Since the beginning of this century, core stability has attracted much attention from sport practitioners and researchers (8). A number of studies have examined the effect of unstable condition on force generation. Behm et al. (7) revealed that unstable condition could lead to decrease in the force generation of the limb and increase in antagonist muscle activation. Anderson and Behm (2) identified that unstable condition decreased force generation but did not change muscle activations of upper body during bilateral contractions. Anderson and Behm (4) found that unstable conditions increased the activities of trunk stabilizers and postural muscles, but only negligible increases of the prime movers.
A variety of unstable platforms have been used in different studies. Swiss ball was one of the most popular devices (2,7,9,24). Because of the difficulty of performing upright exercises on a Swiss ball, most exercises were performed at supine or prone positions. Some studies examined upright movements on unstable platforms, such as Dyna Disc, BOSU ball, wobble board, and Airex cushion. Wahl and Behm (25) found that not all instability training devices enhanced muscle activation in highly resistance-trained individuals. The use of moderately unstable training devices (i.e., Dyna Disc, BOSU ball) did not provide sufficient challenges to the neuromuscular system for these individuals. Krause et al. (19) showed that the gluteus medius electromyographic (EMG) did not change when exercises were performed on a stable verses unstable surface.
Squatting is one of the most widely used exercises for strength development of the lower limb extensors during general fitness and rehabilitation (1,11,22). As described by Kornecki and Zschorlich (18), greater instability would stress the neuromuscular system to a greater extent than traditional training methods using more stable benches and floors. Therefore, squat under unstable conditions, such as on Dyna Disc, is preferred by some athletes during training. It is believed that less load on unstable surface could achieve the same level of muscular activation compared with on stable surface. Anderson and Behm (4) examined the muscular activities of squat under different unstable conditions and identified that the activities of trunk stabilizers and postural muscles increased with level of instability. Hamlyn et al. (13) compared the activations of trunk muscles in squat and some selected instability exercises (i.e., superman and side bridge exercise) and found that the trunk muscular activities in 80%RM (repetition maximum) squat was much higher than in body weight squat, and in some instability exercises.
To our best knowledge, the debate still exists between resistance training under stable and unstable conditions, and there are limited studies that emphasized on the effect of unstable resistance training under different weight loads. Therefore, the objective of this study was to compare the muscular activities of lower body and trunk on unstable and stable platforms under different weight loads. Based on the previous studies, it was hypothesized that the acute effect of instability will increase the muscle activities, but the effect of instability might be limited compared with the effect of external weight load.
Experimental Approach to the Problem
All subjects were asked to squat on Reebok Core Board (RCB; with second level of instability, Figure 1A, unstable) and ground (Figure 1B, stable) under body weight, 30%RM and 60%RM, respectively. The sequence of 6 conditions (3 levels of weight load × 2 levels of instability) for each subject was randomized to eliminate the possible fatigue and learning effects. Subjects had 2- to 3-minute rest between 2 conditions. A Polar S610 heart rate monitor (Polar Electro, Kempele, Finland) was used to make sure that subjects’ heart rate was under 100 b·min−1 after each rest. Electromyographic activities of soleus (SO), vastus lateralis (VL), vastus medialis (VM), rectus femoris (RF), biceps femoris (BF), gluteus maximus (GMa), gluteus medius (GMe), and upper lumbar erector spinae (ULES) muscles were recorded with surface electrodes throughout the test.
Thirteen male students (19.4 ± 1.2 years, 176.9 ± 4.8 cm, 67.9 ± 4.8 kg, Table 1) from a physical education college volunteered to participate in the study with written consent. All subjects had an experience of 3 years traditional resistance training during high school, but no experience of resistance training on unstable platforms. All participants were free of any injury or otorhinolaryngological diseases during the past year. The participants were included according to the consideration of enough training experience of squat, and without much influence from sport-specific training. Detailed procedure of study and the possible risk were informed before the experiment. This study was approved by the National Sport Science Society.
