1. Introduction
In recent years, for enhancing the training effect in sports and rehabilitation exercises, various intervention methods have been implemented, such as providing an unstable environment, whole body vibration, progressive resistance exercise, and task-oriented training [1]. Among the intervention methods, exercising in an unstable environment stimulates the neuromuscular system to induce joint contraction of muscles, and improves mobility and stability through co-contraction of agonistic and antagonistic muscles, thus maximizing the exercise effect. In addition, through the exercise, neuromuscular control training and sensorimotor integration for a single joint can be obtained; muscle activation also increases [2,3]. Therefore, exercising in an unstable environment improves neuromuscular adaptation by providing effective training stimuli.
There are various devices that provide an unstable environment, such as the Togu, Bosu ball, elastic band, and the recently developed sling and total resistance exercise system (TRX), which can control the intensity of exercise using the mechanism of a suspension point [4]. The sling exercise is an active neuromuscular control technique performed using a dangling rope and appropriate auxiliary devices. It reduces the pressure on the joints by pulling the weight, and simultaneously stimulates the muscles suppressed by pain to facilitate reactivation of the muscles [5]. While performing the sling exercise, the position of the suspension point can be changed for active involvement in the exercise, while minimizing the patient’s pain. The intensity and form of the exercise is controlled using the helping hand principle. Compared to exercises on a fixed surface, it has the advantage of simultaneously strengthening multiple muscles involved in balance and stability [2]. Therapeutic exercise using a sling includes relaxation, sensorimotor integration training, stabilization, muscle-strengthening, and muscular endurance. Recently, the sling exercise has been adopted in various fields such as preventive medicine, industrial accident prevention, and for addressing lifestyle problems, as well as for fitness improvement in hospitals, home training, fitness centers, and rehabilitation [6]. According to previous studies comparing lunge exercises with and without a sling, activation of the rectus femoris, hamstring, and gluteus medius increased in the latter [7].
The whole body vibration (WBV) exercise was first conducted by the Russian Aeronautics and Space Administration in the 1970s for training astronauts. Since then, active research has been conducted on the WBV exercise, mainly in Europe. It is used in various fields to restore the physical function of patients with nervous system and musculoskeletal disorders [8,9]. WBV stimulates the muscle spindles of sensory receptors through short and fast mechanical vibrations by controlling the length of the muscles and tendons, causing the tonic vibration reflex (TVR) to increase muscle tone [10]. When muscle tone increases, sensitivity in the neurotransmitter system of the spinal cord increases, and the firing rate responses of the motor units increases. As a result, the overall functions of the neuromuscular system, such as muscle strength, flexibility, and balance, are improved [11].
The WBV machine is driven in various ways, such as the vertical method, seesaw method, and sonic vibration method. In the vertical vibration method, the entire platform moves in a vertical direction, providing vibration of the same intensity to bilateral lower limbs, allowing for balanced vibration stimulation for the body. In the seesaw vibration method, the platform moves in a vertical direction, alternating between left and right with the center of the vibration platform as the axis. In this method, only frequency control is possible; control of the vibration intensity is impossible. In addition, the method has disadvantages in that it is considerably noisy and can cause strain on joints [12]. The sonic vibration method generates sound waves using a speaker mechanism, and generates vertical vibration on the vibration platform using sound wave fields. The advantage of this method is that the precise control of the frequency and amplitude is possible in units of 1 Hz and 1 mm using a sonic actuator [13].
Research on the effects of WBV on muscular function has been conducted over the last ten years. According to Hong et al. [14], the peak torque of the external and internal rotations of the shoulder joint increase significantly during a vibration exercise at 30 Hz. Bosco et al. [15] reported an increase in the muscle activation as a result of conducting WBV at 26 Hz for 60 s. Roelants et al. [16] divided menopausal women into a WBV group, resistance exercise group, and control group. When examined for knee extensor strength, there was an improvement in the WBV group, but no significant difference in the other two groups. Kim et al. [17] reported that during push-up plus exercises with a sling, compared to the exercise without vibration, that with a vibration at 0–90 Hz it increased the activation of the serratus anterior, with the highest increase at 50 Hz.
As described here, research on the methods for improvement in the training effect has been actively underway. However, there have been few studies on the most effective vibration frequency for each muscle and the most appropriate exercise method combining unstable surface and WBV. Therefore, the change in muscular functions showing differences according to various vibration frequencies of WBV needs to be investigated. In this regard, this study aims to investigate the impact of vibration and an unstable environment, such as that with a swing, on the activation of lower limb muscles during the performance of a Bulgarian split squat (BSS).
