Tongue pressure production and orofacial muscle activities during swallowing are related to palatal morphology in individuals with normal occlusion

ABSTRACT

Purpose

The relationships between motor function, including the orofacial muscles coordinated with the tongue during swallowing, and palatal morphology are still unclear. This study aimed to identify the relationships between tongue pressure production and orofacial muscle activities during swallowing and palatal morphology.

Materials and methods

The participants were 20 healthy volunteers with individual normal occlusion (8 men, 12 women; mean age: 25.0 ± 2.9 years). Tongue pressure and masseter, orbicularis oris, mentalis and supra-infrahyoid muscle activities during 4 ml gel swallowing were recorded simultaneously by a sensor sheet system, with five measuring points and surface electromyography, respectively. Onset, offset and duration of each parameter and the maximum magnitude of tongue pressure were analysed. Palatal width and depth were measured using three-dimensional digital images of maxillary dental models and examined for correlations with tongue pressure and orofacial muscle activities.

Results

The temporal coordination between tongue pressure production and orofacial muscle activities during swallowing was observed. The maximum magnitude of tongue pressure was significantly positively correlated with the palatal width. Palatal depth showed significant correlations with the time sequences of tongue pressure and orbicularis oris and mentalis muscle activities.

Conclusion

These results suggest that tongue pressure production and orofacial muscle activities during swallowing are related to palatal width and depth.

KEYWORDS:

• Swallowing
• tongue pressure
• orofacial muscle activity
• palatal morphology

1. Introduction

Harmony between maxillofacial morphology and stomatognathic function contributes to the stability of occlusion after orthodontic treatment. Therefore, understanding the characteristics of stomatognathic function and its relationship with maxillofacial morphology is crucial for diagnosis and the establishment of suitable treatment policies for orthodontic treatment. Swallowing is performed by the coordinated movements of the tongue and the intra-extra orofacial muscles, being involved in the stability of occlusion. The tongue contacts the palate during swallowing [1], and the tongue movement during swallowing is associated with maxillofacial morphology [2]. Palatal shape and linear dimensions vary with maxillofacial morphology [3]. However, the relationships between tongue movement and the orofacial muscles and maxillofacial morphology remain unclear.

The contact pressure between the tongue and palate, i.e., tongue pressure, has a strong correlation with tongue movement [4] and is one of the useful quantitative parameters of tongue-palate contact during oropharyngeal swallowing [5]. Tongue pressure production during swallowing is coordinated with orofacial muscle activities [6], showing specific patterns depending on the maxillofacial morphology or habits [7–10]. In addition, it has been reported that tongue pressure during swallowing is related to the depth and width of the palate [11,12] and is influenced by palatal shape [13]. However, there have been no reports of the relationships among tongue pressure, orofacial muscle activities and palatal morphology. We hypothesized that the tongue pressure production and orofacial muscle activities during swallowing are modulated by the palatal width and/or palatal depth. The purpose of this study was to investigate the relationships between tongue pressure and orofacial muscle activities during swallowing by simultaneous measurement and palatal morphology in subjects with individual normal occlusion.

2. Materials and methods

2.1. Participants

The study participants were 20 volunteers with individual normal occlusion (8 men, 12 women; mean age, 25.0 ± 2.9 years). Inclusion criteria were as follows: appropriate overjet and overbite; bilateral Angle Class I molar relationship in centric occlusion; dental arch width and length within standard range [14]; and arch length discrepancy >−3 mm and <3 mm on both maxillary and mandibular dental arches (Table 1). Exclusion criteria included the following: congenital abnormalities; history of orthodontic treatment; abnormality in the number of permanent teeth (excluding third molars); significant facial asymmetry; abnormal findings in the tongue or soft tissue morphology; dysphagia on the repetitive saliva swallowing test [15]; abnormal swallowing habits including tension around the lips; and nasopharyngeal disease with mouth breathing. Sample size was calculated as 17 from preliminary experiment data (α = 0.05, detection power = 0.8 and correlation coefficient ρ = 0.6).

Table 1. Details of the participants.

