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Determination of chewing efficiency using muscle work
archives of oral biology 53 (2008) 533–537
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/arob
Determination of chewing efficiency using muscle work Jarin Paphangkorakit *, Nayiga Chaiyapanya, Penprapa Sriladlao, Sutasinee Pimsupa Department of Oral Biology, Faculty of Dentistry, Khon Kaen University, 123 Mitraparp Road, Muang District, Khon Kaen 40002, Thailand
article info
abstract
Article history:
Objective: A new method was proposed to evaluate ‘true’ chewing efficiency in which the
Accepted 21 December 2007
‘cost’ of chewing was accounted for. Design: Twenty-three subjects were asked to chew an almond for 5 cycles, after which the
Keywords:
chewed particles were air-dried and passed through a 1.4-mm aperture sieve. The activity of
Chewing efficiency
both superficial masseter muscles was simultaneously recorded with surface EMG. Inte-
Masticatory performance
grated EMG (IEMG) was used to calculate burst amplitude, burst duration and maximum
Muscle work
voluntary contraction (MVC). The percentage weight of particles passing the sieve was used to represent the conventional chewing efficiency (or masticatory performance). Muscle work (integral of IEMG bursts), muscle effort (muscle work normalized to maximum work) and masticatory effectiveness (the ratio between masticatory performance and muscle work) were also calculated. Results: The results showed that (1) masticatory performance was significantly correlated with muscle work (R = 0.45; p < 0.005), MVC (R = 0.31; p = 0.04), but not correlated with muscle effort; (2) masticatory effectiveness was significantly correlated with MVC (R = 0.58, p < 0.001), but not correlated with masticatory performance. Conclusion: Persons with good masticatory performance were not necessarily effective (or efficient) chewers. They seemed to have larger MVCs and use more muscle work during the chewing task. # 2008 Elsevier Ltd. All rights reserved.
1.
Introduction
The term ‘chewing efficiency’ has long been used to describe how well food is fragmented after a fixed number of chewing cycles. Recently a more specific term, ‘masticatory performance’, has been introduced as the median size of food particles after a fixed number of chewing cycles and ‘masticatory efficiency’ as the number of chewing cycles needed to fragment food to a certain average size (e.g. half of the original size).1 Two methods have frequently been used to quantify masticatory efficiency and masticatory performance, a sieving method2–4 and an optical scanning method.5–7 The above methods only measure the output of the chewing test whereas true efficiency is related to the ‘cost’ of chewing. One way to measure the ‘cost’ of chewing is to record the activity of the masticatory muscles. This idea, adopted here, has been used in
a few previous studies where the area beneath integrated EMG quantified the ‘work’ done by the masticatory muscles.4,8 No previous study, however, has attempted to relate muscle work to masticatory performance. A person who has a good masticatory performance is not an efficient chewer if his/her jaw muscles have to do a lot of work. It was the objective of the present study to investigate the association between the masticatory performance (determined from the size of chewed particles) and the work done by masticatory muscles.
2.
