Effect of sitting posture with and without sole-ground contact on chewing stability and masticatory performance

Abstract

Purpose: To verify the effect of sitting posture with and without sole-ground contact on chewing stability and masticatory performance.

Methods: Thirty healthy subjects were evaluated. The Conformat was used to analyze the center of sitting pressure (COSP), and the three-dimensional motion analysis system was used to analyze changes in head and trunk postures while subjects remained in a sitting position with and without sole-ground contact. The parameters of masticatory performance and movement were calculated as follows. For evaluating masticatory performance, the amount of glucose extraction (AGE) during chewing of a gummy jelly was measured. For evaluating masticatory movements, the movement of the mandibular incisal point was recorded using the Motion Visi-Trainer V1, and parameters of the stabilities of movement path and rhythm were calculated.

Results: Head and trunk sway values and the displacement of COSP were significantly smaller with sole-ground contact than those without sole-ground contact. The masticatory movement path with sole-ground contact showed less variation in the opening distance and more stable movement path compared to those without sole-ground contact. The AGE was significantly greater with sole-ground contact than that without sole-ground contact.

Conclusion: Sitting posture with and without sole-ground contact affects chewing stability and masticatory performance.

Introduction

Mastication is a sensory-motor activity aimed at the preparation of food for swallowing, that involves rhythmically repeated and coordinated movements of the jaw, tongue and perioral soft tissues of the lips and cheeks. Smooth masticatory movements are generated through the cooperative activities of these organs [1,2,3]. Mastication affects many functions of the whole body. It has been reported that masticatory movements can physiologically improve the cerebral blood flow [4], and also improve cognition, and mood, and reduce stress by relieving anxiety [5,6]. Further studies have discussed the relationships between masticatory movements and static [7,8] and dynamic [9] balance of body posture, leg muscle activity [10], neck muscle activity [11], head posture [12], and upper body movement [13].

It is clinically desirable for the sitting posture during eating to ground the feet as much as possible to engage in efficient chewing and achieving safe swallows [14]. However, these issues have not been adequately examined, based on life science data, especially regarding posture for efficient chewing. Examination of the effect of changes in sitting posture on masticatory function is meaningful and helpful in understanding the interrelationship between stomatognathic function and posture control system in sitting posture, and moreover, may provide important suggestions based on physiological evidence of appropriate posture during mastication. Posture when masticating is very important in older people, in particular those requiring nursing care [15,16].

Thus, the purpose of this study was to verify the hypothesis that in healthy subjects sitting posture with and without sole-ground contact affects chewing stability and masticatory performance.

Materials and Methods

Study population and ethics

In total, 30 healthy male subjects with an average age of 25.3 years (range, 22-32 years) were recruited among the students and staff members of the Graduate School of Dental Medicine, Hokkaido University. The sample size was calculated using the software program G*Power 3.1.9.2 (Heinrich-Heine-Universität Düsseldorf, Düsseldorf, Germany). When the sample size was calculated by setting α = 0.05, β = 0.8, and effect size = 0.8, 26 participants were needed. All subjects met the following inclusion criteria: (1) healthy condition, (2) no history of head and neck or back problems, (3) no history of orthopedic or otolaryngologic problems affecting body balance, (4) no history of signs or symptoms of temporomandibular joint or orofacial pain, (5) absence of prosthesis (i.e., crowns, bridges, implants, or removable prosthetics) and Class I dental occlusion, and (6) no loose or broken teeth, fillings, or crowns that could be further damaged during the course of this study.

This study was approved by the ethical committee of the Graduate School of Dental Medicine, Hokkaido University (approval number 2019-No. 2) and was conducted in accordance with the ethical principles of the Declaration of Helsinki. The study methodology was explained, and written consent was obtained from all participants prior to their inclusion in the study.

Measurement postures

Two postures were used for the measurement: a sitting posture without the soles of the feet grounded (A) and a sitting posture with the soles of the feet grounded (B) (Fig. 1).