	Mean ± S.D
Overjet [mm]	2.53 ± 0.55
Overbite [mm]	2.30 ± 0.68
Dental arch width [mm]
Maxillary	44.05 ± 3.55
Mandibular	35.73 ± 2.03
Dental arch length [mm]
Maxillary	38.33 ± 1.85
Mandibular	34.25 ± 1.82
Arch length discrepancy [mm]
Maxillary	−0.18 ± 0.61
Mandibular	−0.48 ± 0.64

This study was approved by the Ethics Committee of Niigata University (approval no. 2015–3055). All study protocols were conducted in accordance with the Declaration of Helsinki, and informed consent was obtained from all study participants prior to the start of measurements.

2.2. Measurement of tongue pressure and orofacial muscle activities

Tongue pressure was measured using a 0.1-mm-thick tongue pressure sensor sheet (Swallow-Scan; Nitta, Osaka, Japan) with five measurement points (Ch1, anterior-median part; Ch2, mid-median part; Ch3, posterior-median part; Ch4, left posterior-lateral part; and Ch5, right posterior-lateral part) [5]. The sensor sheet was secured to the palatal mucosa with denture adhesive (Touch Correct II; Shionogi, Osaka, Japan) (Figure 1(A)). Tongue pressure measurements were recorded at a sampling rate of 100 Hz. Small skin electrodes (NT-611T; Nihon Kohden, Tokyo, Japan) were used for the measurement of surface electromyography (sEMG). sEMG activity was monitored with five surface electrode placement sites; (a) the left masseter muscle, superficial central part; (b) left orbicularis oris muscle, central part of the upper lip; (c) mentalis muscle, median part of the mental region; (d) left suprahyoid muscles, line bisecting the left mandibular plane and facial median line and (e) left infrahyoid muscles, corresponding part of the sternohyoid muscle (Figure 1(B)). The small skin electrodes were attached to the skin over each muscle with a distance of 20 mm between the electrodes. The sEMG recordings were made at a sampling rate of 1 kHz and converted via an A/D converter (PowerLab 8/35; ADInstruments, Dunedin, New Zealand).

Figure 1. (A) occlusal view of the sensor sheet attached to the palate, (B) positioning of surface electrodes over the orofacial muscles.

Ch, channel; Ch1, anterior-median part; Ch2, mid-median part; Ch3, posterior-median part; Ch4, left posterior-lateral part; Ch5, right posterior-lateral part; a, masseter muscle; b, orbicularis oris muscle; c, mentalis muscle; d, suprahyoid muscles; e, infrahyoid muscles.

The participant was seated on a chair in a sealed room, and the Frankfort horizontal plane was set parallel to the floor. Then, 4 ml of gel (Yasashiku-oishiku-suibunhokyu, Balance, Toyama, Japan) were dispensed into the mouth with a syringe, and the participant was instructed to swallow the gel in a single swallow. Tongue pressure and orofacial muscle activities were measured simultaneously, and five measurements were conducted for each participant. To synchronize the tongue pressure and sEMG recordings, synchronization signals from the Swallow-Scan were recorded on all computers.

3. Data analysis

3.1. Tongue pressure and orofacial muscle activities

The sEMG waveforms were processed by 50-Hz low-cut processing, full-wave rectification and nine-point smoothing. The peak time of suprahyoid muscle activity was set as time 0 s, by defining it as the time of the highest value in the sEMG waveform derived by full-wave rectification and smoothing, and the onset, offset and duration of tongue pressure and sEMG waveforms at each measurement point were calculated (Figure 2). The maximum magnitude of tongue pressure was also calculated. The final value used for each participant was the mean of five measurements.

Figure 2. A typical example of tongue pressure waveforms, raw and rectified waveforms of orofacial muscle activities during swallowing.

▲, the peak time of suprahyoid muscle activity was set as time 0 s (black arrowhead); ●, onset; ○, offset; the line from onset to offset shows duration; ▽, maximum magnitude of tongue pressure.

3.2. Palatal morphology

Alginate impressions of the maxillary dentition were taken, and a hard plaster model was made. A three-dimensional scanner (3Shape E3 3D scanner, 3shape, Copenhagen, Denmark) was used to scan the maxillary dentition into three-dimensional digital data. The data were imported into three-dimensional morphometry software (Body-Rugle, Medic Engineering, Kyoto, Japan) for model measurement.

