The core has been defined as the lumbopelvic hip complex (7), and the core muscles are considered to be the muscles that produce or restrict motion of the core (37). Several recent reviews on core training have suggested that core training is important for rehabilitation, injury prevention, and improvement of athletic performance (4,18,22,24,29,35). For example, ground-based free-weight training with high loads and few repetitions were recently recommended for athletic training of the core (4). Ground-based free-weight exercises (e.g., squats and dead lifts) may be executed with greater external loads and mimic sports activities more than isolated specific core exercises can (4,16,23). Further, it has been suggested that such exercises induce greater stability requirements of the core than do most machines (3).
Training with free weights, however, may not be the optimal approach for activating muscles because this mode of exercise provides constant resistance without taking into account changes in the lever arm and angle: torque relationship (12,20). The maximal weight that can be lifted with constant resistance largely depends on how much a person is able to lift through the heaviest part (sticking point) of the concentric movement (30). In consequence, the use of elastic bands, in combination with free-weight exercises, has become popular, and more recently also scientifically examined (6,27,28,34). Adding elastic bands to free-weight squats provides greater external resistance during the upper part of the movement (greater stretch of the band) and a lower external load during the lower part, compared with constant resistance free-weight squats (2,6,8,13,27,32,34). This variable resistance should, in theory, provide a greater neuromuscular stimulus than should constant resistance across the whole range of motion (ROM) (14,19), and especially in the upper part of the movement. Previous studies have used weights in combination with elastic bands to examine quadriceps electromyography (EMG) activity during squat and leg extension exercise (1,9,31,33). However, to the best of our knowledge, no study has investigated core muscle activation during free-weight squats with and without external load from elastic bands. Therefore, the aim of the study was to compare the core muscle activation in several lifting phases in the squat using free weights and free weights combined with elastic bands, with matched relative resistance. We hypothesized that core muscle activation would be higher with elastic bands in the whole movement, and in the upper eccentric and concentric lifting phases, and lower in the lower concentric and eccentric lifting phases.
Experimental Approach to the Problem
Within-subject repeated-measure designs were used to examine the differences in the 6 repetition maximum (6RM) strength and core (erector spinae, rectus abdominis, and external oblique) muscle activation achieved during the execution of 2 modalities of squats (free weights and free weights + elastic bands). The relative intensity between the modalities was equilibrated (6RM). In the free-weight + elastic band squat, some of the weights used in traditional free-weight squats were replaced with 2 elastic bands. The elastic bands were attached to the lowest part of the squat rack and provided progressively less resistance in the descending phase and progressively increasing resistance in the ascending phase, when the band was stretched more. Three to 10 days before the experimental test, the participants attended a 6RM test of squats using free weights with and without elastic bands in a randomized order. Both squat modalities were executed in 1 session in the experimental test, because it is difficult to replace EMG electrodes at the exact same location.
Twenty-five healthy women (age = 24.3 ± 4.9 years, stature = 1.68 ± 0.06 m, body mass = 65.5 ± 8.6 kg) participated in this study. All the participants had resistance training experience (4.6 ± 2.1 years) but were not competitive power lifters or weight lifters. The participants were accustomed to the squat exercise and had been trained in it regularly (approximately twice a week) for the past 3 months. The participants' relative strength (6RM load per body mass) in squats was 1.1. The participants were instructed to refrain from any additional resistance training in the 72 hours before participating in the test. Ethics approval for this study was obtained from the local research ethics committee and conformed to the latest revision of the Declaration of Helsinki. Before the study, each subject was informed of the testing procedures and possible risks, and a written consent was obtained from each participant. None of the participants reported having any musculoskeletal pain, injury, or illness that could reduce their maximal effort, and none of the participants experienced any pain during the testing.
The free-weight squat was executed in a power rack (Gym 2000, Modum, Norway) with an Olympic barbell (diameter = 2.8 cm, length = 1.92 m). The exercises started with fully extended knees and a natural sway in the lower back, which was maintained throughout. Using a self-paced but controlled tempo, the participants lowered the load such that the fulcrum of the hip was equal to the fulcrum of the knees (full squat). The lowest position was individually measured and controlled for each participant using a horizontal elastic band. A test leader gave a verbal signal when the participants touched the band with the distal part of the gluteus muscles, which indicated that the participants could start the concentric lifting phase.