Materials and methods
The study was approved by Khon Kaen University’s Human Research Ethical Committee. Twenty-three dental students, aged 20–26 years, who had at least 28 natural teeth, with Class I
* Corresponding author. Tel.: +66 43 202405; fax: +66 43 202862. E-mail address: [email protected] (J. Paphangkorakit). 0003–9969/$ – see front matter # 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.archoralbio.2007.12.014
534
archives of oral biology 53 (2008) 533–537
molar relationship and without any pain in the masticatory system, participated in the study. Each subject was asked to chew an almond placed in a closed latex bag (cut from a finger of a latex examination glove) for 5 cycles. This method has been shown to give comparable results with chewing directly on almonds.6 Three trials were tested on the right and three on the left side. Chewed almonds were uncovered and air-dried on a tissue paper for 1 h and sieved through a 1.4-mm aperture brass sieve (Endecotts, London, UK), assisted by a vibrator. The almond particles passing the sieve were weighed by a digital microscale. The weight of chewed almond particles smaller than 1.4 mm was normalized to the original weight of the unchewed almond. This value was used to represent masticatory performance (expressed in %) since it was shown to be strongly correlated with the median particle size (x50) obtained from a 12-sieve system.9 During the chewing trials, surface EMG electrodes (Ag/AgCl) were also attached to the facial skin over the centre of right and left superficial masseter muscles. Electrical signals from the muscles were sampled with an acquisition system (Biopac, Harvard Scientific, USA) at 1000 Hz sampling rate (frequency range 10–500 Hz). The raw EMG was then rectified and integrated by stepwise-averaging over every
100 samples to give IEMG (integrated EMG). At the end of the test, each subject was asked to clench maximally in their maximum intercuspation (maximum voluntary contraction, MVC). In each trial, ‘muscle work’ (expressed in mV s) was calculated from the integral of IEMG bursts summed from right and left masseter muscles during 5 chewing cycles. ‘Muscle effort’ (expressed in %) was calculated from muscle work normalized to the possible ‘maximum muscle work’, being MVC multiplied by the duration of the 5 cycles chewed by the subject from both right and left muscles. Finally, ‘masticatory effectiveness’ was calculated by dividing masticatory performance with muscle work. In addition, mean duration (burst duration) and mean amplitude (burst amplitude) during each trial were averaged from IEMG bursts of the right and left masseter muscles to provide further details about factors governing the magnitude of the muscle work. In the data analysis, mean values from both right- and leftside chewing were pooled. Any possible associations between studied parameters were tested using linear regression and Pearson’s coefficients.
Table 1 – Summary of masticatory performance, MVC, muscle work, muscle effort and masticatory effectiveness on each chewing side from 23 normal subjects Subject number
Right chewing
Left chewing
MVC (mV)
Masticatory performancea (%)
Muscle workb (mV s)
Muscle effortc (%)
Masticatory effectivenessd
Masticatory performancea (%)
Muscle workb (mV s)
Muscle effortc (%)
Masticatory effectivenessd
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
51.6 56.6 43.0 51.9 38.7 56.8 57.4 34.7 64.6 52.4 45.9 40.7 45.3 42.8 63.0 42.1 52.1 59.8 48.3 43.6 65.0 63.5 52.3
42.3 61.8 41.5 39.4 43.3 64.8 119.1 54.2 73.4 69.8 38.5 37.6 97.5 58.1 68.8 70.5 53.4 134.9 128.5 55.0 80.7 79.1 135.1
58.7 32.6 41.6 30.7 36.4 52.9 60.5 36.1 31.9 31.3 25.9 31.8 35.4 21.3 21.5 40.2 25.6 43.9 25.2 14.0 25.3 17.2 27.8
1.23 0.92 1.03 1.33 0.90 0.86 0.49 0.59 0.88 0.75 1.20 1.14 0.47 0.74 0.92 0.59 0.98 0.45 0.38 0.81 0.81 0.80 0.40
46.2 63.4 34.5 45.9 39.8 56.2 66.3 51.1 64.5 55.5 48.9 38.0 31.8 53.9 58.2 41.8 48.7 64.5 52.9 40.2 72.9 67.2 60.0
34.1 51.4 39.9 30.9 48.6 60.0 113.6 43.8 88.8 41.1 25.1 47.7 75.7 57.8 89.9 90.6 51.7 150.7 127.5 74.6 90.5 95.5 158.4
51.9 34.3 39.7 23.4 40.9 49.5 54.3 28.0 41.3 19.8 17.7 34.4 35.6 21.8 28.1 45.7 26.3 45.1 29.3 16.8 27.2 19.0 22.3
1.37 1.25 0.86 1.54 0.82 0.94 0.58 1.17 0.73 1.35 2.02 0.81 0.43 0.93 0.65 0.48 0.95 0.43 0.42 0.54 0.83 0.71 0.38
12.6 14.3 16.7 20.0 21.3 29.8 32.0 32.2 34.1 34.8 35.1 35.6 36.1 36.8 37.2 39.2 46.2 47.5 48.4 51.8 52.7 70.3 88.9
Mean S.D.