The reference plane was defined by the incisive papilla and the deepest point of the palatal margin of the right and left first molars (M and M’) [11]. The measurement plane was the coronal section perpendicular to the reference plane and containing the line M–M’. On the measurement plane, palatal width was defined as the line M–M’ (Figure 3). Palatal depth at points 1–9 was defined as the distance between the palate and the line perpendicular to the palatal width drawn from each of the 10 equally divided points along the palatal width, in ascending order from the right side, as point 1, point 2 … point 9. The palatal depth ratio was defined as the ratio of palatal depth at each point to the palatal depth at point 5. The palate was then divided into three areas: right lateral area, from M to palatal depth at point 3; central area, from palatal depth at point 3 to point 7; and left lateral area, from palatal depth at point 7 to M.

Figure 3. Assessment of palatal morphology on the plaster model of the maxilla.

M and M’: the deepest point of the palatal margin of the right and left first molars; Measurement plane: a coronal section perpendicular to the reference plane (incisive papilla and M and M’ planes) and containing the line M–M’; palatal width: the line M–M’; palatal depth at points 1–9, the distance between the palate and the line perpendicular to the palatal width drawn from each of the 10 equally divided points along the palatal width, in ascending order from the right side, as point 1, point 2 … point 9. Palatal depth ratio was defined as the ratio of palatal depth at each point to the palatal depth at point 5. The palate was then divided into three areas: right lateral area, from M to palatal depth at point 3; central area, from palatal depth at point 3 to point 7; and left lateral area, from palatal depth at point 7 to M.

3.3. Statistical analysis

The normality for each parameter was examined by Shapiro–Wilk test. Friedman’s test was used to compare the onset, offset and duration of tongue pressure and orofacial muscle activities and the maximum magnitude of tongue pressure at each measurement point during swallowing, and post hoc analysis of Bonferroni was performed for multiple comparisons. Pearson’s product-rate correlation coefficient or Spearman’s rank correlation coefficient was used for correlations between each measurement item of tongue pressure and orofacial muscle activities and palatal width and depth. The significance level was set at α = 0.05 for all parameters. IBM SPSS Statistics Version 29 (IBM Japan, Tokyo, Japan) was used for statistical analyses.

4. Results

4.1. Tongue pressure and orofacial muscle activities

The onset of masseter, orbicularis oris, mentalis and suprahyoid muscle activities were significantly earlier than that of Ch1–5 (Figure 4(A)). The onset of Ch1 was significantly earlier than that of Ch3. The onset of infrahyoid muscle activities was significantly later than that of orbicularis oris muscle activity, and significantly earlier than that of Ch3. The offset of Ch3 disappeared significantly earlier than that of Ch4 and Ch5. The offset of supra-infrahyoid muscle activities disappeared significantly later than that of Ch1–3, masseter, orbicularis oris and mentalis muscle activities. The duration of masseter muscle activity was significantly longer than that of Ch2 and Ch3 (Figure 4(B)). The duration of orbicularis oris, mentalis and suprahyoid muscle activities were significantly longer than that of Ch1–5. The duration of infrahyoid muscle activity was significantly longer than that of Ch2–5. The maximum magnitude of tongue pressure at Ch1 was the highest among all pressure-sensitive points, but there was no significant difference among them (Figure 4(C)).

Figure 4. (A) time sequences for tongue pressure and orofacial muscle activities, (B) duration of tongue pressure and orofacial muscle activities, (C) maximum magnitude of tongue pressure.

Ch, channel; Ch1–5, see in Figure 1(A). *p < 0.05; n.s., Not significant.

4.2. Relationships of tongue pressure and orofacial muscle activities with palatal morphology

Table 2 and Figure 5 show the analysis of palatal morphology. The maximum magnitude of tongue pressure at Ch1 and Ch5 was significantly positively correlated with palatal width (Table 3).

Figure 5. Palatal depth.

M, M’, Points 1–9, Right and left lateral areas and Central area, see in Figure 3.

Table 2. Analysis of palatal morphology.

Palatal width [mm]		Palatal depth [mm]	Palatal depth ratio [%]
Median (IQR)		Median (IQR)
35.75 (34.10–38.23)	Point 1	4.67 (4.02–5.27)	38 (30–42)
	Point 2	9.02 (8.75–9.71)	72 (66–79)
	Point 3	12.20 (11.32–13.15)	97 (91–102)
	Point 4	12.62 (11.62–13.83)	100 (98–104)
	Point 5	12.60 (11.39–13.57)	100
	Point 6	12.52 (11.19–13.62)	99 (97–100)
	Point 7	12.01 (10.65–12.89)	94 (88–96)
	Point 8	9.18 (7.67–10.30)	72 (62–75)
	Point 9	4.74 (3.72–5.71)	37 (31–43)

Table 3. Relationships between the maximum magnitude of tongue pressure and palatal morphology.