Bipolar surface EMG electrodes were used to measure signals from the SO, VL, VM, RF, BF, GMa, GMe, and ULES. All electrodes were placed on the right side of the body. The subjects’ skin was prepared by gentle local shaving and abrasion and cleaned with alcohol before attachment of the surface electrodes, in accordance with the SENIAM recommendations for skin preparation (14). Noraxon dual electrodes (Noraxon USA, Inc., Scottsdale, AZ, USA) were placed on the skin surface according to the manual from Noraxon USA, Inc. Location for ULES was identified 6 cm lateral to the L1–L2 spinous processes in accordance with Behm et al. (9).
Electromyographic data were collected with the Noraxon Telemyo 2400R (Noraxon USA, Inc.) EMG system, a frequency-modulated telemetry system. Electromyographic signals were collected at 1,500 Hz from the electrodes, amplified (1,000×), filtered (5–1,000 Hz), and smoothed with MyoResearch software (Noraxon USA, Inc.). The stored data were then normalized by squat cycle. Thereafter, with the help of goniometric signals from Noraxon 2D Goniometer (Noraxon USA, Inc.), electromyographic data of each squat cycle were identified, normalized, and integrated. Normalization was only operated in accordance with knee angles. There was no need to normalize the signal to the maximal voluntary contraction because the experiment was a repeated measures design comparing within individuals with all conditions performed in a row, and each subject finished his test without moving the electrodes. Squat cycles with a correlation coefficient lower than 0.9 were deleted during normalization of squat cycles in each subject. The same step was also executed during normalization between subjects but with a correlation coefficient of 0.5. Integration was performed for the complete squat movement after averaging all the relevant saved squat cycles.
According to McNeely (21), subjects had an indirect maximal strength test 1 week before experiment. Subjects attended an orientation session 5 days before the experiment to familiarize the RCB. The participants were required to follow the normal nutrition routine, but not to conduct exercise intensively the day before test. No abnormal sleep in the past day was reported by subjects. On the day of testing, no food, except drinking water, was allowed 2 hours before the test. Detailed procedure was repeated again to the subjects before the start of test. Thereafter, subjects performed randomly under the 6 squat circumstances mentioned above in a row. All the subjects participated in the test between 900 and 1200 in the morning and 1400 and 1700 in the afternoon in a given day of January.
Because the width of feet (20), direction of toes (12), depth of squat (23), and direction of gaze (10) have an influence on squat performance, the movement of squat was strictly required with fixed feet width, toe direction (15° outside), squat depth (upper leg to horizontal), and gaze direction (horizontal). Before each condition of squat, the subject was asked to perform a standard squat, which served as a reference for the following squats. After the standard squat, the subjects performed 2 sets of 5 continuous times in accordance with the cadence (1-second down, 1-second up) of a metronome, without any encouragement or 'oral guidance. Two- to 3-minute rest was arranged for recovery between the 2 sets and after the 2 sets. In consideration of safety, 2 spotters were arranged to stand by the subject whenever load was carried (Figure 1).
Electromyographic data were analyzed with 2-way analyses of variance with repeated measures (3 levels of weight load × 2 levels of instability). Difference was considered significant at the p < 0.05 level. Effect sizes were reported in parentheses within the Results. Reliability was assessed with a Cronbach’s α model intraclass correlation coefficient. Descriptive statistics included means and SEMs.
The primary findings from this study indicated that the effect of instability on the muscles concerned depended on the levels of weight load, and the muscles, under the conditions as in this study. However, all the differences resulted from the instability were not significant (p < 0.05) under 3 different levels of weight load used in this study. Furthermore, weight load brought increase of activities to all tested muscles, even though some of the increases were not significant.
Difference Induced by Unstable Surface
The RCB did not bring significant change of integrated EMG (iEMG) for all the tested muscles (p > 0.05, Figure 2) under all weight conditions (body weight, 30%RM, 60%RM).
