Comparison of Kinematics and Muscle Activation in... : The Journal of Strength & Conditioning Research (original) (raw)
Introduction
Traditional free-weight exercises provide constant resistance in the range of movement (ROM). Because of this constant resistance, in multijoint exercises, such as bench press and squat, there are often regions in which subjects can produce less force than in other regions such as the sticking region (11,29,31). The sticking region is referred to as the region from the initial maximum upward velocity to the region associated with the lowest concentric velocity of a barbell after which the barbell velocity increases again (18) and has only been reported in near-maximal to maximal loads during the upward movement in multijoint exercises (9,10,17,24,28), or when fatigued (12,26,30,31). In this region, failure often occurs during lifting (9,17,24,30); however, in barbell heights above the sticking region (poststicking region), more force can be produced (28).
It has been proposed that the cause of the sticking region in multijoint exercises is a poor mechanical force position in which the maximal force generation declines because of reduced lengths and mechanical advantages of the muscles involved (18,28,32). It has been demonstrated that some muscles (the pectoralis major, triceps, and deltoids during bench press) are responsible for getting the loads through the sticking region (28,33,34). For example, van den Tillaar et al. (28) demonstrated that, in the bench press, some muscles are more active and can produce more force in the poststicking region than in the sticking region. Differences in muscle activation were recently reported in squats in the sticking region (29).
Thus, when performing free-weight exercises, there is a mismatch throughout the ROM between the torque created by the weights and the muscles' ability to produce torque because of the constant resistance. To maximize the force and torque generation throughout the ROM, variable resistance has been applied using cam-based machines, chains, or elastic rubber bands (5,19,22). These systems cause more force in regions in which the muscles can produce more force (i.e., the poststicking region). However, there is controversial evidence regarding the effects of variable resistance. Comparably, increased muscle activation and force output have been demonstrated using variable instead of constant resistance throughout the ROM (1,2,16,36). Increased resistance throughout the ROM increased the stress and the neuromuscular activation in squats (15), which might enhance the long-term training effects more than using constant resistance. However, to the best of our knowledge, no studies have analyzed the sticking region comparing effects of including elastic rubber bands, kinematics, and muscle activation in free-weight multijoint exercises. For example, Ebben and Jensen (8) reported similar muscle activity and ground reaction force comparing constant and variable resistance (chains and elastic bands) in free-weight squats, but the authors only included the eccentric vs. concentric phases in the kinematics analyses.
Therefore, the aim of the study was to compare the kinematics and muscle activation in 2-legged free-weight back squats using constant or variable resistance with similar relative intensity to fatigue (6 repetition maximum [6RM] loads). It was hypothesized that the length of the sticking region would increase due to the increased resistance of the elastic band. In addition, it was hypothesized that muscle activity would be the same in the presticking region but would be increased in the poststicking region using variable resistance.
Methods
Experimental Approach to the Problem
To compare the effect of elastic bands on kinematics and muscle activation in 6RM free-weight back squats, repeated-measures designing was conducted. After a familiarization session with the 6RM loads, the participants tested 6RM in the back squat using constant resistance (free weights) and variable resistance (free weights + elastic bands) in a randomized order. The barbell velocity, barbell displacement, the time spent in the different lifting regions, and associated muscle activation performing the 2 squat modalities were compared.
Subjects
Twenty healthy recreationally trained women (mean age = 23.3 ± 2.6 years, age range = 21–27 years, mean stature = 1.68 ± 0.06 m, mean body mass = 65.3 ± 8.5 kg) who demonstrated a clear sticking region (29) in the sixth repetition using both constant and variable resistance were included as participants in the study. All participants had resistance training experience (4.6 ± 2.1 years) but were not competitive powerlifters or weightlifters. The participants were trained in back squats twice a week as part of their training program. The participant's relative strength (6RM load/body mass) in squats was 1.1. Seventy-two hours before testing, the participants were instructed to refrain from any additional resistance training.
