 0.01 0.03 (−0.06 to 0.12) 0.16 0.17 (0.02 to 0.32) 0.98 −1.45 (−1.55 to −1.34) 0.97 −1.30 (−1.42 to −1.18)
ConVGRF 14.37 ± 0.92 14.12 ± 1.01 12.81 ± 0.85§ 12.41 ± 0.82§ 0.02 0.25 (−0.45 to 0.95) 0.06 0.40 (−0.21 to 1.00) 0.83 1.56 (1.17 to 1.96) 0.95 1.71 (1.50 to 1.92)
ConHGRF 0.20 ± 0.14 0.19 ± 0.16 0.58 ± 0.36§ 0.21 ± 0.23 0.00 0.003 (−0.11 to 0.11) 0.29 0.37 (0.15 to 0.58) 0.55 −0.39 (−0.58 to −0.20) 0.02 −0.02 (−0.12 to 0.07)
*Horizontal forces are all posteriorly directed.
GBS = goblet squat; LMS = landmine squat; PeakVGRF = peak vertical force; EccVGRF = eccentric vertical force; EccHGRF = eccentric horizontal force; ConVGRF = concentric vertical force; ConHGRF = concentric horizontal force.
Significant (p < 0.05) difference between men and women.
§Significant (p < 0.05) difference between squat conditions.

Discussion

To the best of our knowledge, this is the first study comparing the differences between these commonly used exercises and detailing characteristics of the LMS. This study investigated the muscle activity and kinetics of the GBS and LMS among college-aged, recreationally active, men and women. The main findings were that the LMS, compared with the GBS, reduced activity in the quadricep muscles and vertical ground reaction forces, while increasing posterior horizontal ground reaction forces. Along with the decreased quadriceps activity in the LMS, men showed a decrease in ST whereas women showed a decreased BF. In addition, women exhibited a higher ST in the LMS than men. Women showed greater VM activity compared with men overall (squat conditions combined) and in the GBS. In the LMS, women showed greater activity of both VM and VL compared with men. Women produced a greater eccVGRF in both the GBS and LMS and less posterior horizontal forces in the LMS. Between squat conditions, the only force variable not significantly different was the conHGRF for women in the LMS.

In both men and women, the VM and VL showed decreased activity in the LMS compared with the GBS, which may coincide with previous studies that showed increasing the load anteriorly decreases the quadriceps activity (4,22). Previous research has demonstrated more trunk flexion during a drop landing may shift the center of mass toward the knee joint and decrease the moment arm of the knee. The result of the forward trunk flexion was reduced vertical and posterior horizontal ground reaction force and decreased quadriceps EMG activity (4). It is possible that the increased anterior load and forward lean of the LMS causes a similar shift in the center of mass, resulting in reduced thigh muscle activity. Similar to Blackburn and Padua (4), this study showed a decrease in vertical ground reaction force and quadriceps EMG activity, although we also showed an increased posterior horizontal ground reaction force, which they did not. This increased posterior horizontal ground reaction force is likely a result of the LMS forward lean, while still maintaining the decrease in vertical force and quadriceps activity. Although joint angles and moments were not assessed in this study, the reduced quadricep activity of the LMS may be beneficial when a more balanced response between quadriceps and hamstring activity is required. This may be an important consideration for injury prevention or reducing strain on the knees. Conversely, the GBS may be better suited when increased quadriceps activity and development is a focus. Further research into the mechanisms and kinematics of the LMS is warranted.

In addition to decreased quadriceps activity in the LMS, men had a lower activity in ST, whereas women had a lower activity in BF. Women also had a significantly higher ST response in the LMS compared with men, and although not significant, men had slightly greater BF activity. Previous research of lower-body muscle activity between men and women in the back squat found that men produced greater activity of the BF muscle, whereas the ST muscle was not significantly different (25). Although there is some similarity in the opposing hamstring response, women in this study had higher ST activity compared with men. In addition, differences were found in the LMS, and the back squat from Mehls et al. (25) is mechanically similar to the GBS, which showed no hamstring differences in this study. It is worth noting that the Mehls et al. (25) used 85% of a 1 repetition maximum, and this study used a much lighter intensity relative to body mass, which could explain some differences. Results from this study also somewhat contradict Lynn and Noffal (22) that found women produced greater BF activation in both the regular and counterbalanced squat, although ST was not measured. Functionally speaking for the squat exercise, the hamstring muscles, ST and BF, work together initially with gastrocnemius to unlock the knee and initiate the eccentric descent phase and then act as antagonists for the eccentrically acting quadriceps (35). During the concentric ascent phase, ST and BF have shown increased activity while lengthening and may contribute by pulling the knee rearward into extension (35). More importantly, the hamstring muscles provide posterior shear force on the tibia during dynamic knee flexion movements (2). Weakened hamstring muscles may not exert enough posterior tibial shear force to resist the opposite anterior shear force, increasing the risk for ACL injury (2). Sex differences in hamstring muscle activity during the LMS should be taken into advisement when programming for different populations.