51.0 8.9
71.6 31.2
33.4 12.1
0.81 0.27
52.3 11.4
73.4 37.4
32.7 11.5
0.88 0.41
38.0 17.5
Each value is an average of three trials on each side of a subject and the order of subjects is sorted by MVCs. Percentage weight of particles sized smaller than 1.4 mm. This value was used to represent ‘masticatory performance’. b The integral (area beneath the curve) of IEMG bursts summed from both masseter muscles during 5 chewing cycles. c The percentage of muscle work from both masseter muscles normalized to maximum muscle work (assuming every cycle had an amplitude equal to MVC) during 5 chewing cycles (MVC: maximum IEMG during maximum voluntary tooth clenching). d The ratio between masticatory performance and muscle work. A larger value indicates more effective chewing. a
archives of oral biology 53 (2008) 533–537
535
with burst duration (R = 0.36). Masticatory performance was significantly correlated with muscle work (R = 0.45; p < 0.005) but not correlated with muscle effort (Fig. 2(A) and (B)). A significant correlation between masticatory performance and
Fig. 1 – Scatter plots showing significant correlations between burst amplitude and muscle work (A) and between burst duration and muscle work (B). Each point represents the average of three trials on each side of a subject (n = 46).
3.
Results
A large variation in masticatory performance was noted among the subjects. Detailed values are summarized in Table 1. In brief, masticatory performance ranged from 34.7% to 65.0% (mean = 51.0, S.D. = 8.9) on the right side and ranged from 31.8% to 72.9% (mean = 52.3, S.D. = 11.4) on the left. Muscle work ranged from 37.6 to 135.1 mV s (mean = 71.6, S.D. = 31.2) on the right side and ranged from 25.1 to 158.4 mV s (mean = 73.4, S.D. = 37.4) on the left. Muscle effort ranged from 14.0% to 60.5% (mean = 33.4, S.D. = 12.1) on the right side and ranged from 16.8% to 54.3% (mean = 32.7, S.D. = 11.5) on the left. Muscle effectiveness ranged from 0.38 to 1.33 (mean = 0.81, S.D. = 0.27) on the right side and ranged from 0.38 to 2.02 (mean = 0.88, S.D. = 0.41) on the left. There were no significant differences of the above values between right- and left-side chewing ( p > 0.05, Wilcoxon test). It was found that muscle work was dependent on both burst amplitude and burst duration ( p’s < 0.001; Fig. 1) but the correlation was higher with burst amplitude (R = 0.78) than
Fig. 2 – Scatter plots showing the association between masticatory performance and (A) muscle work, (B) muscle effort, and (C) MVC. Each point represents the average of three trials on each side of a subject (n = 46). A significant positive correlation between masticatory performance and muscle work is observed, suggesting that persons with good masticatory performance might not be effective chewers. A significant correlation between masticatory performance and MVC in (C) is noted.
536
archives of oral biology 53 (2008) 533–537
Fig. 3 – Scatter plots showing (A) a non-significant correlation between masticatory effectiveness and masticatory performance and (B) a significant correlation between masticatory effectiveness and MVC. Each point represents the average of three trials on each side of a subject (n = 46).
MVC (R = 0.31; p = 0.04) was also found (Fig. 2(C)). Finally, masticatory effectiveness was correlated with MVC (R = 0.58, p < 0.001), but not correlated with masticatory performance (Fig. 3).
4.
