## 4 Discussion  
The results of this study show that there was a difference in COP displacement between participant groups only in the eyes-open condition, and there was a strong positive correlation between the COP displacement and the landing position. Therefore, the null hypothesis was rejected. Postural muscles are responsible for standing control against forward-leaning posture and they increase the sensitivity of somatosensory input in postural control [21; 22]. Because occlusion is related to somatosensory and vestibular input, there are many reports on the effects of occlusal conditions and the wearing of oral appliances on postural control function [14; 19; 23; 24]. Moreover, because visual input provides the relative positional information about surrounding objects, it serves as a source of information about head position and movement, reduces COP sway, and is crucial for postural orientation [3; 25; 26]. In addition, the threshold for sensory input due to postural sway differs according to the sway speed [21]. It has been reported that the proprioceptive threshold is lowest for static or slow motion, while the proprioceptive and visual input thresholds are low for rapid motion. This suggests that the prioritization of sensory input in sports adapts to the characteristics of the sport [27; 28]. Because gymnasts focus on balance training [12], their balance abilities tend to be superior to those of the general public and athletes in other sports [4; 5; 6; 7; 8; 15]. Gymnastics events include artistic gymnastics, rhythmic gymnastics, trampoline, acrobatic gymnastics, and parkour [29]. Athletes in each event have a characteristic body shape that matches the sport [12; 14], and from this we can see the difference in body structure and the skills of each part of the body that result in specialization for an event. Following on from a previous study which revealed that equalization of the occlusal contact state by wearing a mouthguard contributed to the improvement of the static postural control function [14], the focus was on trampoline competition, where postural control function is reflected in competition skill [14; 15]. As a next stage of investigation, the purpose of this study was to clarify the relationship between trampoline performance and standing  
Figure 8: Correlation between left–right displacement of the center of foot pressure (COP-LR) and left–right displacement of the landing position in the straight jump (H-LR). A strong positive correlation was observed between COP-LR and H-LR (R = 0.702, \(p\) < 0.05), and the movement directions of the two measures tended to match. postural control function. First, we compared the center of gravity sway of healthy men and trampoline gymnasts, with a prerequisite that occlusal balance was stable and there were no differences in occlusal conditions. There is a previous report indicating that the postural control function is exhibited only under conditions that match the practice environment [6]. In addition, because the prioritization of sensory input in postural control differs according to the level of competition [30; 31], it was considered desirable to recruit high-level athletes. Therefore, male gymnasts who received practical training from coaches of the Japanese Olympic team and were supported by physical therapists were selected as participants for this study. There was no significant difference in COP displacement between the eyes-open and eyes-closed conditions in either the healthy men or the trampoline gymnasts. From this result, it was confirmed that the importance of visual input in maintaining standing posture is low, as previously reported [3]. Differences between participant groups were observed in COP-FB only in the eyes-open condition, and the trampoline gymnasts had better postural control function compared with the healthy men. It has been reported that postural control function differs according to an athlete's sport and level of competition [4; 5; 6; 7; 8].