# Effect of Whole Body Vibration Conditions on Lower Limb Muscles during Sling Exercise
## 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 [PERSON] 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. [PERSON] 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  
Figure 3: Results of gluteus medius muscle activation according to vibration frequency conditions. 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 [PERSON] 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 [PERSON] 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].