Introduction
Posture control is essential for human life and involves controlling the position of the body with respect to the environment for the dual purposes of stability and orientation. Thereby, multiple sensory signals from visual, somatosensory, and vestibular systems are used. The sensory information is transformed into motor commands to control posture in a task-specific manner (Shumway-Cook & Woollacott, 2017). Previous studies showed that sensory input originating from the stomatognathic motor system (resulting from different oral motor activities, e. g., jaw clenching in different positions of the lower jaw, tongue pressing or chewing) may influence postural control under static conditions (e. g. Ringhof, Leibold, Hellmann, & Stein, 2015; Sakaguchi et al., 2007) drinking and speaking. The purpose of this study was to evaluate the possible effects of tongue position on the postural control system.
Materials and method: We compared the mean center of gravity (COG. Particularly, controlled jaw clenching activities during upright standing have been shown to reduce the body sway (Ringhof et al., 2015) as well as the muscular co-contraction variability in the lower extremities (Hellmann et al., 2015). ... mehrHowever, the effects of stomatognathic motor activities on dynamic balance have not yet been investigated. On this basis, the aim of this study was to investigate the influence of different functional stomatognatic activities on postural performance during a dynamic reactive balance task. It was hypothesised that jaw clenching and tongue pressing would influence dynamic reactive balance performance.
Methods
A total of 48 physically active adults (25 female, 23 male; age: 23.8 ± 2.5 years) participated in the study. Dynamic balance was investigated in single-leg stance on their dominant leg by means of a Posturomed oscillating platform (Haider-Bioswing). Mechanical perturbations in four different directions (Back (B), Front (F), Ipsilateral (I) and Contralateral (C) to the supporting leg) were applied in a randomized order (Fadillioglu et al., 2022). The task was to compensate the perturbation as quickly as possible. The participants were assigned into one of the three groups (JAW: jaw clenching group; TON: tongue pressing group; and HAB: habitual stomatognathic behavior group) according to their initial balance performance. JAW had to submaximally clench their jaws, whereas TON had to apply a submaximal force with the tip of the tongue against the anterior hard palate during the balance tasks. HAB did not receive any additional instructions. Building three groups with different oral-motor tasks were preferred over a single group of participants with repeated measurements containing different oral-motor tasks, in order to avoid the potential carry-over effects between the tasks. A 3D motion capture system (Vicon Motion Systems; 200 Hz) was used to collect the kinematics data of the platform. A wireless EMG system (Noraxon, 2000 Hz) was used to collect EMG data of the masseter for JAW and HAB and of the suprahyoid muscles of the floor of mouth (FoMM) for TON. Data processing and statistical calculations were done in MATLAB R2020a (MathWorks) and SPSS 25.0 (IBM Corporation), respectively. For the operationalization of the dynamic reactive balance performance, damping ratio (DR) was calculated (Kiss, 2011). The normality of data was confirmed by Kolmogorov-Smirnov tests. For each direction, a one-way ANOVA and Tukey post-hoc tests were performed for the group comparisons. The level of significance was set 194 a priori to p < 0.05. Partial eta squared and Cohen’s d were calculated to estimate the effect sizes for ANOVA and post-hoc tests, respectively.
Results
All the results are shown in Table 1. The ANOVA results for DR revealed significantly different dynamic reactive balance performances between groups in the direction F with a high effect size. According to the post-hoc tests, JAW had a significantly higher DR compared to both HAB (p = 0.001, d = 1.03) and TON (p < 0.001, d = 1.40) with high effect sizes. There were no significant differences in the remaining directions. The results showed that JAW had a better dynamic reactive balance performance compared to other two groups only in the direction F.
JAW HAB TON p ƞ2
Back (B) 6.2 ± 0.3 5.5 ± 0.3 5.5 ± 0.3 0.226 0.064
Front (F) 6.6 ± 0.3 4.6 ± 0.3 4.9 ± 0.3 < 0.001 0.349
Ipsilateral (I) 4.5 ± 0.3 4.6 ± 0.8 4.8 ± 0.5 0.920 0.004
Contralateral (C) 4.3 ± 0.5 4.0 ± 0.4 3.7 ± 0.4 0.607 0.022
Table 1: Damping ratio results are given as mean ± standard deviation in %. Significant differences are highlighted in bold. The p-values and the effect sizes for group comparisons are represented in the last two columns. JAW, jaw clenching; TON, tongue pressing; HAB, habitual.
Discussion
The results revealed that jaw clenching may improve the dynamic reactive balance performance significantly in the forward direction of perturbation, whereas tongue pressing did not seem to have any effects in any directions. However, jaw clenching improved the dynamic reactive balance performance only in one of the four perturbation directions. This finding can be attributed to the perturbation direction dependency of postural control (Chen et al., 2014; Nonnekes et al., 2013) compared the postural responses to backward and forward perturbations and found positive effects of a startling auditory stimulus only in the backward body sway condition. They suggested that postural responses to backward and forward perturbations may be processed by different neural circuits. Based on these findings, dynamic reactive balance performance improvement in direction F in this study may be attributed to the effects of jaw clenching, which are specifically recruited during forwards acceleration of the platform. There are several possible explanations for the found effects, including the increased excitability of the human motor system (Boroojerdi, Battaglia, Muellbacher, & Cohen, 2000) due to the stimulation of periodontal mechanoreceptors. However, underlying mechanisms have not been yet investigated explicitly and remained to be answered. This study can contribute to the understanding of postural control, particularly in relation to stomatognathic motor system.