Difference Induced by Weight Load
Whether under stable or unstable circumstances, there were significant increases in muscular activities of SO, VM, GMa, and ULES as the weight load increased (p < 0.05). In contrast, the significance did not exist for the iEMG of BF in any case of weight load increase (p > 0.05). Furthermore, the iEMG of VL, RF, and GMe also increased with weight loads; however, not all the differences caused by increase of weight load were significant (p < 0.05, Table 2).
The current study demonstrated that the unstable platform RCB did not significantly change the activations of major lower limb muscles and some trunk muscle during squat, regardless of the level of weight load. According to the results from Wahl and Behm (25), there was significantly more EMG activity in the SO during the wobble board and Swiss ball conditions than during stable, Dyna Disc, BOSU up, and down conditions. However, there was no significant difference in RF and BF EMG activity. The RCB is more similar to the wobble board; however, RCB can return to the original position when external force is removed because of its elasticity and has 3 levels of instability. Considering the safety of squat under maximal 60%RM, second level of instability was chosen in this study. Reebok Core Board used in this study was probably more stable than the wobble board used by Wahl and Behm (25). The nonsignificant changes in RF and BF in the current study agreed with the results from Wahl and Behm.
The lack of significance in this study might be also related to the testing muscles. The testing muscles were mostly superficial muscles and limb muscles. Many deep muscles, such as multifidus and transverse abdominis, play an important role in stabilizing posture (15–17). However, no deep muscles were evaluated in the current study. Previous studies used surface EMG to measure deep muscle activities. It was found that activity of trunk stabilizers and postural muscles (e.g., multifidus and transversus abdominis) increased with the instability, whereas only negligible increase in prime movers (4). However, we observed that the EMG signals for those deep muscles were largely affected by dynamic movements and sweating at the skin. Therefore, deep muscles were not measured in the current study.
Third, the lack of significant differences observed in this study might result from subjects’ experience of resistance training. The subjects in the current study had an average of 3 years of experience in resistance training that usually involved free weight squat (25). It was postulated that the subjects had a relatively high level of stability control, and therefore, RCB of second level did not generate sufficient stimulus for the resistance-trained subjects.
In addition, it was found that the increases of muscular activity induced by RCB were much less compared with the increases of muscular activity by weight load (Figure 2). This finding was not consistent with the previous reviews on instability resistance. Previous reviews reported that high level of muscular activity could be achieved with less resistance when performing exercises under unstable conditions. It was suggested that the unstable surface training might have positive implication in progressive muscle and joint rehabilitation and sport-specific training (3,6).
The force was proved in this investigation, where it seemed more effective to stimulate the prime mover in way of weight load instead of instability of surface. As a matter of fact, most sports are performed on solid and stable surface (e.g., track and field ground). Based on the concept of training specificity (5), the optimal method to promote increases in balance, proprioception, and core stability for any given sport is to practice the skill itself on the same surface on which the skill is performed in competition (26). It could be inferred that the increase of muscular activity be achieved more effective with weight load than with instability of platform, at least for prime movers in squatting as in this study.
Unstable surface simulated by RCB did not significantly change major muscle activations during a squat task in resistance-trained males. However, the muscle activations increased as the weight load increased. The RCB might not be an effective unstable platform to stimulate the superficial muscles in squat, especially when the subjects are resistance trained. The effects of external loading might not be replaced by unstable surface. Increasing weight load should be considered if the goal is to increase muscle activation levels.
All the tests under 6 squat conditions were performed for each subject in a row, which potentially produced different stimulations to the nervous system because of the different weights and different stabilities occurring in succession and might also be the possible reason for no significant difference between stable and unstable surface.
The authors appreciate Dr. Boyi Dai for his assistance for revising the manuscript. No funding is provided for this research. The results of this study do not constitute endorsement of the product by the authors.
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