# Muscle Activity of Superimposed Vibration in Suspended Kneeling Rollout
## 4 Discussion
These results are relevant because the concern for reducing shoulder injuries in overhead athletes has led to studies on the involvement of the infraspinatus in several exercises [48; 49; 50; 51; 52]. These studies reported higher activation of this muscle, thus reaching very high values (\(>\)60% MVIC) in some of them. In fact, the infraspinatus acts as a fixator of the shoulder joint in overhead actions, as it does in suspended kneeling rollout or triceps dips. It should be noted that suspended kneeling rollout was not performed at high intensity in the present study (Figure 6); therefore, the exercise could be repeated in a more challenging way, for instance, by changing the position, range of motion, the pace, etc. The anterior deltoid showed the lowest activation (\(<\)5% MVIC). However, it was also stimulated by superimposed vibration, showing a small effect at 25 Hz (\(d\) = 0.29, CI: \(-\)0.3; 0.87) and 40 Hz (\(d\) = 0.41, CI: \(-\)0.18; 0.99) compared to the non-vibration condition (Table 1). [PERSON] and associates [21] and [PERSON] and associates [14] also recorded the effects of vibration on the anterior deltoid, observing significant differences at all frequencies used (25 Hz, 35 Hz, and 40 Hz) when performing the exercises studied with superimposed vibration or a vibration platform, respectively. In the aforementioned studies, the anterior deltoid was identified as one of the primary agonist muscles during exercises, such as push-ups, triceps dips, and isometric shoulder flexion. Consequently, the findings from these studies are pertinent to the present investigation, wherein superimposed vibration was found to stimulate the anterior portion of the deltoid, even though it was not a targeted muscle. The pectoralis activity increased when superimposing vibration at 40 Hz in its sternalis portion (\(d\) = 0.3, CI: \(-\)0.4; 1) compared to the non-vibration condition. Although the clavicularis portion did not show statistical significance, the effect of the vibration was similar (\(d\) = 0.4, CI: \(-\)0.21; 0.96) (Table 1). This result is relevant because of the involvement of each portion in this exercise, given that the shoulder joint made flexion and extension in the suspended kneeling rollout. In this movement, the pectoralis clavicularis is the predominant portion [47], even though the portion that was stimulated by the vibration was the sternalis portion. [PERSON] and associates [14] found significant differences in the pectoralis when performing press-ups and triceps dips on a 35 Hz vibration platform, even though the portion of the muscle was not specified. [PERSON] and associates [21] obtained significant differences at 25 Hz and 40 Hz compared to the non-vibration condition in the pectoralis sternalis, but only at 40 Hz compared to the non-vibration condition in the pectoralis clavicularis. These findings may support the proximity of the resonance frequency hypothesis [45]. These results suggest that vibration is a useful strategy for stimulating the pectoralis muscle, but it seems that when high degrees of shoulder flexion are reached, this effect is reduced in the pectoralis clavicularis muscle because the maximum pectoralis clavicularis activity was during the eccentric phase, decreasing in the last degrees of shoulder flexion when the pectoralis sternalis and latissimus dorsi reached the highest muscle activation. Finally, the serratus anterior showed no significant differences between conditions (Table 1). Presumably, this muscle does not have a direct effect on the shoulder joint, and the vibration frequency is absorbed by the arm musculature, in accordance with the findings of [PERSON] and associates [14]. [PERSON] and colleagues performed various exercises that achieved very high activation (>60% MVIC), indicating that vibration is an ineffective strategy to increase the activation of the serratus anterior muscle [48; 49; 50]. The observed results might be attributable to the distance between the serratus anterior and the vibration device, because it is absorbed by the arm muscles.