The same procedure was used in the free-weight + elastic band condition. Two elastic bands (ROPES 3002 Bungee; Norway) were attached from the power rack to both sides of the barbell. The external load provided by the elastic bands decreased with increasing knee flexion, whereas the external forces provided by the elastic bands increased linearly as the elastic bands were stretched (Table 1). Resistance provided by the elastic band was measured using a force cell (Ergotest Technology AS, Langesund, Norway). An example is given in Figure 1.
Before the 6RM test, the participants performed a 10-minute warm-up on a cycle ergometer or treadmill at a light-moderate intensity (comfortable to talk). The participants then performed 3 warm-up sets of traditional squats based on the load from their latest training session (6 repetitions) to calculate the warm-up resistance: 20 repetitions at 25%, 10 repetitions at 50%, and 8 repetitions at 70%. Before the squat testing using the elastic bands, the participants executed 3 nonfatiguing habituation sets (4–8 repetitions) of squats with the elastic bands. The load in the 6RM was increased to either a load that resulted in failure to complete the exercise or to a load that the participants and test leaders agreed was 6RM (i.e., would not have been able to complete 6 repetitions with an additional 2.5 kg). The 6RM was achieved within 1–4 attempts. During the experimental test, adjustments were made when necessary. The intraclass correlation coefficient between the familiarization session and experimental test was 0.952 for free-weight squats and 0.912 for free-weight + elastic band squats.
The surface EMG electrodes were positioned on the rectus abdominis (3 cm lateral to the umbilicus), the external abdominal oblique (∼15 cm from the umbilicus), and the erector spinae (at L1 and 3 cm lateral to the spinous process) (5,17,25). 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 (17). The electrodes (contact diameter = 11 mm, center-to-center distance = 20 mm) were placed on the core contralateral to the side of the dominant leg (5,26). A commercial EMG recording system was used to measure the EMG activation (MuscleLab 4020e; Ergotest Technology AS). To minimize the noise induced from external sources through the signal cables, the raw EMG 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 raw EMG signal was then bandpass filtered (fourth-order Butterworth filter) with cut-off frequencies of 8 and 600 Hz. The filtered EMG signals were converted to RMS signals using a hardware circuit network (frequency response = 0–600 kHz, averaging 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). Commercial software (MuscleLab V8.13; Ergotest Technology AS) was used to analyze the stored EMG data.
Analysis of Different Phases
A linear encoder (100-Hz sampling frequency, synchronized with EMG, ET-Enc-02; Ergotest Technology AS) was attached to the barbell to assess the total lifting time and the different vertical positions of the barbell such that the beginning, midpoint, and the end of each repetition and the eccentric and concentric lifting phases could be identified. The eccentric and concentric lifting phases were both divided in 2 lifting phases (upper and lower) based on the vertical displacement with the middle position used as the cut-off (Figure 1). The external resistance (free-weight load + elastic band resistance) of the lifting phases was calculated for each participant. The taller the participant was, the longer the elastic bands were stretched, and therefore, the greater was the resistance provided from the elastic bands (Table 1). The knee angle at the transition from the upper and lower lifting phases was close to a half squat (middle position). The beginning and the end of each of the 6 repetitions were identified, and the EMG RMS activities were calculated for each repetition and averaged to determine the mean of the 6 repetitions (i.e., short stops between repetitions with the knees extended were removed from the analysis). The same procedure was performed for the eccentric and concentric lifting phases and the upper/lower lifting phases in the eccentric and concentric movement.
To assess the differences in the muscle activity between the squat modalities (free weights and elastic bands + free weights), paired t-tests were used for each of the lifting phases (whole, eccentric, concentric, upper eccentric, lower eccentric, upper concentric, and lower concentric). To assess the differences in the 6RM strength and lifting time between the squat modalities, paired t-tests were used. Unless otherwise specified, results are presented as mean ± SDs and Cohen's d effect sizes (ES). An ES of 0.2 was considered small, 0.5 was considered medium, and 0.8 was considered large. The SPSS software (v20; Chicago, IL, USA) was used for the statistical analyses. A p value ≤0.05 was considered statistically significant.