Fig. 1

Measurement postures. A, a seated posture on a height-adjustable chair without both soles of the feet grounded. B, a sitting posture on a height-adjustable chair with hip and knee joints at 90° flexion position, and with both soles of the feet fully grounded. Both upper limbs were lightly crossed at the anterior chest to minimize the effect of their arms on the posture.

Test food and habitual chewing side

The test food was a cylindrical-shaped gummy jelly with a diameter of 14 mm, height of 10 mm, and weight of 2.3 g. Gummy jelly comprised 41% of reduced sugar syrup, 22% maltitol, 20% sorbitol, 5% glucose, and 8% gelatin [17]. Before the experiment, the habitual chewing side was confirmed by having the participants chew the test food freely and selecting the side that felt easier to chew on.

Analysis of simultaneous measurements of head sway and sitting pressure distributions

The Conformat (Tekscan Inc., South Boston, MA, USA; Nitta Corp., Osaka, Japan) [18,19] was used to analyze the pressure distribution on the sitting surface (Fig. 2). This pressure mapping device measures sitting pressure distribution and changes in the position of the center of sitting pressure (COSP) on the sitting surface during a standard measuring period, and consists of 1,024 pressure sensors that are connected in a flexible way to minimize the hammocking effect. The pressure-mapping device was calibrated according to the manufacturer’s guidelines. The COSP is the center of vertical force acting on the sitting surface, and indicates the center shifts of sitting pressure distribution in the anteroposterior and lateral directions. The mean coordinate of all COSPs during the measuring period was defined as the virtual central coordinate of COSP (VCC-COSP).

The three-dimensional motion analysis system (Library Co., Ltd., Tokyo, Japan) was used to analyze head and trunk sways. This instrument enabled measurements of three-dimensional movements of target points on the surface of the facial skin and body surface simultaneously. The movements of target points were recorded by three charge-coupled device (CCD) cameras, and the three-dimensional coordinates were calculated by using analyzing software (Library Co., Ltd.). Target points on the skin of the face and the trunk were marked by attaching four points respectively (Fig. 3). The center of four target points was calculated in each sampling frame. The mean coordinate of all the center of four target points on the face was defined as the virtual central coordinate of the head (VCC-h). In the same way, the mean coordinate of all the center of four target points on the trunk was defined as the virtual central coordinate of the trunk (VCC-t). The head sway was analyzed based on the coordinate system located on the trunk (A trunk coordinate system). The trunk sway was analyzed based on the coordinate system on the ground.

Fig. 2

Analysis of simultaneous measurements of head and trunk sways, and sitting pressure distribution. Data sampling was performed simultaneously at a sampling rate of 50 Hz using a self-made external synchronization device. For head and trunk sway measurements, a three-dimensional motion analysis system was used to analyze the motion of target points set on the head and trunk respectively. In the head sway analysis, the coordinates were transformed to a coordinate system, trunk coordinate system, based on the trunk to eliminate the trunk sway. Sitting pressure distribution was measured using a pressure mapping device, Conformat.

Fig. 3

Target points set on the head and trunk. Four target points were set on the head (No. 1-4) and trunk (No. 5-8) respectively for the motion analysis. No. 1 Nasion, No. 2 Top of the nose, No. 3 and 4 Right and left zygomatic bones, No. 5 Jugular notch, No. 6 Xiphoid process, No. 7 and 8 Right and left clavicle middle point. Round reflecting markers (10 mm in diameter) were used as target points to be recognized by using their luminance values, and setting these markers on the head and trunk was done using double-sided tape.

Analysis of masticatory function

Masticatory performance

Subjects were asked to chew the test food on their habitual chewing side for 20 s. After chewing, the subjects were asked to hold 10 mL of distilled water in their mouth for a moment and to spilt into a cup with a filter. The glucose concentration of the filtrate containing glucose eluted from gummy jelly was measured using a glucose-measuring device (GS-II, GC Corp., Tokyo, Japan) [17]. The measured value was taken as the amount of glucose extraction (AGE).

Masticatory movements

The movement of the mandibular incisal point during mastication of the test food for 20 s on the habitual chewing side was recorded by the optical jaw motion tracking device, Motion Visi-Trainer (MVT V1, GC Corp.) and was analyzed using the overlapping of each cycle and average path [20].

