## 4 Discussion  
This study investigated the effects on the postural control function of wearing a mouthguard to equalize the occlusal contact state by examining differences in COP displacement among gymnasts, weightlifters, and non-athletes with and without a mouthguard. Our results indicated that COP displacement differed among participant groups and was reduced by wearing a mouthguard. In particular, the postural control function of gymnasts was higher than that of untrained adults and weightlifters. The gymnasts had a high baseline, and the reduction rate of COP displacement when wearing a mouthguard was higher in this group compared with the other groups. Therefore, the null hypothesis was rejected. For measurement of the COG sway, the COP displacement was analyzed separately in the front-and-back and left-and-right directions because both are managed based on different control systems [14; 23]. Front-and-back COP displacement is affected by the stiffness of the ankle joint. Left-and-right COP displacement, on the other hand, is influenced by ankle varus/valgus and hip joint varus/abduction, and there is a tendency for less sway compared with the front-to-back direction [24; 25]. In addition, postural muscles are responsible for standing control against forward-leaning posture and are increased by the sensitivity  
Figure 8: Comparison of the reduction rate of center of pressure displacement in the left-and-right direction (COP-LR) among participant groups when wearing mouthguards. Measurements are expressed as means \(\pm\) SD. Error bar indicates standard error of the mean. * \(p\) < 0.05: denotes statistically significant difference. Figure 9: Comparison of the reduction rate of area enclosed by the trajectory of the center of pressure fluctuation (COP-Area) among participant groups when wearing mouthguards. Measurements are expressed as means \(\pm\) SD. Error bar indicates standard error of the mean. of the somatosensory input in postural control [25, 26]. Postural stability is affected by the fascial chain; the masseter muscle, which maintains clenching, belongs to the deep front line, and the sternocleidomastoid muscle belongs to the lateral line, superficial front line, and superficial back line, with both contributing to body stabilization [27]. Although visual input is the most important factor in posture localization [28], it is less important for maintaining standing posture [3]. Therefore, the visual condition in this study was set as closed eyes. The threshold of sensory input due to postural sway depends on the sway speed; sensory thresholds at static or slow sway have the lowest proprioception [26], and the importance of sensory input differs according to the characteristics of the sport [3, 11, 16, 17, 18, 19, 29, 30]. Based on the above, the present study targeted untrained adults who do not exercise regularly, gymnasts who emphasize balance training, and weightlifters who mainly do strength training. Then, the difference in COP displacement due to the difference in the occlusal contact state (i.e., somatosensory input) was investigated. Participants with a difference of 15% or more in the occlusal force between the left and right sides were excluded to avoid the possible difficulty in comparison among participants or between mouthguard conditions due to a large bias in COP displacement under the \"without mouthguard\" condition. Differences in COP displacement among participants were observed in all evaluation indices, with the COP displacement of gymnasts tending to be smaller than that of untrained adults and weightlifters. It has been reported that gymnasts, for whom postural training is emphasized, have a superior postural control function compared with other athletes and that this function is particularly evident under conditions that match the training environment [11]. The gymnasts targeted in this study had trained to maintain a posture in which the shoulders, hip joints, knees, and lateral malleolus were in a straight line and the COG was distributed over the entire sole.