The 6RM load with only free weights and the mean total resistance with added elastic bands were similar (p = 0.172, Figure 2, Table 2). Further, the 6RM free-weight load compared with the 6RM load with elastic bands was similar in the middle position (p = 0.172), lower in the upper position with elastic bands (p < 0.001, ES = 1.36), and tended to be greater in the lower position (p = 0.083, Table 2). The percentage attributions of the resistance from elastic bands to the total resistance are given in Table 2. Similar lifting time executing 6RM in the measured lift phases using free weights and elastic bands were observed (p = 0.136–0.952).
When comparing the EMG activity between the 2 squat modalities, similar muscle activities were observed in the erector spinae (free weights: 0.209 ± 0.096 mV vs. free weights and elastic bands: 0.210 ± 0.098 mV, p = 0.660), external oblique (0.057 ± 0.050 mV vs. 0.064 ± 0.083 mV, p = 0.608), and rectus abdominis (0.057 ± 0.066 mV vs. 0.057 ± 0.079 mV, p = 0.585).
Similar EMG activities in the eccentric lifting phase in the squat modalities were observed in the erector spinae (free weights: 0.230 ± 0.108 mV vs. free weights and elastic bands: 0.205 ± 0.112 mV, p = 0.215), external oblique (0.061 ± 0.054 mV vs. 0.062 ± 0.084 mV, p = 0.977), and rectus abdominis (0.063 ± 0.072 mV vs. 0.063 ± 0.086 mV, p = 0.584).
Similarly, in the concentric lifting phase, there were no differences in the muscle activity in the erector spinae (free weights: 0.230 ± 0.107 mV vs. free weights and elastic bands: 0.224 ± 0.107 mV, p = 0.782), external oblique (0.060 ± 0.055 mV vs. 0.071 ± 0.093 mV, p = 0.494), or rectus abdominis (0.062 ± 0.073 mV vs. 0.077 ± 0.105 mV, p = 0.315).
A similar muscle activity was observed between the 2 squat modalities (free weights vs. elastic bands + free weights) for both the upper and lower eccentric phases, respectively, erector spinae: 0.150 ± 0.071 mV vs. 0.170 ± 0.083 mV, p = 0.122 and 0.167 ± 0.069 mV vs. 0.184 ± 0.089 mV, p = 0.335; external oblique: 0.034 ± 0.031 mV vs. 0.025 ± 0.043 mV, p = 0.396 and 0.051 ± 0.056 mV vs. 0.031 ± 0.058 mV, p = 0.225; and rectus abdominis: 0.040 ± 0.049 mV vs. 0.035 ± 0.056 mV, p = 0.399 and 0.045 ± 0.054 mV vs. 0.044 ± 0.061 mV, p = 0.729.
A similar muscle activity was observed between the 2 squat modalities (free weights vs. elastic bands + free weights) for both the upper and lower concentric phases, respectively, erector spinae: 0.194 ± 0.092 mV vs. 0.194 ± 0.094 mV, p = 0.996 and 0.219 ± 0.101 mV vs. 0.222 ± 0.100 mV, p = 0.869; external oblique: 0.057 ± 0.063 mV vs. 0.062 ± 0.078 mV, p = 0.681 and 0.065 ± 0.058 mV vs. 0.069 ± 0.085 mV, p = 0.820; and rectus abdominis: 0.057 ± 0.063 mV vs. 0.044 ± 0.054 mV, p = 0.417 and 0.065 ± 0.058 mV vs. 0.053 ± 0.065 mV, p = 0.609.
The main finding of this study was that a similar core (erector spinae, rectus abdominis, and external oblique) muscle activity was observed for free-weight squats with and without added elastic bands to replace some of the resistance. This was the case for all analyzed lifting phases (whole repetition, eccentric and concentric phases, and upper and lower phases).
Theoretically, adding the variable resistance component by elastic bands should allow for a higher muscle activity throughout the whole movement by creating greater resistance in regions at which the musculoskeletal leverage is greater. However, albeit theoretically plausible, we found no evidence of such effects on core muscle activation.