The pressure mapping mat of the Conformat was placed under the subject’s buttocks. To assist in obtaining a natural sitting posture, the subjects were asked to look directly into a reflected image of their eyes two meters away, and to remain in this position during the measurements. Simultaneous measurements of the head and trunk sways and sitting pressure distribution were conducted under the following two conditions: (1) The subjects were asked to chew the test food on their habitual chewing side with the soles of the feet grounded, (2) The subjects were asked to chew the test food on their habitual chewing side without the soles of the feet grounded. The subjects were requested not to swallow the test food for the time tested. These two conditions were randomly conducted in each subject, based on the table of random numbers. Testing under each condition was recorded for 20 s. The recording was started after the subjects were seated in the chair, ready for the simultaneous measurements of head and trunk sways, and sitting pressure distribution, and the investigator confirmed that their head and body positions were stable. Each trial was recorded three times with a one-minute rest period. The sampling rate for simultaneous measurements was 50 Hz. Recording of masticatory performance and masticatory movements were also conducted under the above two conditions and in the same manner as the simultaneous measurements of the head and trunk sways and sitting pressure distribution.

Figure 4 shows an example of the trajectory data set during chewing the test food with and without sole-ground contact respectively of the head and trunk sways and displacements of COSP obtained from one subject. Figure 5 shows an example of overlapping of cycle and average path during chewing the test food on the right side obtained from one subject.

Fig. 4

An example of the trajectory data set during chewing the test food with and without sole-ground contact respectively of the head and trunk sways and displacements of COSP obtained from one subject. For head sway, the trajectories in the horizontal plane of the center of four target points set on the skin of the face were shown. Similarly for trunk sway, the trajectories in the horizontal plane of the center of four target points set on the skin of the trunk were shown. Head and trunk sways, and the displacement of COSP when sole-ground contact tended to show the centripetal type, and those when sole-ground off the floor tended to show the anterior-posterior type. Head and trunk sways, and the displacement of COSP with sole-ground contact were small and stable compared to those without sole-ground contact. A, a seated posture on a height-adjustable chair without both soles of the feet grounded. B, a sitting posture on a height-adjustable chair with hip and knee joints at 90° flexion position, and with both soles of the feet fully grounded.

Fig. 5

An example of overlapping of cycle and average path during chewing the test food on the right side obtained from one subject. A, a seated posture on a height-adjustable chair without both soles of the feet grounded. B, a sitting posture on a height-adjustable chair with hip and knee joints at 90° flexion position, and with both soles of the feet fully grounded. Using the centric occlusion of each cycle as the standard, coordinates for each cycle were determined by vertically dividing the opening and closing paths into 10 equally spaced sections in the frontal view. From these coordinates, the average path and SD (standard deviation) were calculated. The method used to calculate the average path is as follows: (a) 5-14 cycles on the habitual side chewing were recorded, and the coordinates for each cycle were determined by vertical division into 10 equally spaced sections. (b) Overlapping of each cycle and average path. (c) Average path and SDs of each level. The masticatory movement path with sole-ground contact showed less variation in the opening distance and more stable movement path compared to that without sole-ground contact. On the other hand, the masticatory movement path without sole-ground contact showed more variation in the opening distance and movement path, especially closing path.

Parameters

Head and trunk sway values were used to evaluate the stability of head and trunk positions respectively. Each trial of three-dimensional motion analysis system was recorded in 1,000 frames for 20 s. The three-dimensional coordinate of the center of four target points of the head was acquired for every frame. Head sway value (mm) was defined as the mean distance between VCC-h and each center of four target points. The trunk sway value (mm) was obtained in the same manner as the head sway value. The shorter the head sway value and trunk sway value was, the more stable was the head position and trunk position. Conversely, the longer the head sway value and trunk sway value was, the less stable was the head position and trunk position.

The mean displacement of the COSP (MD-COSP) was used to evaluate the stability of the displacement of COSP, that is, sitting pressure distribution. Each trial of the Conformat was recorded in 1,000 frames for 20 s. The 2-dimensional coordinates of COSP were acquired for every frame. MD-COSP (mm) was defined as the mean distance between each COSP during a recording period and VCC-COSP. The shorter the MD-COSP was, the more stable was the sitting pressure distribution. Conversely, the longer the MD-COSP was, the less stable was the sitting pressure distribution.