Discussion
In the present study, ‘muscle work’ was used to represent the ‘cost’ of chewing done by masticatory muscles. We have demonstrated that persons with good masticatory performance tend to use more muscle work. Although muscle work values could be varied by the properties of the electrode–skin interface, it was unlikely that persons with low performance were the only ones who had poor electrode–skin electrical conductivity (resulting in lower muscle work values). The larger muscle work values seen in subjects with higher performance suggested that they either might have larger burst amplitude or longer burst duration in masseter muscles (Fig. 1). This may result in larger chewing forces since burst amplitude (in mV) was shown to be correlated with bite
forces.10 Alternatively, with longer burst duration they could have either chewed with a longer power stroke or more grinding movement. An increased activity in masseter muscles could also indicate more anteriorly directed chewing forces11 but it was not clear how this would affect masticatory performance. Placing an almond inside a latex glove in the present study reduced the importance of tongue in food manipulation, hence confirming the role of dentition and chewing forces on the masticatory performance. Despite more muscle work used by good-performance chewers, it was not necessary that they used more muscle effort (Fig. 2). This could be explained by their larger MVCs. It also suggested that increasing muscle work in poor-performance chewers (yielding larger muscle effort) might not necessarily improve their performance. On the other hand, large MVC values suggest that such persons might have larger muscles or more stable intercuspation during maximum clenching and might explain good masticatory performance found in our subjects who had large MVCs. Taken the above together, we propose that in order to obtain a ‘true’ masticatory efficiency, the ‘cost’ of the chewing task must be considered. It was shown here that when ‘muscle work’ was used to represent the cost of chewing, there is no correlation between masticatory performance and masticatory effectiveness, suggesting that good-performance chewers may not be necessarily classified as effective chewers. Some limitations were encountered in the present study. First, masticatory performance obtained from the one-sieve system might not be the best measure of masticatory performance. However, it has been shown that the proportion of particles passing through a one-sieve system (designated as ‘‘masticatory performance index’’) is strongly associated with the median particle size obtained from a 12-sieve system (R = 0.91 for 1-mm sieve, R = 0.97 for 2-mm sieve) and the index is acceptable for categorizing good and bad chewers when its value ranged between 20% and 80%9 which was the case in the present study. Secondly, muscles other than the masseters could have contributed to a chewing task, but due to our technical constraints only the activity of masseter muscles was measured. However, previous studies have shown that the activity of the anterior temporalis muscle correlates well with that of the masseter muscle during natural chewing8,12 and the activity of masseter muscles seems to best predict the force produced by the masticatory system during simulated vertical chewing.13 Another way to measure the ‘‘cost’’ of chewing might be the measurement of chewing forces but at present, this seems impractical during a chewing test in a fully dentate subject. Although the term ‘chewing efficiency’ has generally been used to describe how well a person can fragment test food in a given time or after a number of chewing cycles, the present study has argued that this does not reflect the actual ‘efficiency’ of the chewing task. We prefer to use the term ‘masticatory performance’ to represent the fineness of chewed particles and further introduce a new term ‘masticatory effectiveness’ to represent actual ‘efficiency’. Masticatory effectiveness was defined as the ratio between masticatory performance and the work of masticatory muscles in accomplishing the tasks. It has been shown in the present study that a person who has good masticatory performance is
archives of oral biology 53 (2008) 533–537
not necessarily an effective (or efficient) chewer. It would, therefore, be of interest to re-analyze the effect of various factors on masticatory performance in previous studies by taking muscle work into account.
5.
6.
Acknowledgements The authors thank Prof. Andries van der Bilt and Prof. Jeff Osborn for giving valuable comments to the original version of this manuscript. We also thank all subjects for participating in this study. The study was supported by Student Research Grant, Faculty of Dentistry, Khon Kaen University.
7. 8.
9.
references 10.
