# Effect of Whole Body Vibration Conditions on Lower Limb Muscles during Sling Exercise
## 3 Results and Discussion
In addition, this result was consistent with the results of [PERSON] 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 4: Results of biceps femoris muscle activation according to vibration frequency conditions (\({}^{*}p<0.05\)). 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 slip and that at 0 Hz and 4 Hz vibration frequency with the slip (\(p\) < 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 [PERSON] 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 5: Results of rectus femoris muscle activation according to the vibration frequency conditions **_p_ < 0.01). Figure 6: Results of vastus medialis muscle activation according to the vibration frequency conditions (*_p_ < 0.05, **_p_ < 0.01). Figure 8 shows the muscle activation during BSS exercise with the sling and sonic vibration at 30 Hz. According to the research results of [PERSON] 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 \(\alpha\)-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 [PERSON] 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 [PERSON] [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.