Before the study, each subject was informed of the testing procedures and possible risks, and written consent was obtained from each participant. The participants had to be free of any musculoskeletal pain, injury, or illness that might reduce their maximal effort. None of the participants experienced pain during the test. Ethical approval for this research study was obtained from the local research ethics committee (31359/3/SSA) and conformed to the latest revision of the Declaration of Helsinki.
Procedures
The free-weight back squat was performed in a power rack (Gym 2000, Modum, Norway) with an Olympic barbell (diameter = 2.8 cm, length = 1.92 m). The exercise started with fully extended knees and a natural sway in the lower back, which was maintained throughout the entire execution. Using a self-paced but controlled tempo, the participants lowered themselves to 80° knee flexion (180° fully extended knee) measured with a protractor (femur–fibula). When the participants had the correct knee angle, a horizontal elastic band was adjusted (4,25). The participants had to touch the band (midthigh) in every repetition before starting the concentric phase. A test leader gave oral confirmation when the participants touched the band. If the participants successfully lifted six repetitions, the loads were increased until the true 6RM in the 2 squat modalities were achieved. The interclass coefficient between the familiarization session and experimental session was 0.952 using constant resistance (free weights) and 0.912 using variable resistance (free weights + elastic bands).
The procedures used in the squats performed with variable resistance were identical to the constant resistance condition with 1 exception: 2 elastic bands (R.O.P.E.S 3002 Bungee, Norway) were attached at the bottom of the power rack on both sides of the barbell creating variable resistance (Figure 1). The external load provided by the elastic bands decreased with decreasing knee angles and increased with increasing knee angles. The barbell and the elastic bands did not provide sufficient 6RM loads alone. Therefore, weight plates were added to increase the total resistance.
The placement of the elastic bands performing squats with variable resistance.
The force provided from the elastic bands with different lengths when stretched was measured using a force cell (Ergotest Technology AS, Langesund, Norway). The external forces provided by the elastic bands as they stretched were close to having a linear relationship (Figure 2). Therefore, the total resistance in the variable resistance group during the different concentric lifting phases was the sum of the weight of the barbell, external loads, and the force provided by the stretch length of the elastic bands (25).
Relationship between the stretching length of the elastic band and the resistance provided by the elastic bands.
Before the 6RM test, the participants performed a 10-minute warm-up on a cycle ergometer or treadmill while talking. The participants then performed 3 warm-up sets of traditional back squats using their self-estimated 6RM loads to calculate the warm-up resistance: 20 repetitions at 25%, 10 repetitions at 50%, and 8 repetitions at 70% of the 6RM load. The testing order was randomized. The load in the 6RM test was increased to either a load that resulted in failure to complete the final repetition or to a load where the participants and test leaders agreed that it was the true 6RM load. The 6RM was achieved within 1–4 attempts with 4–5 minutes of rest between each attempt (13). Half of the participants started with the constant resistance, while the other half started with the variable resistance 6RM attempts. After shifting from the first squat modality to the other, the participants executed 2 or 3 nonfatigue habitation sets (4–8 repetitions at 50% of familiarization 6RM loads with 3 minutes of rest between each set).
The surface electromyographic (EMG) bipolar electrodes (contact diameter = 11 mm and center-to-center distance = 20 mm) were positioned on the preferred foot (4) on the vastus medialis (80% of the distal distance between the anterior spina iliaca superior and the joint space in front of the anterior border of the medial ligament), vastus lateralis (2/3 of the distal distance between the anterior spina iliaca superior and the lateral side of the patella), and biceps femoris (50% of the distance between ischial tuberosity and the lateral epicondyle of the tibia) according to the SENIAMs recommendations (14). Before the placement of the gel-coated self-adhesive electrodes (Dri-Stick Silver Circular sEMG Electrodes AE-131; NeuroDyne Medical, Cambridge, MA, USA), the skin was shaved, washed with alcohol, and abraded, as recommended in a previous study (14).