2. Materials and Methods
2.1. Subjects
The subjects participating in this study were 20 healthy adult males (age: 25.95 ± 2.42 years, height: 172.59 ± 5.56 cm, weight: 77.74 ± 10.82 kg) attending a university in J city, who regularly participate in sports-related activities at least for 30 min, twice a week. Those treated for musculoskeletal disorders in the last six months and those with a history of neurological problems or cardiopulmonary disease were excluded from the study. Before participation in the experiment, all subjects were briefed on the purpose of the study and the experimental procedure, and completed a consent form expressing their willingness to participate. In addition, participating subjects were asked to refrain from vigorous physical activities for 24 h, and from consuming stimulants (e.g., caffeine) for 3–4 h before the experiment. This research was approved by the Institutional Review Board (IRB) of Jeonbuk National University (IRB file no. JBNU 2021-07-002-001).
2.2. Experimental Method
The experiment was performed in the position of BSS, a type of lunge exercise. While the subject was performing BSS, a sling device (Newton 3D Sling, Easy Step Co., Gyeonggi-do, Korea) was connected to the hindlimb and sonic vibration was applied to the forelimb. At this time, activation of the lower limb muscles was measured. In the experiment, repetitions of BSS with a sling and sonic vibration (BSS–sling with vibration frequency: 0, 4, 8, 12, 20, and 30 Hz) were performed under six conditions in total. A single motion of BSS–sling with vibration refers to when the forelimb is placed on a vibration platform, the hindlimb is propped against the sling device support, the rectus femoris is bent down to a point parallel to the ground, and then the knee is extended to return to the starting position (Figure 1). The selection criterion for the forelimb was the leg used when performing kicking. The sequence of each condition was randomly assigned to exclude the effect that may occur due to the order of sequence. Before the experiment, the subjects performed sufficient stretching for at least 15 min to prevent injury and were asked to practice the movements performed in the experiment. In addition, to ensure the consistency of motions during the experiment, the rear step length was measured in consideration of the length of the lower limb of each subject, and the experiment was conducted by controlling the motion for each subject at a constant level based on the measured step length. Subjects who completed maximum voluntary isometric contraction (MVC) measurement participated in the main experiment after sufficient rest. BSS motions were performed at a set timing according to the beat of the metronome (60 BPM, four beats), and each condition was repeated five times in total. To minimize muscle fatigue, a rest time of 60 s was given for each condition. A block diagram of the experimental procedure is shown in Figure 2. Surface electromyography (sEMG) was used to measure the activation of the lower limb muscles during the experiment, and electrodes were attached to gluteus medius, biceps femoris, rectus femoris, vastus medialis, and vastus lateralis, respectively.
A telemetry system for EMG (Telemyo DTS, Noraxon Co., Arizona, USA) was used for EMG measurement, and the attachment positions of the electrode were gluteus medius, biceps femoris, rectus femoris, vastus medialis, and vastus lateralis of the forefoot, respectively, with reference to the guidelines of Surface Electromyography for the Non-Invasive Assessment of Muscles (SENIAM). To minimize the range of errors due to skin resistance during measurement, the surface for the attachment of electrodes was wiped with alcohol, and the electrodes were attached to the center of the muscle, along the direction of the muscle fibers, observed when maximum voluntary isometric contraction (MVIC) was induced using the manual muscle test (MMT) method [1].
To prevent errors in data analysis due to differences in the muscle strength level between subjects, and to increase the objectivity of the measured values, the data were quantified using the MVIC value. Each muscle was measured thrice for about 5 s, and the average value was used as the MVIC value. The MVIC of gluteus medius was measured by hip abduction, applying maximum force in the lateral position. For biceps femoris, MVIC was measured by fixing the upper body and pelvis in the prone position and applying resistance to the upper ankle joint. The MVIC values of rectus femoris, vastus medialis, and vastus lateralis were measured by extending the knee joint while maintaining 90˚ flexion of the knee joint while sitting on a table with the feet off the ground.
2.3. Analysis Method
All of the measured EMG signal data were analyzed with EMG analysis software (Noraxon MR3 3.14.52, Noraxon Co., USA) through the following process. Muscle activation was analyzed for three stabilized repetitions of BSS, excluding the first and last performance out of data obtained from five repetitions of BSS in total. The MVIC values were analyzed by (1) filtering the raw EMG data (FIR Filter, 20 Hz low frequency, 400 Hz high frequency), (2) data value rectification, (3) smoothing (RMS algorithm, 200 ms window) process, and (4) amplitude normalization. As for the EMG signal data, raw EMG was analyzed through the same process as MVC, and with amplitude normalization values from other records. The mean value of the EMG signal data for each subject was normalized by MVIC (%MVIC) for representation.