AGE (mg/dl) was used as a quantitative parameter for masticatory performance [17].

Parameters of the stability of masticatory movements (stabilities of movement path and movement rhythm) were calculated from 10 cycles of mastication beginning with the fifth cycle after the start of chewing the test food [21]. From the opening and closing paths consisting of vertical component and lateral component of mandibular movements for 10 cycles from the fifth cycle, the average path was calculated. The opening distance was used as a parameter representing the masticatory movement path. The opening distance was defined as the vertical distance from the maximum intercuspal position to maximum jaw opening. The average of the 11 SDs (standard deviations) from level 0 to the 10th level in the horizontal direction during the opening movement, in the horizontal direction during the closing movement, and in the vertical direction were calculated as the opening lateral component, closing lateral component and vertical component, respectively. These values were divided by the opening distance to obtain the SD/OD (standard deviation/opening distance). Each SD/OD (%) of opening lateral component, closing lateral component and vertical component were used as the parameters of the stability of movement path [21]. For 10 cycles from the fifth cycle, the opening time, closing time, occluding time and cycle time were calculated. The coefficient of variations (CVs) were then determined from the mean time of the ten cycles and its standard deviation, and the mean values (%) were used as the parameters of the stability of movement rhythm [21].

Each trial was repeated three times and the average value of the three trials was used as the representative value for each subject.

Statistical analysis

All quantitative parameters were compared between with and without sole-ground contact to evaluate whether changes in sitting posture affected masticatory function. All comparisons were performed using Wilcoxon signed-rank test (P < 0.05). SPSS version 21 (IBM Japan, Ltd., Tokyo, Japan) was used for statistical analysis.

Results

The results of the comparisons (median values) in head and trunk sway values and MD-COSP between with and without sole-ground contact are shown in Fig. 6. Medians (Interquartile range [IQR]) of A and B in the head sway value were as follows: A 1.92 (1.68-2.04), B 1.44 (1.19-1.76). Medians (IQR) of A and B in the trunk sway value were as follows: A 2.23 (1.97-2.33), B 1.80 (1.62-1.90). Medians (IQR) of A and B in the MD-COSP were as follows: A 0.71 (0.56-0.87), B 0.29 (0.26-0.32). Head and trunk sway values and MD-COSP were significantly smaller with sole-ground contact than without sole-ground contact.

The results of the comparisons (median values) in the SD/ODs of opening lateral component, closing lateral component and vertical component between with and without sole-ground contact are shown in Fig. 7. Medians (IQR) of A and B in the SD/OD of opening lateral component were as follows: A 7.03 (5.74-9.07), B 4.90 (3.11-7.57). Medians (IQR) of A and B in the SD/OD of closing lateral component were as follows: A 6.15 (4.81-8.39), B 5.04 (4.10-6.47). Medians (IQR) of A and B in the SD/OD of vertical component were as follows: A 5.89 (5.48-6.14), B 5.45 (4.50-5.97). No significant differences were found in the SD/ODs of opening lateral component and closing lateral component between with and without sole-ground contact. However, the SD/OD of vertical component was significantly smaller with sole-ground contact than without sole-ground contact.

The results of the comparisons (median values) in the CVs of the opening time, closing time, occluding time and cycle time between with and without sole-ground contact are shown in Fig. 8. Medians (IQR) of A and B in the CV of opening time were as follows: A 11.11 (9.13-15.19), B 9.81 (8.43-14.79). Medians (IQR) of A and B in the CV of closing time were as follows: A 9.15 (6.80-9.78), B 7.95 (6.20-10.65). Medians (IQR) of A and B in the CV of occluding time were as follows: A 7.85 (6.10-13.97), B 9.95 (7.38-12.68). Medians (IQR) of A and B in the CV of cycle time were as follows: A 5.45 (4.10-7.70), B 5.41 (4.73-6.24). There were no significant differences in the median CVs of the opening time, closing time, occluding time and cycle time between with and without sole-ground contact.