1. van Kampen FM, van der Bilt A, Cune MS, Fontijn-Tekamp FA, Bosman F. Masticatory function with implant-supported overdentures. J Dent Res 2004;83:708–11. 2. Edlund J, Lamm CJ. Masticatory efficiency. J Oral Rehabil 1980;7:123–30. 3. Tate GS, Throckmorton GS, Ellis E, Sinn DP. Masticatory performance, muscle activity, and occlusal force in preorthognathic surgery patients. J Oral Maxillofac Surg 1994;52:476–81. 4. Weijnen FG, van der Bilt A, Kuks JB, van der Glas HW, Oudenaarde I, Bosman F. Masticatory performance in
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patients with myasthenia gravis. Arch Oral Biol 2002;47: 393–8. Mahmood WA, Watson CJ, Ogden AR, Hawkins RV. Use of image analysis in determining masticatory efficiency in patients presenting for immediate dentures. Int J Prosthodont 1992;5:359–66. Mowlana F, Heath R. Assessment of masticatory efficiency: new methods appropriate for clinical research in dental practice. Eur J Prosthodont Restor Dent 1993;1:121–5. Wilding R. The association between chewing efficiency and occlusal contact area in man. Arch Oral Biol 1993;38:589–96. Mioche L, Bourdiol P, Martin JF, Noel Y. Variations in human masseter and temporalis muscle activity related to food texture during free and side-imposed mastication. Arch Oral Biol 1999;44:1005–12. van der Bilt A, Fontijn-Tekamp FA. Comparison of single and multiple sieve methods for the determination of masticatory performance. Arch Oral Biol 2004;49:155–60. Lindauer SJ, Gay T, Rendell J. Effect of jaw opening on masticatory muscle EMG-force characteristics. J Dent Res 1993;72:51–5. Mao J, Osborn JW. Direction of a bite force determines the pattern of activity in jaw-closing muscles. J Dent Res 1994;73:1112–20. Veyrune J, Mioche L. Complete denture wearers: electromyography of mastication and texture perception whilst eating meat. Eur J Oral Sci 2000;108:83–92. Ottenhoff FA, van der Bilt A, van der Glas HW, Bosman F, Abbink JH. The relationship between jaw elevator muscle surface electromyogram and simulated food resistance during dynamic condition in humans. J Oral Rehabil 1996;23:270–9.
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/arob
Determination of chewing efficiency using muscle work Jarin Paphangkorakit *, Nayiga Chaiyapanya, Penprapa Sriladlao, Sutasinee Pimsupa Department of Oral Biology, Faculty of Dentistry, Khon Kaen University, 123 Mitraparp Road, Muang District, Khon Kaen 40002, Thailand
article info
abstract
Article history:
Objective: A new method was proposed to evaluate ‘true’ chewing efficiency in which the
Accepted 21 December 2007
‘cost’ of chewing was accounted for. Design: Twenty-three subjects were asked to chew an almond for 5 cycles, after which the
Keywords:
chewed particles were air-dried and passed through a 1.4-mm aperture sieve. The activity of
Chewing efficiency
both superficial masseter muscles was simultaneously recorded with surface EMG. Inte-
Masticatory performance
grated EMG (IEMG) was used to calculate burst amplitude, burst duration and maximum
Muscle work
voluntary contraction (MVC). The percentage weight of particles passing the sieve was used to represent the conventional chewing efficiency (or masticatory performance). Muscle work (integral of IEMG bursts), muscle effort (muscle work normalized to maximum work) and masticatory effectiveness (the ratio between masticatory performance and muscle work) were also calculated. Results: The results showed that (1) masticatory performance was significantly correlated with muscle work (R = 0.45; p < 0.005), MVC (R = 0.31; p = 0.04), but not correlated with muscle effort; (2) masticatory effectiveness was significantly correlated with MVC (R = 0.58, p < 0.001), but not correlated with masticatory performance. Conclusion: Persons with good masticatory performance were not necessarily effective (or efficient) chewers. They seemed to have larger MVCs and use more muscle work during the chewing task. # 2008 Elsevier Ltd. All rights reserved.
1.
Introduction
The term ‘chewing efficiency’ has long been used to describe how well food is fragmented after a fixed number of chewing cycles. Recently a more specific term, ‘masticatory performance’, has been introduced as the median size of food particles after a fixed number of chewing cycles and ‘masticatory efficiency’ as the number of chewing cycles needed to fragment food to a certain average size (e.g. half of the original size).1 Two methods have frequently been used to quantify masticatory efficiency and masticatory performance, a sieving method2–4 and an optical scanning method.5–7 The above methods only measure the output of the chewing test whereas true efficiency is related to the ‘cost’ of chewing. One way to measure the ‘cost’ of chewing is to record the activity of the masticatory muscles. This idea, adopted here, has been used in
a few previous studies where the area beneath integrated EMG quantified the ‘work’ done by the masticatory muscles.4,8 No previous study, however, has attempted to relate muscle work to masticatory performance. A person who has a good masticatory performance is not an efficient chewer if his/her jaw muscles have to do a lot of work. It was the objective of the present study to investigate the association between the masticatory performance (determined from the size of chewed particles) and the work done by masticatory muscles.