A commercial EMG recording system was used to measure EMG activation (MuscleLab 4020e, Ergotest Technology AS). To minimize the noise induced from external sources through the signal cables, the EMG raw signal was amplified and filtered using a preamplifier located as close as possible to the pickup point. The preamplifier had a common mode rejection ratio of 100 dB. The EMG raw signal was then bandpass-filtered (fourth-order Butterworth filter) with cut-off frequencies of 8 and 600 Hz. The bandpassed EMG signals were converted to root mean square (RMS) signals using a hardware circuit network (frequency response = 0–600 kHz, average constant = 100 milliseconds, total error = ± 0.5%). Finally, the RMS-converted signal was sampled at 100 Hz using a 16-bit A/D converter (AD637). A commercial software program (MuscleLab V8.13; Ergotest Technology AS) was used to analyze the stored EMG data.
Only the final repetition of the 6RM lift was included in the analyses because the sticking region occurs only during the upward phase at near-maximal load or fatigue (9,17,29,31). A linear encoder (sampling frequency of 100 Hz, ET-Enc-02; Ergotest Technology AS) was attached to the barbell during the squats to measure barbell velocity, lifting time, and vertical displacement. The linear encoder synchronized the EMG measurements using MuscleLab 4020e (Ergotest Technology AS) and used to identify the different lifting regions using the same approaches as previous studies (29,34). The following regions were identified and used to calculate the EMG activity: (a) the presticking region from the lowest barbell position until the first barbell peak velocity (Vmax1), (b) the sticking region from the first barbell peak velocity until the lowest barbell velocity (Vmin), and (c) the poststicking region from the first located lowest barbell velocity until the second barbell peak velocity (Vmax2) for the sixth repetition (9,17). The mean muscle activity (RMS) of the 3 regions was calculated and used for further analysis. The RMS values were not normalized as the aim of the study was to compare the muscle activity between the 2 squat modalities and the relative muscle activation values from normalization would not provide any further information (20).
Statistical Analyses
To assess differences in the EMG activity between the constant and variable resistance, a 2-way (lifting phase: presticking and poststicking phase × squat modality: constant vs. variable) analysis of variance (ANOVA) with repeated measurements was used. If differences were detected by the ANOVA, paired t post hoc tests with Bonferroni post hoc corrections were used to determine the identity of the differences. To assess the differences in barbell velocity, barbell displacement, and the time spent in the different lifting phases between constant and variable resistance, paired sample _t_-tests were used. To compare the total resistance in the 3 lifting phases between the squat modalities, a 1-way ANOVA with Bonferroni post hoc corrections was used. Where sphericity assumptions were violated, Greenhouse-Geisser adjustment of the _p_-values was reported. The criterion level for significance was set at p ≤ 0.05. Effect size was evaluated with η2 (eta partial squared), where 0.01 < η2 < 0.06 constitutes a small effect, 0.06 < η2 < 0.14 constitutes a medium effect, and η2 > 0.14 constitutes a large effect (7). Statistical analysis was performed in SPSS, version 21.0 (SPSS, Inc., Chicago, IL, USA) and differences with p ≤ 0.05 were considered statistically significant. All results are presented as mean ± SD values.
Results
The average 6RM lifting weight with the constant resistance was 72.7 ± 8.6 kg and 76.3 ± 11.2 kg with the variable resistance weight (loads: 49.9 ± 9.7 kg + mean resistance from the elastic band: 26.4 ± 15.7 kg). A 1-way ANOVA for repeated measures indicated a significant effect for the resistance during the different lifting phases with the constant resistance (F = 87.0; p < 0.01; η2 = 0.82). The post hoc comparison showed that the constant resistance load (712 N) was comparable with the variable resistance load at the presticking region (697; p = 0.78), while the variable resistance was significantly higher at the sticking (105%; p = 0.02) and poststicking regions (113%; p < 0.01) when compared with the constant resistance load (Table 1). The first peak barbell velocity (Vmax1) was significantly higher when using variable resistance than constant resistance (21.0%), whereas the opposite was found for the second barbell peak velocity (−22.8%). No significant differences in velocity were observed at Vmin (p = 0.42; Table 1; Figure 3). When comparing vertical displacement and time occurrence of the different regions, only a significantly higher barbell displacement at Vmax2 was found for the variable resistance compared with the constant resistance (Table 1). No other significant differences in measured kinematics were found (Table 1).