The results of the comparisons (median values) in the AGE between with and without sole-ground contact are shown in Fig. 9. Medians (IQR) of A and B in the AGE were as follows: A 172.51 (154.88-201.26), B 207.75 (191.50-215.75). The median AGE was significantly greater with sole-ground contact than without sole-ground contact.

Fig. 6

Comparison of the head sway value, trunk sway value, and MD-COSP between with and without sole-ground contact. Differences between with and without sole-ground contact were tested with the Wilcoxon signed-rank test (P < 0.05). *P < 0.05, **P < 0.01 and n = 30. A, a seated posture on a height-adjustable chair without both soles of the feet grounded. B, a sitting posture on a height-adjustable chair with hip and knee joints at 90° flexion position, and with both soles of the feet fully grounded.

Fig. 7

Comparison of the SD/ODs of opening lateral component, closing lateral component and vertical component between with and without sole-ground contact. Although there were no significant differences in the SD/ODs of opening lateral component and closing lateral component between with and without sole-ground contact (P > 0.05), the SD/ODs of opening and closing lateral components without sole-ground contact showed more variation compared to those with sole-ground contact, *P < 0.05 and n = 30. A, a seated posture on a height-adjustable chair without both soles of the feet grounded. B, a sitting posture on a height-adjustable chair with hip and knee joints at 90° flexion position, and with both soles of the feet fully grounded.

Fig. 8

Comparison of the CVs of opening time, closing time, occluding time and cycle time between with and without sole-ground contact. No significant differences were found in the CVs of opening time, closing time, occluding time and cycle time between with and without sole-ground contact (P > 0.05), and n = 30. A, a seated posture on a height-adjustable chair without both soles of the feet grounded. B, a sitting posture on a height-adjustable chair with hip and knee joints at 90° flexion position, and with both soles of the feet fully grounded.

Fig. 9

Comparison of the AGE between with and without sole-ground contact. **P < 0.01 and n = 30. A, a seated posture on a height-adjustable chair without both soles of the feet grounded. B, a sitting posture on a height-adjustable chair with hip and knee joints at 90° flexion position, and with both soles of the feet fully grounded.

Discussion

The present study investigated whether changes in sitting posture, with and without sole-ground contact, affect masticatory function. Results for the head sway value, trunk sway value, and MD-COSP (Fig. 6) indicated that when healthy subjects chewed the test food with sole-ground contact, head and trunk movements and the displacement of COSP were smaller and the sitting posture was more stable compared to when they chewed the test food without sole-ground contact. The result of the SD/OD of vertical component (Fig. 7) indicated that when they chewed the test food with sole-ground contact, the vertical variation of movement path was smaller and masticatory movement path was more stable compared to when they chewed the test food without sole-ground contact. Moreover, analysis of AGE indicated that when they chewed the test food with sole-ground contact, AGE was greater and masticatory performance was higher compared to when they chewed the test food without sole-ground contact (Fig. 9).