2.
Materials and methods
The study was approved by Khon Kaen University’s Human Research Ethical Committee. Twenty-three dental students, aged 20–26 years, who had at least 28 natural teeth, with Class I
* Corresponding author. Tel.: +66 43 202405; fax: +66 43 202862. E-mail address: [email protected] (J. Paphangkorakit). 0003–9969/$ – see front matter # 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.archoralbio.2007.12.014
534
archives of oral biology 53 (2008) 533–537
molar relationship and without any pain in the masticatory system, participated in the study. Each subject was asked to chew an almond placed in a closed latex bag (cut from a finger of a latex examination glove) for 5 cycles. This method has been shown to give comparable results with chewing directly on almonds.6 Three trials were tested on the right and three on the left side. Chewed almonds were uncovered and air-dried on a tissue paper for 1 h and sieved through a 1.4-mm aperture brass sieve (Endecotts, London, UK), assisted by a vibrator. The almond particles passing the sieve were weighed by a digital microscale. The weight of chewed almond particles smaller than 1.4 mm was normalized to the original weight of the unchewed almond. This value was used to represent masticatory performance (expressed in %) since it was shown to be strongly correlated with the median particle size (x50) obtained from a 12-sieve system.9 During the chewing trials, surface EMG electrodes (Ag/AgCl) were also attached to the facial skin over the centre of right and left superficial masseter muscles. Electrical signals from the muscles were sampled with an acquisition system (Biopac, Harvard Scientific, USA) at 1000 Hz sampling rate (frequency range 10–500 Hz). The raw EMG was then rectified and integrated by stepwise-averaging over every
100 samples to give IEMG (integrated EMG). At the end of the test, each subject was asked to clench maximally in their maximum intercuspation (maximum voluntary contraction, MVC). In each trial, ‘muscle work’ (expressed in mV s) was calculated from the integral of IEMG bursts summed from right and left masseter muscles during 5 chewing cycles. ‘Muscle effort’ (expressed in %) was calculated from muscle work normalized to the possible ‘maximum muscle work’, being MVC multiplied by the duration of the 5 cycles chewed by the subject from both right and left muscles. Finally, ‘masticatory effectiveness’ was calculated by dividing masticatory performance with muscle work. In addition, mean duration (burst duration) and mean amplitude (burst amplitude) during each trial were averaged from IEMG bursts of the right and left masseter muscles to provide further details about factors governing the magnitude of the muscle work. In the data analysis, mean values from both right- and leftside chewing were pooled. Any possible associations between studied parameters were tested using linear regression and Pearson’s coefficients.
Table 1 – Summary of masticatory performance, MVC, muscle work, muscle effort and masticatory effectiveness on each chewing side from 23 normal subjects Subject number
Right chewing
Left chewing
MVC (mV)
Masticatory performancea (%)
Muscle workb (mV s)
Muscle effortc (%)
Masticatory effectivenessd
Masticatory performancea (%)
Muscle workb (mV s)
Muscle effortc (%)
Masticatory effectivenessd
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
51.6 56.6 43.0 51.9 38.7 56.8 57.4 34.7 64.6 52.4 45.9 40.7 45.3 42.8 63.0 42.1 52.1 59.8 48.3 43.6 65.0 63.5 52.3
42.3 61.8 41.5 39.4 43.3 64.8 119.1 54.2 73.4 69.8 38.5 37.6 97.5 58.1 68.8 70.5 53.4 134.9 128.5 55.0 80.7 79.1 135.1
58.7 32.6 41.6 30.7 36.4 52.9 60.5 36.1 31.9 31.3 25.9 31.8 35.4 21.3 21.5 40.2 25.6 43.9 25.2 14.0 25.3 17.2 27.8
1.23 0.92 1.03 1.33 0.90 0.86 0.49 0.59 0.88 0.75 1.20 1.14 0.47 0.74 0.92 0.59 0.98 0.45 0.38 0.81 0.81 0.80 0.40
46.2 63.4 34.5 45.9 39.8 56.2 66.3 51.1 64.5 55.5 48.9 38.0 31.8 53.9 58.2 41.8 48.7 64.5 52.9 40.2 72.9 67.2 60.0
34.1 51.4 39.9 30.9 48.6 60.0 113.6 43.8 88.8 41.1 25.1 47.7 75.7 57.8 89.9 90.6 51.7 150.7 127.5 74.6 90.5 95.5 158.4
51.9 34.3 39.7 23.4 40.9 49.5 54.3 28.0 41.3 19.8 17.7 34.4 35.6 21.8 28.1 45.7 26.3 45.1 29.3 16.8 27.2 19.0 22.3
1.37 1.25 0.86 1.54 0.82 0.94 0.58 1.17 0.73 1.35 2.02 0.81 0.43 0.93 0.65 0.48 0.95 0.43 0.42 0.54 0.83 0.71 0.38
12.6 14.3 16.7 20.0 21.3 29.8 32.0 32.2 34.1 34.8 35.1 35.6 36.1 36.8 37.2 39.2 46.2 47.5 48.4 51.8 52.7 70.3 88.9
Mean S.D.