Maximum magnitude of tongue pressure	Palatal width		Palatal depth
			Point 1		Point 2		Point 3		Point 4		Point 5		Point 6		Point 7		Point 8		Point 9
Ch1	0.45	*	0.23		0.25		0.36		0.31		0.22		0.14		0.16		0.15		0.15
Ch2	0.39		−0.06		−0.04		0.20		0.18		0.03		0.01		0.00		−0.03		−0.11
Ch3	0.40		0.16		0.00		−0.29		−0.33		−0.36		−0.37		−0.27		−0.13		−0.02
Ch4	0.37		0.25		0.19		−0.05		−0.13		−0.19		−0.20		−0.15		−0.12		−0.03
Ch5	0.51	*	0.09		0.10		−0.02		−0.07		−0.14		−0.14		−0.15		−0.14		−0.12

Values show correlation coefficients. *p < 0.05, **p < 0.01.

Significant negative correlations were found between the onset of Ch2 and Ch3 and the palatal depth at point 3, between the onset of orbicularis oris muscle activity and the palatal depth at points 5–7 and between the onset of mentalis muscle activity and the palatal depth at points 3, 6 and 7 (Table 4). These results indicated that the deeper the palate, the earlier the onset of tongue pressure and perioral muscle activities. Significant negative correlations were found between the offset of Ch3 and the palatal depth at points 3–6, showing that the deeper the palate, the earlier the offset of tongue pressure (Table 5).

Table 4. Relationships of the onset of tongue pressure production and orofacial muscle activities with palatal morphology.

Onset	Palatal width		Palatal depth
			Point 1		Point 2		Point 3		Point 4		Point 5		Point 6		Point 7		Point 8		Point 9
Ch1	−0.16		−0.15		−0.18		−0.36		−0.29		−0.18		−0.17		−0.26		−0.18		−0.13
Ch2	−0.39		−0.14		−0.29		−0.57	**	−0.37		−0.20		−0.18		−0.27		−0.20		−0.10
Ch3	−0.38		−0.08		−0.25		−0.56	*	−0.34		−0.16		−0.13		−0.23		−0.17		−0.07
Ch4	−0.24		0.01		0.06		−0.03		0.06		0.19		0.21		0.11		0.09		0.03
Ch5	−0.24		−0.03		0.03		−0.11		−0.03		0.12		0.17		0.11		0.12		0.07
Masseter muscle	0.14		0.03		−0.14		−0.30		−0.34		−0.33		−0.35		−0.31		−0.18		0.05
Orbicularis oris muscle	0.13		−0.03		−0.15		−0.35		−0.44		−0.52	*	−0.51	*	−0.47	*	−0.40		−0.19
Mentalis muscle	−0.20		−0.11		−0.15		−0.45	*	−0.40		−0.45		−0.48	*	−0.51	*	−0.44		−0.28
Suprahyoid muscles	0.05		0.02		0.04		0.00		−0.10		−0.14		−0.15		−0.14		−0.14		−0.07
Infrahyoid muscles	−0.43		−0.05		−0.23		−0.37		−0.27		−0.17		−0.17		−0.12		−0.03		0.11

Values show correlation coefficients. *p < 0.05, **p < 0.01.

Table 5. Relationships of the offset of tongue pressure production and orofacial muscle activities with palatal morphology.

Offset	Palatal width		Palatal depth
			Point 1		Point 2		Point 3		Point 4		Point 5		Point 6		Point 7		Point 8		Point 9
Ch1	−0.16		0.11		−0.11		−0.40		−0.31		−0.20		−0.20		−0.26		−0.25		−0.15
Ch2	−0.26		−0.02		−0.22		−0.34		−0.23		−0.15		−0.14		−0.17		−0.20		−0.17
Ch3	−0.14		−0.24		−0.41		−0.65	**	−0.61	**	−0.51	*	−0.46	*	−0.39		−0.27		−0.19
Ch4	−0.20		−0.04		−0.04		−0.35		−0.33		−0.27		−0.21		−0.29		−0.35		−0.43
Ch5	−0.19		0.05		0.00		−0.34		−0.28		−0.21		−0.14		−0.21		−0.25		−0.29
Masseter muscle	−0.30		0.15		0.04		−0.02		0.02		0.13		0.06		0.12		0.20		0.32
Orbicularis oris muscle	0.01		0.04		−0.03		0.10		0.28		0.33		0.27		0.13		0.12		0.20
Mentalis muscle	0.05		0.09		0.13		0.14		0.25		0.29		0.28		0.23		0.19		0.10
Suprahyoid muscles	−0.12		0.13		0.11		−0.16		−0.25		−0.18		−0.14		−0.22		−0.27		−0.26
Infrahyoid muscles	−0.17		0.08		0.06		−0.22		−0.27		−0.18		−0.14		−0.24		−0.30		−0.31