The existence of three-dimensional rhythmic coordinated movements of the head in response to the masticatory rhythm of mandibular movements during mastication has been reported [11,22,23,24]. In contrast, it has been reported that the center of gravity of the head is in front of the pinna and near the articular tubercle of the temporal bone [25]. Further, the center of rotation of the head movement in healthy individuals is at the top of the cervical spine [26]. Based on these findings, when the coordinated movements of the head occurred during mastication, it may have caused rotation-like movements of the head in the anteroposterior direction because the center of gravity of the head was located at the forward position for the center of rotation of the head movements, and moreover, these movements of the head during mastication may have acted as an anteroposterior disturbance to the sitting posture. In addition, the base of support (BOS) in the sitting posture with sole-ground contact was a large area that included the seat surface and both soles, whereas the BOS in the sitting posture without sole-ground contact was only the seat surface and had a smaller area in the anteroposterior direction than that in the sitting posture with sole-ground contact. Therefore, the degree of freedom in the anteroposterior direction of head and trunk movements in the sitting posture without sole-ground contact with a narrow BOS may have been higher than that in the sitting posture with sole-ground contact with a wide BOS. Based on these reasons, the present results for the evaluation of the sitting posture (Fig. 6) suggest that when subjects chewed the test food in the sitting posture without sole-ground contact, head and trunk movements may have become larger to the anterior-posterior direction and the head and trunk postures, i.e., the sitting posture may have been less stable compared to when they chewed the test food with sole-ground contact. Figure 4, which shows an example of the trajectory data set during chewing for the test food with and without sole-ground contact respectively of the head and trunk sways and displacements of COSP obtained from one subject, suggests that the head and trunk movements without sole-ground contact were larger in the anterior-posterior direction and more unstable compared to those with sole-ground contact. The present results for the stability of masticatory movement path (Fig. 7) suggest the following possibility: when subjects chewed the test food in the sitting posture without sole-ground contact, the anteroposterior movements of their head may have been larger than when they chewed the test food with sole-ground contact, and their head position may have become unstable in the anteroposterior direction, which may have affected the vertical component of the masticatory movement path, resulting in the unstable masticatory movement path in the sitting posture without sole-ground contact. Yamada et al. [27] reported that head posture affected the direction and stability of the mandibular closing movement, and they suggested that the forward bending of the head may produce resistance to the inframandibular soft tissue when the mandible opens, resulting in the anterior placement of the opening and closing paths. Therefore, the present results (Fig. 7) may also suggest that the vertical component of masticatory movement path during chewing without sole-ground contact may have become unstable due to the increased anteroposterior movements of the head and the influence of the resistance of the inframandibular soft tissue during the opening and closing of the masticatory movements compared to during chewing with sole-ground contact. With regard to the stabilities of masticatory movement path and rhythm during mastication in the sitting posture with and without sole-ground contact, although there are no previous investigations that can be used in comparison, the results of this study did not find statistically significant differences in indicators representing the stabilities of masticatory movement path (opening and closing lateral components) (Fig. 7) and rhythm (Fig. 8) with and without sole-ground contact. These results suggest that, regardless of the sitting posture with and without sole-ground contact, normal healthy adults are regular with respect to the lateral component of masticatory movement path and have rhythmic masticatory movements. Figure 5, which shows an example of the overlapping of cycle and average path during chewing for the test food on the right side with and without sole-ground contact obtained from one subject, suggests that the masticatory movement path with sole-ground contact showed less variation in the opening distance and more stable movement path compared to that without sole-ground contact. The present result for the AGE (Fig. 9) suggests that when subjects chewed the test food in the sitting posture with sole-ground contact, the BOS of the sitting posture was wider in the anteroposterior direction due to the grounded feet, and thus the sitting posture, especially the head position, was stable, and the masticatory movement path, especially the vertical component of movement path, was stable, and the reproducible mandibular movements were performed, which resulted in higher masticatory performance than when chewing without sole-ground contact.

Collectively, the results of the present study suggest that the sitting posture during chewing with sole-ground contact showed the stability of masticatory movement path and the higher masticatory performance compared to the sitting posture during chewing without sole-ground contact.

This study has some limitations. Simultaneous measurements of head, trunk, and body sways were carried out to evaluate the effect of changes in sitting posture on masticatory function in the present study. However, analyses were not done on the motion analysis of lower legs and muscle activities in the head, neck, trunk, and lower legs. Future studies should include the motion analysis of the lower legs and the analysis of electrical activities of craniocervical and whole-body muscles to elucidate the relationship between stomatognathic function including mastication and body posture in more detail. The findings in this study could provide important suggestions based on physiological evidence of appropriate and efficient posture during mastication. However, it is necessary to further study the selection of subjects and the setting of the test postures in the future to take into consideration treatment and rehabilitation planning for some patients with postural instability due to balance disorders and elderly individuals requiring support and care. Despite these limitations, this study clarified the effect of sitting posture with and without sole-ground contact on chewing stability and masticatory performance.

Acknowledgments

This study was supported by JSPS KAKENHI Grant Numbers JP15K11188 and JP19K10219.

Conflict of Interest

The authors declare no conflict of interest.

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