Influence of different jaw positions on dynamic balance using Y‐balance test

Abstract Background Jaw sensory‐motor system has been shown to affect static balance of the body. It would be interesting to know whether it can influence dynamic balance as well. The objective of this study is to examine the influence of different jaw positions on dynamic balance using the Y‐balance test. Methods Eighty healthy male participants aged 20–35 years were invited to participate in this study. Dynamic balance was measured by the Y‐balance test in three directions (anterior, posteromedial, and posterolateral) for each leg separately in three jaw positions: resting jaw (control), open‐jaw, and clenched jaw. Results There were no significant differences in reach distances between the different jaw positions except in the posterolateral direction. In comparison with resting jaw position, reach distance was significantly higher in open‐jaw position for the right leg and in clenched and open‐jaw positions for the left leg in the posterolateral direction. Conclusions Although various studies have shown direct or indirect influence of jaw sensory‐motor system on static postural control, results of this study point to limited relation with dynamic postural control among healthy subjects. However, it supports the potential of the jaw sensory‐motor system to affect motor control during functional tasks in patients with postural instability or similar disorders.

Having established direct or indirect links between jaw sensory-motor system and static balance (Alghadir, Zafar, Whitney, & Iqbal, 2014;Alghadir, Zafar, & Iqbal, 2015), it would be interesting to know whether it can influence dynamic balance as well. This study was done to see the influence of different positions of jaw on dynamic balance. We studied the effect of three jaw positions, that is, resting jaw, open-jaw, and clenched jaw on dynamic balance using YBT. We hypothesized that variation in jaw sensory-motor system can affect reach distance significantly among healthy adults.

| Participants
Eighty healthy male participants aged 20-35 years were invited for this study. Any case of musculoskeletal injury in the last 1 year, back pain in last 6 months or history of surgery or temporomandibular joint disorders, or any other neurological problem was excluded. All participants were briefed about the need of the study and asked to sign a consent before participation. An ethical approval according to Declaration of Helsinki was obtained from research committee of our institution.
Participants' data including age, weight, height, and leg length were recorded. Leg length for the dominant side was measured from the anterior superior iliac spine to the most distal part of the medial malleolus in supine position (Plisky et al., 2006).

| Dynamic balance
Dynamic balance was measured using YBT (Move2Perform, Evansville, IN). Test was conducted as described in previously published studies (Alhusaini et al., 2017;Alnahdi et al., 2015;Plisky et al., 2009;Smith et al., 2015). Barefooted participants were advised to perform practice trials before actual data collection. YBT was conducted in three positions: resting jaw (natural jaw position with no instructions; control), open-jaw (jaws slightly apart with no contact between tooth), and clenched jaw (jaws tightly closed against each other). The order of the test was random. Three trials were recorded for both legs in each direction. The participants were asked to stand on one leg and reach the indicator as far as they could by using other leg and then return to the starting position without losing their balance. Reach distance was recorded to the nearest 0.5 cm. The trial was repeated if participants failed to return to the starting position without losing balance or they kicked the indicator. The mean of three trials was used for data analysis. Normalized reach distance was calculated by dividing this value by limb length and multiplying by 100 (Gribble & Hertel, 2003).

| Statistical analysis
Data were analyzed using GraphPad Instat 3.0 software. Mean, standard deviation (SD), and 95% confidence interval were presented, and hypothesis of no difference in reach distance between 3 positions was tested by parametric repeated measures ANOVA using Bonferroni multiple comparisons test, and it was rejected if p value was <.05.

| Ethics approval
All subjects were informed about the aims and procedures of the study, and written informed consent was obtained for participation in the study. This study was approved by the Rehabilitation research review board for ethics according to Declaration of Helsinki (Ref no. KSU/RRC/031/01).

| Comparison between three jaw positions
Although reach distance (actual as well as normalized) varied be-  (Figures 1 and 2).  Postural control is static (when attempting to maintain a position with minimum movement) or dynamic (involving completion of a task during movement without compromising base of support) (Gribble & Hertel, 2003;Winter, Patla, & Frank, 1990). It is a complex system that depends on information from the proprioceptive, vestibular, ocular systems, and neck reflexes (Horak, 2006). The role of visual input and standing surface on balance control is well documented (Alghadir et al., 2015;Mohapatra, Kukkar, & Aruin, 2014;Redfern, Yardley, & Bronstein, 2001). Influence of jaw sensory-motor system on vestibular, neck, and ocular systems has been shown (Alghadir, Zafar, Iqbal, & Al-Eisa, 2018;Davies, 1979;Ehrlich, Garlick, & Ninio, 1999;Hellmann, Giannakopoulos, Blaser, Eberhard, & Schindler, 2011;Park et al., 2014), and thus, it has the capacity to affect posture control. Variation in activation pattern of the jaw sensory-motor system while maximum biting, sub-maximum biting, clenching or chewing has been shown to modulate strategies of central postural motor control mechanisms differently (Alghadir et al., 2014;Hellmann et al., 2011;Kushiro & Goto, 2011). These include improvement in sports performance, distal muscle strength, and postural balance (Cherry, Brown, Coburn, & Noffal, 2010;Hosoda et al., 2007). Instant reduction of body sway after using dental splints in patients with whiplash-associated disorders in comparison with healthy subjects further supports the conspicuous role of jaw sensory-motor system (Eriksson, Zafar, & Backén, 2018). Therefore, it can be postulated that the influence of jaw positions on dynamic balance is expected to be more in patients with postural instability or similar disorders rather than healthy subjects.

| D ISCUSS I ON
Although posteromedial component of the SEBT has been shown to highly represent the performance in all its components (Hertel, Braham, Hale, & Olmsted-Kramer, 2006) abductor strength (Hubbard, Kramer, Denegar, & Hertel, 2007;Lee et al., 2014). Larger hip range of motion is needed while reaching in posterior direction (Robinson & Gribble, 2008). Challenging balancing tasks lead to modification of fusimotor drive and muscle tone (Aniss, Diener, Hore, Gandevia, & Burke, 1990). Such mechanisms could bring the most symmetric neuromuscular equilibrium during open-jaw and clenched jaw positions (Gangloff et al., 2000). Y-balance test is the instrumented version of the modified SEBT, and its performance has been shown to vary among different cultures (Butler, Queen, Beckman, Kiesel, & Plisky, 2013;Plisky et al., 2006;Smith et al., 2015). Reach distance values from SEBT have been also shown to be associated with leg length and its normalization or matching paired participants for leg length have been recommended (Gribble & Hertel, 2003). However, same results were found in the analyses of the actual and normalized reach distance values for the leg length in this study.

| CON CLUS IONS
Although various studies have shown direct or indirect influence of jaw sensory-motor system on static postural control, results of this study point to limited relation with dynamic postural control among healthy subjects. However, it supports the potential of jaw sensorymotor system to influence motor control during functional tasks in patients with postural instability or similar disorders and further study is recommended.

ACK N OWLED G M ENT
The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for funding this research through the research group NO. RGP-VPP-209.

CO N FLI C T O F I NTE R E S T
None declared.

AUTH O R CO NTR I B UTI O N S
HZ and AA proposed research idea and design. AA, AHA, and ZI reviewed the manuscript. ZI, AI, and SA executed data collection and analysis. HZ, SA, AI, and ZI prepared and submitted the manuscript.

DATA AVA I L A B I L I T Y S TAT E M E N T
The datasets used in this study are available from the corresponding author on request.