For statistical processing of this study, SPSS program (Version 12.0, SPSS Inc., Chicago, IL, USA) was used to obtain the mean and standard deviation for the analysis, and one-way ANOVA was conducted for comparative analysis of the change in muscle activation difference according to each condition. The Scheffe test was performed for the post hoc test, and the statistical significance level was set to p < 0.05.
3. Results and Discussion
Figure 3 shows gluteus medius activation during BSS–sling with the vibration exercise. In this case, muscle activation showed the highest value during exercise with 30 Hz vibration frequency and the sling. However, there was no significant difference (p > 0.05). Gluteus medius plays the most important role when fixing the lower limb, straightening the pelvis and trunk, and maintaining an upright posture. It was activated during exercise in an unstable environment. This result agrees with a previous study by Krause et al. [2], which reported an increase in gluteus medius activation during lunge exercise. In addition, recent studies have been conducted on the change of muscle activation according to the position of the vibration platform and muscles. Abercromby et al. [3] reported low muscle activation of muscles far away from the vibration platform, as vibration energy was dissipated when moving away from the vibration platform. In agreement with this finding, the result of this study also showed relatively low muscle activation of gluteus medius, a muscle positioned far from the vibration platform, compared to other muscles.
The activation of the biceps femoris during BSS–sling with the vibration exercise is shown in Figure 4. In this case, muscle activation was highest during exercise at 30 Hz vibration frequency with the sling. There was a significant difference in muscle activation between the exercise at 30 Hz vibration frequency with a sling and that at 0 Hz vibration frequency with a sling (p < 0.05). From the results, it can be seen that during exercise in an unstable environment, the activation of the biceps femoris, which plays an important role in providing body stabilization and balance, increased. This result is consistent with a result from a previous study conducted by NASA [4] on muscle activation of lower limb muscles according to vibration type. The study reported that for both vertical and rotational vibration, there was significant increase in the activation of the biceps femoris, rectus femoris, and vastus lateralis when compared to the result from the no-vibration condition.
Figure 5 shows the activation of the rectus femoris during the BSS–sling with the vibration exercise. In the case of rectus femoris, muscle activation was the highest during exercise at 30 Hz vibration frequency with the sling, and there was a significant difference in muscle activation between the exercise at 30 Hz vibration frequency with the sling and that at 0 Hz vibration frequency with the sling (p < 0.01). Rectus femoris is responsible for hip and knee movement, and it was assumed that the increase in neuromuscular activities due to the increase in vibration frequency led to co-activation of the leg extensor during BSS exercise. Therefore, exercising in an unstable environment is an effective method to increase the activation of the rectus femoris. These results are consistent with the findings of Andersen et al. [5], who reported that the activation of the rectus femoris increased during BSS in an unstable environment with a foam cushion, compared to BSS under a stable environment. In addition, this is also consistent with the result by Roelants et al. [6], who reported that activation of rectus femoris was significantly increased during the single-leg squat compared to the double-leg squat.
Figure 6 shows the activation of the vastus medialis during the BSS–sling with the vibration exercise. In the case of vastus medialis, there was a significant difference in muscle activation between the exercise at 30 Hz vibration frequency with the sling and that at 0 Hz and 4 Hz vibration frequency with the sling (p < 0.05). In the case of vastus medialis, high activation was shown overall during the exercise compared to other muscles, which was assumed to be because vastus medialis and the muscles around the knee are mainly used, as BSS involves the motion of bending the knee [7]. In addition, this result was consistent with the results of Jung et al. [8], who reported that the activation of vastus medialis and vastus lateralis was the highest at 30 Hz when vibration in the range of 0–30 Hz was applied.
Figure 7 shows the activation of vastus lateralis during the BSS–sling with the vibration exercise. In the case of vastus lateralis, there was a significant difference in muscle activation between the exercise at 30 Hz vibration frequency with the sling and that at 0 Hz and 4 Hz vibration frequency with the sling (p < 0.05). This result agrees with that of a previous study, which reported that activation of the vastus lateralis was increased with vibration compared to the no-vibration condition [9]. Other previous research by Riccardo et al. [10] reported high activation of vastus lateralis at 30 Hz when a vibration in the range of 0–55 Hz was applied.
