Original Article |
Corresponding author: Vladimir Bogdanov ( vladbogdanov@yahoo.com ) © 2023 Vladimir Bogdanov.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Bogdanov V (2023) Type of correlation between bite force and EMG activity of the temporalis and masseter muscles during maximal and submaximal clenching. Folia Medica 65(6): 975-985. https://doi.org/10.3897/folmed.65.e107180
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Introduction: Maximal bite force is an important determinant of masticatory function and is essential for the estimation of dental status. Bite force is usually measured by gnathodynamometry.
Aim: The aim of the study was to investigate the type of correlation between the electromyographic activity of the masticatory muscles and the bite force during simultaneous measurement and to evaluate the use of EMG activity as an option for indirect determination of the bite force.
Materials and methods: The study included 68 people (33 men and 35 women) with a mean age of 18.4±6.1 years. The bite force and the sEMG were measured in all 68 subjects consecutively on the right and the left side during maximal (1) and submaximal (2/3 and 1/3) clenching. Statistical analysis was performed by IBM SPSS Statistics, version 23.0.
Results: EMG activity and maximal bite force values did not show significant differences on the left and right side. Linear relationship was found for the association bite force: EMG activity for the masseter muscle during maximal and submaximal (2/3) clenching, while for 1/3 clenching force the association was lost. For the temporalis muscle, this relationship was linear for the left side during maximal and submaximal 2/3 clenching and non-linear for the right side. During 1/3 clenching, the linearity was lost on both left and right sides.
Conclusions: The masseter muscle shows stable linear relationship between BF and EMG on both sides and can be considered reliable for indirect estimation of the bite force by measuring the EMG potentials. During low occlusal forces (1/3) the linearity of both masseter and temporalis muscles is decreased or lost.
bite force, masticatory muscles, surface electromyography, type of correlation
The evaluation of bite force is important for the estimation of the masticatory system in physiological and pathological conditions (temporomandibular disorders, bruxism, etc.) and the effects of treatment. The chewing muscles create sufficient bite force to provide normal masticatory function.[
The aim of the present study was to evaluate the type of relationship between bite force and EMG activity of the masticatory muscles during maximal and submaximal clenching bilaterally and to determine a reliable protocol for evaluating the occlusal forces in physiological and pathological conditions.
The study included 68 healthy subjects (33 male and 35 female) with mean age of 18.4±6.1 years. The bite force and the surface EMG were measured simultaneously on the right and on the left side. Individuals who had undergone or were undergoing orthodontic treatment, or who had evidence of bruxism, were not included in the study. The inclusion criteria were: no temporo-mandibular disorders; no pain during clenching the teeth; no missing teeth in the lateral areas and no evidence of an acute inflammatory process.
The bite force was measured by a strain-gauge transducer – a GD500.1 gnatodynamometric system with a test range of 0-700 newtons. The subjects sat on the dental chair with their heads upright in natural position and unsupported. The horizontal planes of the gnathodynamometer were positioned in the area of the first permanent molars. A maximal voluntary clench was performed on the fork of the strain-gauge transducer. After recording the maximal value - 1, the subject was asked to bite with 2/3 and 1/3 of the maximal bite force and these attempts are recorded. The subject observed the display and applied the necessary strain for 3 seconds. The measurement was taken bilaterally, sequentially to the left and right side at an interval of one minute, simultaneously with the recording of the EMG potentials of m. masseter and m. temporalis.
The surface EMG recording was performed using a two-channel electromyograph (Neuro-EMG-Micro-2) with the Neuro-MEP-Ω software. The electrical potentials of temporalis anterior (TA) and the masseter muscle (MM) were measured. Before placing the surface electrodes (Ag/AgCl), the skin was cleaned with 90% ethyl alcohol over the most prominent part of the muscle palpated in contraction, parallel to the fibers according to the Surface EMG for Non-Invasive Assessment of Muscles (SENIAM) program guidelines. All subjects were briefed on the procedure prior to the sEMG measurement. Maximal and mean EMG activity values of every contraction of the right and the left masseter and the anterior temporalis were recorded during maximal (1), 2/3 and 1/3 of the maximal bite force during a 3-second period. The EMG potentials were recorded in microvolts (µV). All participants gave their informed consent.