Kinematics of the sixth repetition for the variable and constant resistance modalities in free-weight back squats.
The barbell kinematics in the sixth repetition in squat using constant or variable resistance. *Significant difference (p ≤ 0.05) between the 2 squat modalities in barbell speed in the different lifting regions.
A 2-way ANOVA for repeated measures performed on EMG and squat modality of the different muscles indicated significant main effects for the lifting phase (F ≥ 5.505; p ≤ 0.008; η2 ≥ 0.23) in biceps femoris, vastus medialis, and vastus lateralis. However, no main effects for squat modality (F ≤ 1.545; p ≥ 0.229; η2 = 0.08) or interaction (F ≤ 2.819; p ≥ 0.072; η2 ≤ 0.13) for any of the muscles were found. Post hoc comparisons revealed that, for the biceps femoris, the activity significantly increased from the presticking to the sticking phase (constant; 50.0%, variable 69.5%) with no significant change from the sticking to the poststicking phase (Figure 4A). The EMG activity decreased significantly from the sticking to the poststicking region in the vastus medialis (constant −12.1%; variable −8.8%) and vastus lateralis (constant −12.6%; variable −9.9%), while no significant changes were observed from the presticking to the sticking region (Figures 4B, C).
A–C) The muscle activity (mean ± SD) in the sixth repetition in squat using constant or variable resistance for the biceps femoris (A), vastus medialis (B), and vastus lateralis (C).
Discussion
The purpose of the study was to compare kinematics and muscle activation in 2-legged free-weight back squats using constant or variable resistance with a similar relative intensity to fatigue (6RM loads). The main findings were that the length of the sticking region was the same for both squat modalities and that the first and second peak barbell velocities were results of the variable resistance, which caused a higher resistance in the poststicking region. In addition, no significant differences in the EMG activity were observed between the modalities.
As hypothesized, a clear sticking region was observed in both squat modalities in the sixth repetition. The results are in line with previous studies examining the bench press at near-maximal effort (9,17,24,31) and, most recently, in squats (29). Furthermore, the variable resistance of the elastic bands influenced the kinematics of the barbell; the first barbell peak velocity (Vmax1) was greater and the second barbell peak velocity (Vmax2) lower than the constant resistance of free-weight 6RM squats. The results are in line with the hypotheses. Using variable resistance in squats, the force needed to move the barbell upward increased with greater knee angles as the elastic bands provide increasing resistance with greater length of the elastic bands (Figure 2) (25). This resulted in a significantly greater total resistance using variable resistance in the sticking and poststicking regions compared with constant resistance (Table 1). This explains the decreased barbell velocity at the Vmax2 (11). The increased Vmax1 with the variable resistance squat modality was not expected, because the resistance at this lifting height was approximately the same between the 2 (Table 1). Possible explanations for these findings can be potentiation, the active state of the muscles, and stability during lifting with the variable resistance modality. Because of the nature of elastic bands, the participants are pulled down more at the beginning of the downward movement. This might cause a higher active state and more potentiation, and/or store more elastic energy at the bottom of the lift, which can result in a higher first peak barbell velocity that has been demonstrated in explosive movements (35). Still, the prolonged transition period between the eccentric and concentric phases would result in loss of any stretch shortening cycling enhancements (23,27,28) and, thereby, not be the explanation of increased Vmax1 using variable resistance.
Another possibility is the lifting movement caused by the elastic bands. These bands are connected from the barbell to the ground, and the participants are pulled down to the attachment point of the ground. It could therefore be speculated that the lifting pattern was more vertical with less hip flexion (decreased horizontal movement) than with free weights and, thereby, could increase the Vmax1 in the variable squat modality. However, 3D analyses were not conducted to support the speculation.
