# Muscle Activity of Superimposed Vibration in Suspended Kneeling Rollout
## 4 Discussion  
This study analysed the effects of superimposed vibration on the periscapular musculature during suspended kneeling rollout, which is considered a core exercise [22; 23; 24], and obtained high activation values in this musculature [26; 27; 28; 29; 30; 31]. Previously, vibration combined with suspension straps was used in the upper extremities [21]; however, this is the first study to use it in an overhead exercise. The results demonstrated an increase in muscle activation in response to vibration; however, no discernible differences were observed between the two vibration frequencies in terms of global activation. The hypothesis that a frequency of 40 Hz would be more demanding was not supported by the data. This finding suggests that, while vibration may enhance muscle activation, higher frequencies do not guarantee greater muscle stimulation. Likewise, the hypothesis that perceived exertion would be greater at higher frequencies was confirmed. The findings of this study indicate that the implementation of superimposed vibrations may be a viable strategy for enhancing the strength of the periscapular muscles in their role as glenohumeral stabilisers. The main hypothesis of the study has been confirmed, showing a significant moderate increase in muscle activation in global activity when vibration is superimposed at frequencies of 25 Hz (\(d\) = 0.71, CI: 0.11; 1.3) and 40 Hz (\(d\) = 0.68, CI: 0.08; 1.28), compared to the non-vibration condition. These results add to the evidence that vibration is a useful tool for increasing upper-body muscle activation [13; 14; 15; 16; 21]. Nevertheless, no significant differences were observed between the two frequencies, although they did not exhibit the same response in all the evaluated muscles. Several authors have found differences between the frequencies used, 25 Hz and 40 Hz [20; 21], and 30 Hz and 50 Hz [18], but these differences were reported in particular muscles in each study and did not represent the general trend. These variations could be explained by the proximity of the resonance frequency, known as the frequency at which a muscle accumulates energy, in this case, vibration [45]. The muscles of the body act as dampers by absorbing this vibration through the damping effect. Therefore, the transmission of vibrations depends on the direction of vibration, body position, stiffness, and damping effect [17; 45; 46]. The muscle activity of the latissimus dorsi significantly increased in both vibration conditions, with a moderate increase at 25 Hz (\(d\) = 1, CI: 0.29; 1.51) and 40 Hz (\(d\) = 0.69, CI: 1; 1.29) compared to the non-vibration condition (Table 1). On the contrary, [PERSON] and associates [14] recorded muscle activation while performing push-ups and triceps dips on  
Figure 6: OMNI-Res for suspended kneeling rollout. a: significantly different compared to non-vibration. A.U. : arbitrary units. a vibrating platform and found significant differences only in the lower latissimus dorsi fibres when performing push-ups at 35 Hz. This finding might be attributed to the low involvement of the latissimus dorsi in horizontal adduction and shoulder flexion compared with the high involvement of the latissimus dorsi in shoulder extension [47]. The infraspinatus has also shown significant differences in both vibratory conditions compared to non-vibration, with a small effect at 25 Hz (\(d\) = 0.62, CI: 0.03; 1.22) and 40 Hz (\(d\) = 0.46, CI: \(-\)0.13; 1.05) (Table 1). Similarly, [PERSON] and associates [14] reported significant differences when performing triceps dips on a vibrating platform at 35 Hz, whereas muscle activity was moderate (21-40% MVIC) in the present study. Moreover, [PERSON] and associates [14] reported moderate (21-40 MVC) and high activation (41-60% MVIC) when performing triceps dips and isometric shoulder flexion with vibration, respectively. However, when performing press-ups, the activation was low (\(<\)21% MVIC).