The statistical analysis was performed using IBM SPSS Statistics version 23.0. The Kolmogorov-Smirnov test was used to check the normality of distribution. Nonparametric tests were applied to variables that did not have a normal distribution. Regression analysis was used to find out what equation describes most precisely the type of association between the bite force and EMG activity of the masticatory muscles during maximal and submaximal force (1, 2/3, 1/3). All statistical tests were performed at a level of 5% for significance at p<0.05.
Table
There was no significant difference between the left and right side of the bite force and the EMG activity of the temporalis and masseter muscles. The EMG values of the temporalis muscle were higher than those of the masseter muscle but without statistical significance.
Bite force and maximal and mean EMG values of the masseter and temporalis muscles on the right (RM, RT) and left (LM, LT) side
BF | BF right | RM max | RM mean | RT max | RT mean | BF left | LM max | LM mean | LT max | LT mean |
1 | 362.5±134.1 | 914.5±516.8 | 272.9±89.5 | 940.3±368.4 | 282.2±63.1 | 396.3±128.1 | 830.3±320.5 | 286.1±83.2 | 877.0±274.9 | 274.7±55.2 |
2/3 | 263.1±114.8 | 678.2±338.6 | 225.8±51.3 | 791.6±390.9 | 244.9±51.1 | 286.5±111.7 | 684.9±264.8 | 234.6±50.7 | 716.5±250.5 | 242.0±48.2 |
1/3 | 174.6±113.8 | 528.5±282.6 | 205.9±61.8 | 642.8±309.3 | 220.5±57.1 | 202.9±125.4 | 537.4±239.9 | 200.9±52.2 | 590.9±261.5 | 217.8±52.9 |
The applied regression analysis and procedure ‘Curve estimation’ of SPSS found an association between the maximal BF and EMG potential of the right masseter muscle (RmMax), described best with the linear model (p=0.002, R2=0.136) with the following parameters: BF=285,042+0.086*RmMax. The curve described a linear relationship. For the mean values, the linear model was (p=0.001, R2=0.158) with the following parameters BF=152,744+0.799*RmMean. A directly proportional linear relationship was found, stronger for the mean values.
The relationship between BF and EMG activity of the right m. masseter (Rm Max, Rm Mean) during maximal clenching is shown in Fig.
Type of correlation between BF and maximal and mean EMG values of the right masseter muscle.
Fig.
Type of correlation between BF and maximal and mean EMG values of the right temporalis muscle during maximal clenching.
The association was best described by the reciprocal model (p<0.001, R2=0.247) with the parameters: BF=575,579−176051,864/RtMax. The type of relationship was not linear. For the mean values of EMG of the right temporal muscle, the association was described by the reciprocal model (p<0.001, R2=0.355) and parameters: BF=753,097−105063,520/RtMean. The association is directly proportional but not linear.
The correlation between BF and EMG of the left masseter muscle during maximal clenching was described by the linear model (p=0.001, R2=0.154) with parameters BF=271,966+0.131*LmMax for the maximal values and with the linear model (p<0.001, R2=0.186) with parameters BF=205,701+0.665*LmMean for the mean values. The relationship is directly proportional and linear (Fig.
Type of correlation between BF and maximal (LmMax), and mean (LmMean) EMG values of the left masseter muscle.
For the temporalis muscle, the relationship was described by the linear model (p<0.001, R2=0.331) with the parameters BF=160,675+0.268*LtMax for the maximal values and by (p<0.001, R2=0.383), and BF=−39.409+1.594*LtMean for the mean values respectively (Fig.
Because the values of the bite force were not normally distributed, they were logarithmically transformed (Ln) to be normalized. The relationship EMG of the right masseter/bite force was best described by the linear model (p<0.001, R2=0.180) with parameters Ln(BF)=4,411+0.001*RmMax for the maximal values and with the logarithmic model (p<0.001, R2=0.235) with parameters Ln(BF)=−2,061+(1.335*ln(RmMean) for the mean values. The curves for the right masseter are shown in Fig.