Values show correlation coefficients. *p < 0.05, **p < 0.01.

Significant positive correlations were found between the duration of orbicularis oris muscle activity and the palatal depth at points 4–6 and between the duration of mentalis muscle activity and the palatal depth at points 4–7 (Table 6).

Table 6. Relationships of the duration of tongue pressure production and orofacial muscle activities with palatal morphology.

Duration	Palatal width		Palatal depth
			Point 1		Point 2		Point 3		Point 4		Point 5		Point 6		Point 7		Point 8		Point 9
Ch1	0.00		0.25		0.18		−0.01		0.04		0.00		0.03		0.03		−0.08		−0.08
Ch2	0.11		0.10		0.06		0.18		0.12		0.04		0.04		0.08		0.00		−0.05
Ch3	0.13		−0.17		−0.22		−0.23		−0.34		−0.37		−0.34		−0.21		−0.14		−0.13
Ch4	0.05		−0.04		−0.08		−0.25		−0.31		−0.37		−0.35		−0.32		−0.35		−0.36
Ch5	0.06		0.06		−0.03		−0.19		−0.21		−0.28		−0.28		−0.29		−0.33		−0.31
Masseter muscle	−0.07		0.11		0.12		0.28		0.35		0.39		0.37		0.31		0.30		0.26
Orbicularis oris muscle	−0.07		0.05		0.07		0.30		0.50	*	0.58	**	0.54	*	0.40		0.35		0.28
Mentalis muscle	0.11		0.11		0.17		0.40		0.46	*	0.52	*	0.54	*	0.52	*	0.43		0.25
Suprahyoid muscles	−0.14		0.08		0.05		−0.12		−0.08		0.00		0.04		−0.03		−0.07		−0.12
Infrahyoid muscles	0.27		0.11		0.26		0.18		0.05		0.03		0.05		−0.07		−0.19		−0.33

Values show correlation coefficients. *p < 0.05, **p < 0.01.

5. Discussion

The present study found that tongue pressure and orofacial muscle activities during swallowing are associated with palatal width and palatal depth in the central area. The present results provide basic knowledge of the relationships of tongue pressure and orofacial muscle activities during swallowing with palatal morphology in subjects with individual normal occlusion where morphology and function are in harmony.

5.1. Methodology

The sensor sheet has multi-channel pressure-sensitive points, evaluating tongue movement related to liquid transfer by measuring tongue pressure during swallowing [4]. Simultaneous measurements of tongue pressure and orofacial muscle activities during swallowing were performed using the multi-channel sensor sheet and sEMG, respectively, thus facilitating the evaluation of a series of time sequences of swallowing movements. Furthermore, a comprehensive evaluation of the motor function of the tongue and orofacial muscles during swallowing and palatal morphology was performed by searching for correlations with palatal morphology. The use of three-dimensional model measurement facilitated the evaluation of palatal depth, which is difficult with conventional model measurement.

The onset of rapid hyoid elevation for swallowing reflex occurs immediately after the peak time of suprahyoid muscle activity during swallowing [16], suggesting the closest timing to the onset of swallowing reflex that could be observed in this study. Since the swallowing reflex is an involuntary movement and was considered to be more stable in timing compared to voluntary movements, the peak time of the suprahyoid muscle activity was set as time 0 s.

Strict selection criteria were established because it was impossible to take cephalometric radiographs of the participants due to X-ray exposure. Therefore, morphologically and functionally harmonious subjects with individual normal occlusion could be selected in a way that is consistent with daily clinical practice. As the palatal width and depth of the participants were average compared with previous study [11,12], palatal morphology was analysed using actual measurements rather than ratios.

