## 4 Discussion
The results of this study support the training effect and are consistent with previous research [11]. On the other hand, the COP displacement of weightlifters was significantly smaller than that of untrained adults, but the difference for gymnasts was not as large, except for COP-FB. The difference between untrained adults and weightlifters in this study may be due to the fact that body sway tends to be smaller with a greater body weight [9]. In addition, because the sway of the COG might be greater in more muscular or stronger athletes than in athletes who focus on balance, there was a tendency for COP displacement to be greater in the weightlifters than in the gymnasts. Furthermore, there may be an effect of foot position during measurement. The foot position during the gravity center fluctuation test significantly affects the measured values, and the closed foot position is able to detect minute changes in the COG [31]. In this study, measurements were taken in the closed foot position; however, this position did not match the practice environment for weightlifters, and the COP displacement may have been increased. It is thus necessary to verify the effect of foot position on the COP displacement of weightlifters. The most significant between-participant differences were detected in COP-FB because the COP displacement is greater in the front-and-back direction than in the left-and-right direction [24, 25]. Differences in COP displacement from wearing a mouthguard were significant under all conditions, except for COP-LR and COP-Area in weightlifters, and COP displacement was smaller when wearing a mouthguard. These results demonstrated that equalization of the occlusal contact state by wearing a mouthguard reduces the COP displacement and that the effect on the postural control function depends on the participating groups. It has been clarified that people with a good occlusal contact state have less COG sway and that equalization of occlusal contact with an oral appliance reduces the sway [5, 6, 12]; the present results also showed similar findings. Weightlifters' occlusal contact area may have influenced the lack of a significant difference in COP-LR and COP-Area due to wearing a mouthguard. The participants in this study were selected based on the difference in the occlusal force between the left and right sides, but weightlifters included those with significant tooth wear due to the characteristics of their sport. Because participants with significant tooth wear and large occlusal contact areas tended to have a small difference in oclusal contact areas between the left and right sides, it is possible that there was not much difference in their occlusal contact state when wearing a mouthguard. In future work, it will be necessary to focus on the distribution of the occlusal contact area and to refine the selection of participants. Furthermore, gymnasts showed the greatest reduction in COP displacement when wearing mouthguards. They showed significantly higher reduction rates in COP-FB compared with untrained adults and in COP-LR compared with weightlifters. This study revealed that gymnasts, the group with the best postural control without a mouthguard, showed the greatest reduction in COP displacement when wearing a mouthguard. In other words, it was suggested that ensuring a well-balanced occlusal contact state in gymnasts with excellent postural control function could lead to further improvement. Furthermore, the stabilization of occlusion by wearing a mouthguard also contributed to the improvement of postural control function in weightlifters, who mainly engage in strength training, as well as in non-athletes. The results of this study indicate that equalization of occlusal contact contributes to the improvement of postural control function, suggesting that oral health management, including the wearing of mouthguards, may lead to the prevention and reduction of sports injuries as well as the improvement of athletic performance.