Although the LMS showed a decrease in quadriceps activity compared with the GBS, women in both squat conditions displayed higher quadriceps activity compared with men. Although women showed greater activity of VM and VL in the LMS, only VM was greater in the GBS. This may indicate the women in this study were quadricep dominant, although percent MVIC is sensitive to the magnitude of the dynamic trial relative to the MVIC trial. This study coincides with Youdas et al. (47) that found women produced greater quadriceps and lesser hamstring muscle activation than men during a single-leg squat. However, this contradicts other studies that have shown no difference in quadriceps activity between men and women in the squat exercise (22,25). Although EMG muscle activity and force production are not the same, they have shown a strong relationship (4,22), and some insight can be gained from studies that measure hamstring and quadriceps strength. Injuries of the ACL can be characterized by an increase in abduction and torque about a nonstable knee joint because of a lack of neuromuscular and skeletal muscle control (27). Myer et al. (27) found 22 female soccer and basketball players who suffered a noncontact ACL injury had weaker hamstrings relative to the quadriceps when compared with 88 female and male control athletes who did not suffer an ACL injury. Although some different and even contradictory research exists on the nature of quadriceps dominance in women, none have been found specifically on the LMS making it more difficult to compare. Regardless, results of this study indicate women have greater quadriceps activity and both the GBS and LMS should be complemented with hamstring strengthening exercises.

Although vertical and horizontal force was relatively constant in the GBS, the LMS showed shifting forces depending on the phase of the movement. At the top of the lift, near the initiation of the eccentric phase, the posterior directed horizontal force was near its peak and the vertical force was reduced. As the lifter descended into the full depth, the vertical force increased near its peak and the posterior horizontal force reduced. Women applied a greater eccVGRF than men in both the GBS and LMS and a lesser eccHGRF in the LMS. A possible explanation for this could be the taller stature of men, who start the LMS with an increased bar height. This would reduce the vertical loading and increase the horizontal loading, at least at the top of the lift. However, this does not explain the greater eccVGRF in the GBS, as both men and women had similar vertical loading throughout the movement. For the concentric phase of the LMS, men produced significantly less conVGRF, and greater posterior conHGRF, compared with the GBS. By contrast, although women also saw a decrease in conVGRF, their conHGRF production in the LMS was significantly less than men and not different from the GBS. At the bottom of the squat, when vertical force was near its peak, men still produced a posterior force in the LMS, whereas women's posterior force production was negligible. This may also be a result of size differences between men and women and that men produced greater posterior horizontal force at the top of the lift as well. However, it is interesting that there are horizontal differences at the bottom of the LMS, as there were no differences in either conVGRF or peakVGRF. This is in contrast to the top of the lift when women's greater eccVGRF was paired with a lesser eccHGRF. Nevertheless, overall kinetic differences between squat condition and sex may be important considerations for exercise prescription.

The force-vector theory implies that training exercises of horizontal force application may be more specific to sports or skills that require horizontal force production (33,48). Recently, this theory has been disputed based of the results of a 14-week hip thrust training study by Fitzpatrick et al. (13) in 11 female athletes. They established that the hip thrust exercise improved performance, but there was no difference between vertical and horizontal jumping ability. This was contradictory to the findings of Contreras et al. (10), who also did not find any effect on horizontal jumping and relied on improved 10 and 20 m sprint times to support the theory. Although current research might be unclear on the nuance of force-vector theory, it has been recommended that the combination of exercises, both vertical and horizontal, is most beneficial for sport performance (10,33,48). To what degree the LMS fits in the theory or contributes to horizontal field performance is unknown. Although it was believed that the LMS could help develop horizontal force production, results from this study may not support this as the horizontal forces were considerably less than the vertical forces. However, the posteriorly directed horizontal force shifts throughout the LMS and as the lifter descends the vertical forces increase while posterior horizonal forces decrease. This continuous change in force throughout the movement requires balance and coordination and could be a good accessory to training programs using both vertical and horizontally loaded movements.