It was hypothesized that the length of the sticking region would increase because of the increased resistance of the elastic band. However, this was not found in this study: the lifting phases occurred with similar vertical displacement of the barbell and time. This surprising result could be the cause of the low variable resistance differences. However, the loads performing variable resistance were 98, 105, and 113% of the constant resistance in presticking and poststicking regions. The differences in loads between the modalities were perhaps not large enough to change the sticking region. Furthermore, the results indicated that the sticking region was caused by a poor mechanical force position at specific joint angles, as previous studies have suggested for the bench press (9,18). This is true even when greater peak barbell velocity (Vmax1) was observed with the variable resistance, which, theoretically, should lead to a greater vertical displacement in the presticking region. However, because of the combination of a small time interval (0.41–0.44 seconds), low maximal barbell velocity (0.19–0.23 m·s−1), and increasing resistance (with the variable resistance) in the presticking region, no significant differences in displacement were observed. Because only the final repetition in 6RM in each squat modality was studied, the participants were close to fatigue and near-maximal effort (28,29,33). This resulted in a sticking region for both squat modalities, which is in line with the findings of earlier studies using multijoint exercises as bench press (30,31) and back squats (29).
No difference in the EMG activity was observed between the variable and constant resistance, which was not hypothesized, because the resistance increased significantly in the last region with more than 100 N. Normally, this would result in higher EMG activity of the prime movers (3,21). However, low differences in resistance between the 2 modalities and the possible different lifting movement in the variable resistance condition could explain that EMG activity is not different between the 2. The results are supported by Ebben and Jensen (8) who reported similar quadriceps and hamstring activation comparing free-weight squats and squats with variable resistance (elastic bands and chains). Unfortunately, Ebben and Jensen (8) only analyzed the lifting movement in the eccentric and concentric phases and the results are therefore not comparable with those of this study. The EMG activity of the biceps femoris and the vastus lateralis and medialis showed the same muscle activation pattern as found by van den Tillaar et al. (17), in which the biceps femoris increased the EMG activity from the presticking to the sticking region, whereas the vastus muscles decreased their activity in the poststicking region. However, both this study and the study by van den Tillaar et al. (17) were limited by not including the glutei muscles. The gluteus muscles are mainly responsible for the hip extension in multijoint exercises as the squat (6), and it can be speculated that the EMG activity can be changed with variable resistance. The majority of the participants refused to participate if the EMG measurement of the gluteus maximus was included. Therefore, it was not possible to include the EMG measurement of the gluteus maximus in this study.
Future studies should include the EMG measurement of the glutei muscles, use 3D analyses, and test 1RM to examine what the limitations are during squat lifting. In addition, to improve the knowledge of what happens in the muscles and kinematics using variable resistance, the authors suggest increasing the percentage resistance from the elastic bands or using chains in the total resistance group.
Practical Applications
Heavy resistance training (i.e., squats) has been shown to be effective for improving maximal strength and jump heights. However, when training with heavy resistance, maximal effort is only required at the beginning of the concentric lifting phase: neuromuscular stress decreases throughout the concentric phase. Theoretically, variable resistance may provide near-maximal neuromuscular stress throughout the whole ROM because of the increasing load in the concentric phase. However, this study did not demonstrate that variable resistance training had an increased effect on muscle activation in the sticking and poststicking regions because of increased resistance when compared with constant resistance. This was probably caused by the use of elastic bands that did not provide enough resistance in these regions, whereas stability during the lift increased. Therefore, the use of chains is suggested, because they also increase the resistance during the lift, but not the stability. When performing squats with heavy resistance, the authors recommend including variable resistance with greater resistance than used in this study.
Acknowledgments
This study was conducted without any funding from companies or manufacturers or outside organizations. The results of this study do not constitute endorsement by National Strength and Conditioning Association.
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Keywords:
EMG; variable resistance; constant resistance; strength training; sticking region
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