It is even more surprising that we did not observe higher muscle activation for the upper parts of the movement with elastic bands, where the resistance was significantly higher. Previous studies investigating the impact of variable resistance in the squat have demonstrated an increased EMG activity with increasing stress throughout the ROM (1,9,15,19,31), although this is not a universal finding (10). However, these studies examined leg muscles only and not the trunk muscles. Previous studies investigating core muscle activation (not with elastic bands) with different intensities demonstrated a decrease in the EMG activity of the erector spinae, with decreasing test intensities (100–50% of 1RM and body weight) in squats (16,23). However, interestingly, the external oblique and the rectus abdominis were similarly activated under different testing intensities (16,23). Finally, contrary to our hypothesis, the lower resistance in the lower lifting phases of squats with elastic band (Figure 2) did not decrease the EMG activity.
To our knowledge, this is the first study to examine the acute effects of replacing constant resistance with variable resistance of a compound leg exercise on EMG activity in the core. The elastic bands we used contributed to approximate values of 42, 35, and 26% of the total resistance in the upper, middle, and lower positions of the squat, respectively. In addition, adding the elastic bands made the total resistance 117, 105, and 93% in the upper, middle, and lower positions, respectively, relative to the constant free-weight resistance. Albeit, our findings show no differences between loading modalities, it could be speculated that a greater variable resistance provided from the elastic bands (i.e., thicker bands and less free weights) may have resulted in greater core muscle activation. Particularly the erector spinae would have to sustain heavier resistance in the upper phases of the squat to avoid unintended flexion of the truncus. At least this seems to be the case for leg muscles (1,33). For example, Walker et al. (33) demonstrated a greater quadriceps EMG activity during leg extension in the last ROM (120–180° knee angle) with elastic tubes compared with constant resistance.
We speculate that the similarity in the external oblique and rectus abdominis muscle activation between the 2 loading modalities could be because of a relatively low level of core muscle activation (percentage of MVC) necessary to stabilize the trunk during 6RM squat lifting. Previous research examining core muscle activation during squats using heavy resistance (>75% of 1RM) reported muscle activation to be approximately 5 and approximately 10% of MVC for the rectus abdominis and the external oblique (36). However, this should not be the case for the erector spinae in heavy squats because the muscle activity has been reported to be between approximately 55 and 130% of the MVC (16,23,36). Nevertheless, a similar muscle activity between the squat modalities in all lifting phases was observed in this study.
A strength of our study is that the participants were familiar with both testing modalities and had several years of resistance training experience. In hindsight, as we speculate that the negative finding of this study could be because of relatively low levels of core muscle activity, we consider it a limitation that we did not perform MVC tests of the core muscles to provide an estimate of the level of muscle activity during the squats. However, an a priori decision was made not to perform MVC tests because the aim of the study was to compare the EMG activity of the 2 different squat modalities and normalizing to MVC would not provide any further information with respect to this aim (21). Further, there are inherent technical limitations in the surface EMG, and it only provides an estimate of the neuromuscular activity (11). In addition, there is a risk of crosstalk occurring between neighboring muscles, even if a small interelectrode distance is used.
In conclusion, for the free-weight squat, there were no effects of substituting free-weight resistance with elastic band resistance, of the magnitude used in this study, on muscle activity of trunk muscles.
Training with free weights may not be the optimal approach for activating muscles because this mode of exercise provides constant resistance without taking into account changes in the lever arm and angle–torque relationship. Adding elastic bands provides a greater external resistance during the upper part of the squat where the musculoskeletal leverage is greater, and a lower external load during the lower part than constant resistance free-weight squats. This variable resistance could in theory provide a greater neuromuscular stimulus than could constant resistance across the whole ROM and especially in the upper part of the movement.
We observed no effects of substituting some of the free weights with elastic band resistance in the free-weight squat on the muscle activity of trunk muscles. Therefore, free-weight squats and free-weight + elastic band squats seem to be similarly effective in activating the core, when the relative intensity is matched (e.g., 6RM). However, it should be noted that we added elastic bands that made the total resistance 117, 105, and 93% in the upper, middle, and lower positions relative to free-weight squats, respectively. Although not experimentally verified, it is possible that using thicker bands could increase the core muscle activation, because this would have increased the total resistance in the upper position and possibly also the mean total resistance.
The authors thank the subjects for their enthusiastic participation.
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