For the right temporalis, the EMG/BF relationship during 2/3 max clenching is described best with the logarithmic model (p<0.001, R2=0.431) with parameters Ln(BF)=−0.050+(0.792*ln(RtMax) for the maximal EMG values, and with the reciprocal model (p<0.001, R2=0.467) with parameters Ln(BF)=6,849−389,725/RtMean for the mean values. The relationship has a directly proportional but not linear character (Fig.
For the left masseter, the relationship is described best with the linear model (p<0.001, R2=0.223) with parameters BF=69,329+0.238*LmMax for the maximal EMG values, and with the linear model (p<0.001, R2=0.277) with parameters BF=−191.909+1,930*LmMean for the mean values. In both cases the relationship is linear (Fig.
For the left temporalis muscle, the relationship during 2/3 max clenching is described best by the linear model (p<0.001, R2=0.432) with parameters BF=20,393+0.299*LtMax for the maximal values and by the linear model (p<0.001, R2=0.509) with parameters BF=−188,236+1.778*LtMean for the mean values. The curves are shown in Fig.
Because the values of the bite force during submaximal clenching (1/3 max) were not normally distributed, they were logarithmically transformed to normalize them. The regression analysis found a linear model of (p=0.023, R2=0.078), with parameters Ln(BF)=5,190+0.0004*RmMax for the maximal values of the right masseter and a linear model (p=0.001, R2=0.148) with parameters Ln(BF)=4,629+0.004*RmMean for the mean values. The correlation is directly proportional and linear (Fig.
Type of relationship between BF and EMG of the right masseter muscle during submaximal clenching (1/3 max).
For the right temporalis muscle, the logarithmic model was (p<0.001, R2=0.333) with parameters Ln(BF)=1,002+(0.678*ln(RtMax), for the maximal values and (p<0.001, R2=0.478) with parameters Ln(BF)=7,035−368,294/RtMean, for the mean values, respectively. The relationship is directly proportional, but not linear. Therefore, when the EMG potentials are above 200 mV, a further increase of the potentials is associated with a lower increase of the biting force (Fig.
Regression analysis did not find association between the maximal EMG activity and the bite force values during submaximal clenching of 1/3 of the left masseter muscle.
As to the mean EMG values of the masseter, the linear model of (p=0.002, R2=0.133) with parameters BF=97,904 + 0.803*LmMean described best the relationship (Fig.
For the left temporalis muscle, this relationship was best described by the model (p<0.001, R2=0.277) and parameters BF=471,755−120072,661/LtMax for the maximal values and by (p<0.001, R2=0.311) and BF=649,443−85463,756/LtMean for the mean values (Fig.
In the present study, we assessed the type of association between bite force and EMG activity of the temporalis and masseter muscles during maximal and submaximal clenching to determine the most reliable results when occlusal forces of varying magnitude are applied. Therefore, different occlusal forces could be recommended and applied in patients with disorders of the masticatory system (TMD, bruxism, crossbites, etc.), when maximal clenching is risky and not desirable.
Some EMG studies suggest that muscles at rest do not display electrical activity, and accordingly, there are no motor units’ contractions, while other authors found minimal electrical activity in muscles at rest, associated with the masticatory process.[
In many studies, the temporalis and masseter muscles are used due to their easy access for localization and measurement and because they exhibit the most definite contraction during maximal occlusion.[
The temporalis and masseter muscles show different BF/EMG relationship curves, and respond differently to different occlusal force. The temporalis muscle’ curves on the left and the right side are not identical. Linear relationship is more typical for the masseter muscle, compared to temporalis, and it could be considered more appropriate for indirect evaluation of BF using EMG when there is a risk of direct measurement of bite force. Concerning the occlusal load, maximal and submaximal 2/3 clenching can be recommended as more reliable, as at low occlusal load (1/3) the linearity is decreased or lost.