Figure 8 shows the muscle activation during BSS exercise with the sling and sonic vibration at 30 Hz. According to the research results of Pollock et al. [13], muscle activation increased with frequency during the vibration exercise. In the same way, in this study, muscle activation increased gradually as the vibration frequency increased. In this short WBV, stimulation input from the vibration platform stimulates the Ia afferent fiber located in the muscle spindle and activates the muscles connected by the α-motor neuron. The muscle spindle was also stimulated by the surrounding muscles directly affected by the vibration, leading to a positive effect [18]. In addition, according to the research results of Riccardo et al. [10], muscle activation was the highest at the frequency balanced between the excitatory input and the inhibitory input. In this study, the highest muscle activation was measured when 30 Hz vibration was applied in all of the muscles. Therefore, when the WBV machine in this study was used for muscle-strengthening, applying a vibration of 30 Hz frequency was the most effective method. Among the recent studies on vibration exercise, studies on the change of muscle activation according to the position of the vibration platform and the muscles have also been conducted. According to the results of Ritzmann [19], during vibration exercise, activation in the muscles around the calf, which are closer to the vibration platform, was higher than that of the muscles around the thigh, far from the vibration platform. In this study, the muscle activation of vastus medialis closest to the vibration platform was high, and the muscle activation of gluteus medius farthest from the vibration platform was low. This is thought to be because vibration energy was dissipated by the knee joint, affecting the neuromuscular responses of gluteus medius, a muscle close to the trunk. However, the reason that the muscle activation of the biceps femoris was the lowest compared to that of the gluteus medius is that during BSS exercise, rectus femoris and gluteus medius acted as agonistic muscles, and biceps femoris acted as an antagonistic muscle, leading to lower muscle activation of biceps femoris than that of gluteus medius.
These results can be used as basic data for clinical rehabilitation and for formulating an effective method for the application of sling exercise with sonic vibration to lower limb muscles. However, this study was conducted on 20 healthy adult male subjects and it is difficult to generalize the result of this study to a wider population. In future research, further investigation needs to be conducted for various age groups and specific groups, such as athletes and people with disabilities. In addition, as this study examined changes in activation of lower limb muscles only through a single-run exercise, further studies are needed for a longer period of more than four weeks.
4. Conclusions
In this study, the effect of BSS–sling with the vibration exercise on lower limb muscle activation according to frequency was investigated for 20 healthy male subjects. The findings are summarized as follows.
In all muscles, compared to the no-vibration condition, in the condition with vibration, muscle activation was increased. The highest activation was observed under the condition of applying vibration at 30 Hz.
Therefore, it was found that during BSS, the unstable environment and WBV were factors with a positive impact on the activation of lower limb muscles. In particular, WBV at 30 Hz vibration frequency had the most positive effect on muscle activation of the lower limb muscles, and accordingly, the optimal vibration frequency was confirmed at 30 Hz. These results can be used as scientific basic data for sports scientists and athletes who need to improve the function of lower limb muscles, and thus improve athletic performance.
Conceptualization, H.-J.W., M.Y. and T.-K.K.; data curation, H.-J.W.; formal analysis, H.-J.W. and M.Y.; investigation, H.-J.W., M.Y. and T.-K.K.; methodology, H.-J.W., M.Y. and T.-K.K.; supervision, M.Y. and T.-K.K.; visualization, H.-J.W.; writing—original draft, H.-J.W.; writing—review and editing, M.Y. and T.-K.K. All authors have read and agreed to the published version of the manuscript.
This study was supported by the Korea Institute for Advancement of Technology (KIAT) grant funded by the Korea government (MOTIE) (project no.: P0004793, project title: Establishment of the Open Lab Ecosystem for Solving Social Problems Based on Senile Disorders.) and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2020R1F1A1076549).
Not applicable.
The authors declare no conflict of interest.
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Enlarge this image.Figure 1. Bulgarian split squat using a sling according to the vibration frequency condition.
Enlarge this image.Figure 3. Results of gluteus medius muscle activation according to vibration frequency conditions.
Enlarge this image.Figure 4. Results of biceps femoris muscle activation according to vibration frequency conditions (* p < 0.05).
Enlarge this image.Figure 5. Results of rectus femoris muscle activation according to the vibration frequency conditions ** p < 0.01).
Enlarge this image.Figure 6. Results of vastus medialis muscle activation according to the vibration frequency conditions (* p < 0.05, ** p < 0.01).
Enlarge this image.Figure 7. Results of vastus lateralis muscle activation according to the vibration frequency conditions. (* p < 0.05, ** p < 0.01).
Enlarge this image.Figure 8. Results of muscle activation according to 30 Hz vibration frequency conditions.
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