Effect of static and dynamic jaw positions on postural stability among people with blindness

Abstract Background In comparison with the people with normal sight, the mean center of gravity (COG) velocity is significantly higher among blind people. A strong relationship has been shown between jaw and neck sensorimotor and postural control. The purpose of this study was to determine the effect of different static and dynamic jaw positions on postural stability among subjects with blindness. Methods Postural stability was measured as COG velocity in 39 blind subjects under the following five conditions: resting jaw (natural jaw position with no instructions, control), open jaw (teeth of both jaws slightly apart), clenched jaw (teeth tightly closed across each other), chewing (a standard bolus of gum at the natural palace), and tongue position (positioned behind the upper incisors) while standing on firm and foam surfaces. Results The mean COG velocity while standing on the firm surface during resting, open jaw position, clenched jaw position, chewing, and tongue positions were 0.54, 0.50, 0.44, 0.59, and 0.46 deg/s, respectively. The mean COG velocity while standing on the foam surface during resting, open jaw position, clenched jaw position, chewing, and tongue positions were 1.42, 1.23, 1.10, 1.14, and 1.06 deg/s, respectively. Compared to the firm surface, the COG velocity was significantly higher on the foam surface in all five conditions (p < .001). In the comparison between the conditions, there were no significant differences in either the firm or foam surface in all five conditions (p > .05). Conclusion People with blindness behave in the same way as sighted subjects on firm and foam surfaces. However, changes in static and dynamic jaw positions do not affect postural stability among them.


INTRODUCTION
People with visual impairment face difficulties while ambulating and are dependent on their daily activities (Alotaibi et al., 2016;Sadowska et al., 2017). Previous studies among such people show that loss of vision has detrimental effects on postural control (Giagazoglou et al., 2009;Ray & Wolf, 2008). The inability to use visual input prevents one's own visual assessment of body position in space and lowers postural stability (Friedrich et al., 2008;Giagazoglou et al., 2009). They experience serious difficulties while performing motor activities and avoiding obstacles and are at high risk of falling (Brooke-Wavell et al., 2002;Nakata & Yabe, 2001;Schmid et al., 2007).
Various studies have indicated a strong connection between the jaw region and postural control (A. Alghadir et al., 2014;A. H. Alghadir et al., 2015a). Various clinical studies show functional, anatomical, biomechanical, neurophysiological, and neuroanatomical relations between the neck and jaw regions (P.-O. Eriksson et al., 2019). A link between the motor system of the jaw and neck has also been shown by simultaneous movements of the head, neck, and mandible during jaw opening and closing (P.-O. Eriksson et al., 2007;P. O. Eriksson et al., 1998, 2002. Changes in the jaw position reorganize the relation between the head, neck, and mandibular region that can further alter the sensory output from high-density muscle fibers of the region (P. O. Eriksson et al., 2004).

Subjects
Forty-five male subjects with visual acuity less than 3/60 in both eyes were invited to participate in this study (Bucci et al., 2009

Procedure
Postural stability was measured as the center of gravity (COG) velocity

Conditions
The COG velocity was recorded in the following five conditions: rest-

Statistical analysis
The mean and standard deviation (SD) were used to present descriptive statistics. GraphPad Instat 3.0 software (GraphPad Software, San Diego, CA, USA) was used for statistical analysis. The normality of the

RESULTS
The  results in difficulty in the control of independent navigation (Surakka & Kivelä, 2011). Due to their disability and limited participation in physical activities, people with blindness are at a disadvantage compared to their sighted counterparts (Oh et al., 2004). Balance improvement is important because it can provide the opportunity to walk, run, turn, and jump independently (Keogh & Sugden, 1985). Interventions are needed for the improvement of balance among them (Häkkinen et al., 2006).
There are limited studies among people who are visually impaired that assess the risk of falling or analyze potential modifications that can improve postural stability among them.
Postural stability is shown to be reduced during quiet standing and while performing dynamic postural tasks with eyes closed (Buchanan & Horak, 1999;Corna et al., 1999;Dichgans, 1976;Gurfinkel et al., 1976;Schieppati et al., 1999). Larger body sway has been reported in the literature among normal subjects while eyes are closed rather than eyes open . Modification in the reciprocal position of the jaws has been shown to be accompanied by variation in head and neck positions in both sighted and blind individuals (Sforza et al., 2003). The literature about the ability of a blind person to maintain balance in different static and dynamic tasks is either limited or inconclusive. Some studies show that blind subjects can maintain better equilibrium than their sighted counterparts, while other studies show opposite results (Juodžbalienė & Muckus, 2006;Portfors-Yeomans & Riach, 1995;Pyykkö et al., 1991;Stones & Kozma, 1987). The results of the current study show that postural stability was disturbed among blind subjects while standing on foam surfaces in all five conditions. This confirms that subjects with visual impairment, regardless of eyes open or closed, behave in the same way as sighted subjects with eyes closed (A. H. Alghadir, Alotaibi, et al., 2019;Schmid et al., 2007).
The jaw function is innate and important for the three basic skills of survival: feeding, attack, and defense (Smith, 1999). Similarly, posture and gait control developed with the evolution of human beings (Stedman et al., 2004). A close link between body balance and head-neck-jaw position has been observed in healthy subjects (A. Alghadir et al., 2014Alghadir et al., , 2017A. H. Alghadir et al., 2015a;Zafar, Alghadir, Iqbal, Iqbal, et al., 2019). Jaw clenching has also been shown to affect the maximal voluntary contraction of limb muscles (A. H. Alghadir, Zafar, et al., 2019). The tongue is supplied by two motor and four sensory cranial nerves that have musculotendinous connections with the mandible, hyoid, palate epiglottis, and cranium, making it highly sensitive and discriminative (Sicher, 1965;Trulsson & Essick, 1997 (Vuillerme et al., 2007). However, the current study did not reproduce similar results among subjects with visual impairment.
The results of the current study differ from the belief that people with blindness have compensatory cross-modal plasticity and further support the obligatory role of vision in the integration of all sensory inputs in choosing an appropriate body balancing strategy (Schmid et al., 2007). People with compromised sensory systems, such as vision, may use sensory augmentation via various rehabilitation devices, for example, vibrotactile cues, to emphasize the available information from uncompromised systems to improve postural control (Sienko et al., 2018;Umphred et al., 2013). Despite the increasing demand and interest in such techniques, a limited number of researchers have investigated their underlying mechanisms and effectiveness (Bach-y-Rita et al., 1969).

CONCLUSION
People with blindness behave in the same way as sighted subjects on firm and foam surfaces. However, changes in static and dynamic jaw positions do not affect postural stability among them.

ACKNOWLEDGMENTS
The authors are grateful to the Researchers Supporting Project number (RSP-2021/382), King Saud University, Riyadh, Saudi Arabia for funding this research.

AUTHOR CONTRIBUTION
The research idea and design were proposed by HZ and ZAI. A review of the literature was done by SA and ZAI. Data collection was done by AI and SA. Data analysis was executed by AI and AHA. Project supervision was done by the AHA. Manuscript preparation and submission were done by HZ and ZAI.

CONFLICT OF INTEREST
The authors declare that they have no competing interests.

DATA AVAILABILITY STATEMENT
The datasets used in this study are available from the corresponding author on request.