An exercise progression is used to teach complex movements, starting with the simplest and progressing to the complex (7). It has been suggested that a squat progression could begin with the plate squat, which is performed with one end of a weight plate on top of the head and the other end held up with both hands. The plate should stay parallel with the ground during each repetition (7). The box squat may also be useful where a taller box is used to teach the exercise and progressed by decreasing the box height over time (42). Recently, a task-based movement progression has been suggested for the rehabilitation process for an ACL repair. After the first step of achieving normal walking gait, the second phase is the bilateral squat to increase lower-body strength and develop motor patterning for more complex movements (5). Suggested variations of the bilateral squat include the wall squat and GBS, performed with gradually increasing load (5). It is also important to consider adequate loading to encourage neuromuscular adaptations but also protect the ACL repair from excessive strain (5). With this in mind, some inference may be drawn from the results of this study and the possible roles of the GBS and LMS in a squat progression. Vertically loaded squats such as the plate squat, taller box squat, or GBS might be a good starting point for a squat progression and the LMS may be a good next step. The leaning nature of the LMS might be slightly more complex and challenge the lifter to maintain a balanced movement as the vertical and horizontal forces change through the lift. In addition, as squat proficiency improves and loading begins to increase the LMS applies less vertical force to the lower body for the same relative load, which then could be followed by the same relative load in a vertically oriented squat such as the GBS or back squat.

Despite some interesting results, several limitations of this study must be acknowledged. First, data from the EMG and force plate were not synced and were recorded and analyzed separately. This makes it more difficult to draw inferences about the relationship between the EMG and kinetics and therefore were treated relatively separate in the results. In addition, EMG was only recorded as the peak, and mean peak, whereas eccentric and concentric phases were derived for the forces. Next, the load of 30% of body mass may have been a relatively higher intensity for women, which may be a possible explanation of the greater quadriceps activity. As mentioned, the LMS is potentially easier to support a greater load than the GBS and differences in this study only reflect the 2 conditions at the same relative load. Also, the greater vertical forces and EMG results for the GBS may reflect increased loading throughout the whole range of motion. Finally, this study used recreationally active men and women who potentially limit inference to other populations such as athletes with more training experience. Further investigations into landmine exercises such as weight distribution of the load during the arced bar path and comparisons with other squat variations are warranted. Elucidating the specific training results and to what degree it compares with other measures and athlete characteristics would be valuable.

Practical Applications

The results of this study indicate that overall, the LMS produced less vertical and more horizontal loading than the GBS, and reduced muscle activity of the quadriceps. In the LMS, men and women had an opposite decrease in ST and BF, respectively, and women exhibited greater quadriceps activity in both squat conditions. Women also produced greater eccentric vertical force for both squat conditions and less posterior horizontal forces in the LMS. The findings from this study can provide practitioners some evidence for including the LMS into their training programs and adds further insight into sex-related differences. The specific kinetic and muscle activity response of the LMS may make it a viable accessory to programs that include vertically and horizontally loaded movements. Attenuating the vertical force may have application in introducing tolerable load to the squat exercise in rehabilitation settings. The LMS may also find application in injury prevention programming, as the reduction in quadriceps activity could improve the ratio of hamstring to quadriceps recruitment. This may be more protective for women considering the hamstring to quadriceps imbalance frequently described in the literature. In this study, women still had more quadriceps activity than men, so both the GBS and LMS would still need focused accessory exercises that target the hamstring muscles. The GBS may also be programmed if the focus is on quadriceps activity and vertical loading. The findings of this study are mostly descriptive in nature and to what degree the LMS improves performance characteristics is unknown. Squat exercises are well studied, so benefits could be extrapolated, but the LMS may have specific adaptations that differ from the vertically loaded squat exercises.

Acknowledgments

This study received internal funding from the Department of Health, Nutrition, and Exercise Sciences, and College of Human Sciences and Education, at North Dakota State University. The results of this study do not constitute endorsement by the authors or the NSCA, and the authors report no conflict of interest.

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Keywords:

horizontal force; electromyography; resistance exercise; sex comparison; injury prevention

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