51.0 8.9
71.6 31.2
33.4 12.1
0.81 0.27
52.3 11.4
73.4 37.4
32.7 11.5
0.88 0.41
38.0 17.5
Each value is an average of three trials on each side of a subject and the order of subjects is sorted by MVCs. Percentage weight of particles sized smaller than 1.4 mm. This value was used to represent ‘masticatory performance’. b The integral (area beneath the curve) of IEMG bursts summed from both masseter muscles during 5 chewing cycles. c The percentage of muscle work from both masseter muscles normalized to maximum muscle work (assuming every cycle had an amplitude equal to MVC) during 5 chewing cycles (MVC: maximum IEMG during maximum voluntary tooth clenching). d The ratio between masticatory performance and muscle work. A larger value indicates more effective chewing. a
archives of oral biology 53 (2008) 533–537
535
with burst duration (R = 0.36). Masticatory performance was significantly correlated with muscle work (R = 0.45; p < 0.005) but not correlated with muscle effort (Fig. 2(A) and (B)). A significant correlation between masticatory performance and
Fig. 1 – Scatter plots showing significant correlations between burst amplitude and muscle work (A) and between burst duration and muscle work (B). Each point represents the average of three trials on each side of a subject (n = 46).
3.
Results
A large variation in masticatory performance was noted among the subjects. Detailed values are summarized in Table 1. In brief, masticatory performance ranged from 34.7% to 65.0% (mean = 51.0, S.D. = 8.9) on the right side and ranged from 31.8% to 72.9% (mean = 52.3, S.D. = 11.4) on the left. Muscle work ranged from 37.6 to 135.1 mV s (mean = 71.6, S.D. = 31.2) on the right side and ranged from 25.1 to 158.4 mV s (mean = 73.4, S.D. = 37.4) on the left. Muscle effort ranged from 14.0% to 60.5% (mean = 33.4, S.D. = 12.1) on the right side and ranged from 16.8% to 54.3% (mean = 32.7, S.D. = 11.5) on the left. Muscle effectiveness ranged from 0.38 to 1.33 (mean = 0.81, S.D. = 0.27) on the right side and ranged from 0.38 to 2.02 (mean = 0.88, S.D. = 0.41) on the left. There were no significant differences of the above values between right- and left-side chewing ( p > 0.05, Wilcoxon test). It was found that muscle work was dependent on both burst amplitude and burst duration ( p’s < 0.001; Fig. 1) but the correlation was higher with burst amplitude (R = 0.78) than
Fig. 2 – Scatter plots showing the association between masticatory performance and (A) muscle work, (B) muscle effort, and (C) MVC. Each point represents the average of three trials on each side of a subject (n = 46). A significant positive correlation between masticatory performance and muscle work is observed, suggesting that persons with good masticatory performance might not be effective chewers. A significant correlation between masticatory performance and MVC in (C) is noted.
536
archives of oral biology 53 (2008) 533–537
Fig. 3 – Scatter plots showing (A) a non-significant correlation between masticatory effectiveness and masticatory performance and (B) a significant correlation between masticatory effectiveness and MVC. Each point represents the average of three trials on each side of a subject (n = 46).