5.2. Tongue pressure and orofacial muscle activities

Time sequences of tongue pressure and orofacial muscle activities in subjects with individual normal occlusion showed normal tongue-palate contact and orofacial muscle activities during the oropharyngeal phase of swallowing, as reported by Ono et al. [6], suggesting that the tongue and orofacial muscles were coordinated with each other.

5.3. Relationships of tongue pressure and orofacial muscle activities with palatal morphology

Significant positive correlations were found between the maximum magnitude of tongue pressure and palatal width, similar to previous reports [11,12]. The narrower the palate, the more difficult it is to elevate the tongue over the anterior-median and the posterior-lateral parts of the palate, which is involved in bolus retention, suggesting that tongue movement during swallowing may be restricted. In the present study, palatal depth ratio at points 3–7 was 90% or more, and points 3 and 7 were palatal inflexion points. Therefore, the palate was divided into three areas by these points.

Significant negative correlations were found between the onset of Ch2 and Ch3 and the palatal depth at point 3 and between the offset of Ch3 and the palatal depth at points 3–6. In the present study, the earlier onset and offset were synonymous with the longer from onset to time 0 and the shorter from time 0 to the offset, respectively. Palatal depth had positive correlation with the vertical motion magnitude of tongue movement during swallowing [17]. Thus, a deep palate was thought to increase the vertical distance between the tongue and palate, which might require more time from the onset of Ch2 and Ch3, being involved in propelling the bolus [16], to the peak time of suprahyoid muscle activity. It was also suggested that the deeper the palate in the central area, the more difficult it was for the tongue to make prolonged contact with the posterior-median part of the palate and the tongue left it earlier.

The deeper the palate in the central area, the earlier the onset and the prolonged duration of orbicularis oris and mentalis muscle activities, related to lip closure [1]. Considering that a series of swallowing movements began with perioral muscle activities, it was suggested that the deeper the palate in the central area, the earlier and longer it keeps closing the anterior part of the mouth by lip closure might be essential for performing the swallowing. However, relationships between palatal depth in the central area and the time sequences of tongue pressure and orofacial muscle activities varied by palatal depth points, suggesting that the fine palatal morphology of each participant might have influenced the results. Further validation with more subjects is needed in the future.

5.4. Limitations

Orofacial muscle activities except for the mentalis muscle were measured only on the left side, since there is no laterality in the orofacial muscle activities in individuals without obvious facial asymmetry [9]. Bilateral measurement of orofacial muscle activities will be necessary to examine patients with malocclusion who have morphological or functional abnormalities in the oral and maxillofacial region that may cause laterality.

Only regarding the maxillary, first molars were measured for palatal morphometric analysis in the present study. More multifaceted model analysis and the effects of gender differences and body size in the future are being considered.

Tongue pressure production during swallowing in patients with malocclusion differs from that in subjects with normal occlusion [7,8]. Palatal width and depth differ according to the sagittal malocclusion and vertical pattern [3], and the relative position of the tongue and palate is thought to vary with maxillofacial morphology. Future studies in patients with malocclusion may lead to the development of effective myofunctional therapy taking palatal morphology into account and contribute to more appropriate diagnosis for orthodontic patients with functional problems.

6. Conclusion

Sequential coordination of tongue pressure and orofacial muscle activities was observed during swallowing in subjects with individual normal occlusion. The narrower the palate, the lower the maximum magnitude of tongue pressure. The deeper the palate in the central area, the more the time sequences of tongue pressure and perioral muscle activities were modulated. These findings indicate that the tongue pressure and the orofacial muscle activities during swallowing are related to palatal width and depth.

Author contributions

Author 1: Design, data acquisition and interpretation, statistical analyses, writing-original draft and editing; Author 2: Statistical analyses and data interpretation; Authors 3 and 5: Provision of guidance on the use of research equipment and data interpretation; Authors 4, 8 and 9: Consideration and discussion of the findings; Author 6: Data acquisition and interpretation; Author 7: Conception and data interpretation; Authors 2–9: Writing-review.

Ethical approval

This study was approved by the Ethics Committee of Niigata University (approval no. 2015–3055).

Informed consent

Informed consent was obtained from all study participants prior to the start of measurements.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

Part of this work was supported by JSPS KAKENHI Grant Numbers [17K11952 and 20K18662].