MVC (R = 0.31; p = 0.04) was also found (Fig. 2(C)). Finally, masticatory effectiveness was correlated with MVC (R = 0.58, p < 0.001), but not correlated with masticatory performance (Fig. 3).
4.
Discussion
In the present study, ‘muscle work’ was used to represent the ‘cost’ of chewing done by masticatory muscles. We have demonstrated that persons with good masticatory performance tend to use more muscle work. Although muscle work values could be varied by the properties of the electrode–skin interface, it was unlikely that persons with low performance were the only ones who had poor electrode–skin electrical conductivity (resulting in lower muscle work values). The larger muscle work values seen in subjects with higher performance suggested that they either might have larger burst amplitude or longer burst duration in masseter muscles (Fig. 1). This may result in larger chewing forces since burst amplitude (in mV) was shown to be correlated with bite
forces.10 Alternatively, with longer burst duration they could have either chewed with a longer power stroke or more grinding movement. An increased activity in masseter muscles could also indicate more anteriorly directed chewing forces11 but it was not clear how this would affect masticatory performance. Placing an almond inside a latex glove in the present study reduced the importance of tongue in food manipulation, hence confirming the role of dentition and chewing forces on the masticatory performance. Despite more muscle work used by good-performance chewers, it was not necessary that they used more muscle effort (Fig. 2). This could be explained by their larger MVCs. It also suggested that increasing muscle work in poor-performance chewers (yielding larger muscle effort) might not necessarily improve their performance. On the other hand, large MVC values suggest that such persons might have larger muscles or more stable intercuspation during maximum clenching and might explain good masticatory performance found in our subjects who had large MVCs. Taken the above together, we propose that in order to obtain a ‘true’ masticatory efficiency, the ‘cost’ of the chewing task must be considered. It was shown here that when ‘muscle work’ was used to represent the cost of chewing, there is no correlation between masticatory performance and masticatory effectiveness, suggesting that good-performance chewers may not be necessarily classified as effective chewers. Some limitations were encountered in the present study. First, masticatory performance obtained from the one-sieve system might not be the best measure of masticatory performance. However, it has been shown that the proportion of particles passing through a one-sieve system (designated as ‘‘masticatory performance index’’) is strongly associated with the median particle size obtained from a 12-sieve system (R = 0.91 for 1-mm sieve, R = 0.97 for 2-mm sieve) and the index is acceptable for categorizing good and bad chewers when its value ranged between 20% and 80%9 which was the case in the present study. Secondly, muscles other than the masseters could have contributed to a chewing task, but due to our technical constraints only the activity of masseter muscles was measured. However, previous studies have shown that the activity of the anterior temporalis muscle correlates well with that of the masseter muscle during natural chewing8,12 and the activity of masseter muscles seems to best predict the force produced by the masticatory system during simulated vertical chewing.13 Another way to measure the ‘‘cost’’ of chewing might be the measurement of chewing forces but at present, this seems impractical during a chewing test in a fully dentate subject. Although the term ‘chewing efficiency’ has generally been used to describe how well a person can fragment test food in a given time or after a number of chewing cycles, the present study has argued that this does not reflect the actual ‘efficiency’ of the chewing task. We prefer to use the term ‘masticatory performance’ to represent the fineness of chewed particles and further introduce a new term ‘masticatory effectiveness’ to represent actual ‘efficiency’. Masticatory effectiveness was defined as the ratio between masticatory performance and the work of masticatory muscles in accomplishing the tasks. It has been shown in the present study that a person who has good masticatory performance is
archives of oral biology 53 (2008) 533–537
not necessarily an effective (or efficient) chewer. It would, therefore, be of interest to re-analyze the effect of various factors on masticatory performance in previous studies by taking muscle work into account.
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Acknowledgements The authors thank Prof. Andries van der Bilt and Prof. Jeff Osborn for giving valuable comments to the original version of this manuscript. We also thank all subjects for participating in this study. The study was supported by Student Research Grant, Faculty of Dentistry, Khon Kaen University.
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