Motor control of jaw muscles in chewing and in isometric biting with graded narrowing of jaw gape

When a certain bite force is applied during unilateral chewing, the combination of jaw elevator muscle activities is different than when a comparable force is applied in unilateral isometric biting, e.g. on a force transducer. Masticatory peak force is generated in a nearly isometric phase of the chewing cycle, with a jaw gape of about 1 mm. In contrast, peak force in isometric biting on force measuring equipment usually induces jaw gapes of 6 mm or even more. Therefore, we tested the hypothesis that the jaw gape influences relative activation of elevator muscles in unilateral isometric biting. We further examined whether such influence could explain the different activity combinations of chewing and isometric biting. In thirty asymptomatic males, masseter and temporalis activities were recorded during intermittent isometric biting with jaw gapes of 6, 5, 3, 2 and 1 mm and during unilateral chewing. Activity combinations were described by working/balancing ratios and by temporalis/masseter ratios. With decreasing jaw gape the working/balancing ratio of the posterior temporalis decreased (P < 0.002) while that of the masseter increased (P < 0.001). Likewise, the temporalis/masseter ratio on the balancing side increased (P < 0.001). With decreasing jaw gape, activity ratios of isometric biting approached ratios of chewing. We conclude that: (i) relative jaw muscle activation in isometric biting depends on the jaw gape, (ii) relative muscle activation in chewing resembles relative activation of isometric biting with a small 'chewing-like' gape. This suggests that characteristic activity combinations in chewing are mainly a result of the approximately isometric contraction during the slow closing phase of the chewing cycle.     

Influence of maximum bite force on jaw movement during gummy jelly mastication

It is known that maximum bite force has various influences on chewing function; however, there have not been studies in which the relationships between maximum bite force and masticatory jaw movement have been clarified. The aim of this study was to investigate the effect of maximum bite force on masticatory jaw movement in subjects with normal occlusion. Thirty young adults (22 men and 8 women; mean age, 22·6 years) with good occlusion were divided into two groups based on whether they had a relatively high or low maximum bite force according to the median. The maximum bite force was determined according to the Dental Prescale System using pressure‐sensitive sheets. Jaw movement during mastication of hard gummy jelly (each 5·5 g) on the preferred chewing side was recorded using a six degrees of freedom jaw movement recording system. The motion of the lower incisal point of the mandible was computed, and the mean values of 10 cycles (cycles 2–11) were calculated. A masticatory performance test was conducted using gummy jelly. Subjects with a lower maximum bite force showed increased maximum lateral amplitude, closing distance, width and closing angle; wider masticatory jaw movement; and significantly lower masticatory performance. However, no differences in the maximum vertical or maximum anteroposterior amplitudes were observed between the groups. Although other factors, such as individual morphology, may influence masticatory jaw movement, our results suggest that subjects with a lower maximum bite force show increased lateral jaw motion during mastication.     

Proclination of lower incisors: a design to maximize food penetration and minimize torque

Human upper and lower incisors are both tilted forward in the sagittal plane. Previous theoretical and in vitro studies have investigated how proclination may help the teeth either to penetrate or to fracture food more effectively or both. We study the proclination of lower incisors in relation to efficiency and to the protection it may offer from potentially damaging torque forces. Lateral cephalographs from 57 normal human subjects were traced. In one study, a line was drawn joining the centre of the condyle to the tip of the lower incisor. The results showed the lower incisor is oriented so that it is closely parallel to the arc of a circle centred at the condyle. In another study, lines were drawn joining the tips of upper and lower incisors at different openings. Each line showed the direction of the force that must be used to bite an object held between the tips of the incisor teeth. Its direction was compared with the direction of the long axis of the lower incisor when the mandible was graphically rotated open. The results showed the long axis of the lower incisor is closest to the direction of the bite force at 12 degrees and 15 degrees of jaw openings (roughly 20-25 mm incisal separation). This is the opening where the maximum incisal force is normally produced. The findings suggest that to reduce the torque, lower incisors implanted or relocated during orthodontic treatment should be oriented parallel to the closing arc.     

Influence of static and dynamic occlusal characteristics and muscle force on masticatory performance in dentate adults

Masticatory performance is the outcome of a complex interplay of several factors. This study was carried out to determine the relationship between masticatory performance and several muscular-related and occlusion-related factors in a population with a full or near-full complement of natural teeth. One-hundred dentate young adults participated in this cross-sectional study. Maximum muscular force with jaw, hand, tongue, and cheek were measured by means of a gnatodynamometer. Occlusal contact area and number of teeth in contact were determined in the maximal intercuspal position and in a 1.5-mm right and left lateral excursion by means of interocclusal registrations that were scanned and analysed using image software. Masticatory performance was determined by sieving the Optosil particles resulting from 20 chewing cycles. Stepwise multiple linear regression analysis showed that the maximum bite force in the region of the first molar had the best correlation with masticatory performance and explained 36% of its variation. Static occlusion characteristics such as occlusal contact area, the lack of lateral crossbite and the number of anterior teeth in contact explained an additional 9% of the variation in masticatory performance. These findings suggest that variables related to dynamic occlusion or tongue or cheek force do not enhance the prediction of masticatory performance.     

叩齿、紧咬运动中正常牙动态[牙合]力变化特点的研究

目的：研究正常牙紧咬、叩齿运动中[牙合]力的动态变化特征。方法：选择25名咬合正常的受试者，用PVDF动态[牙合]力仪测量节律性叩齿和紧咬运动中下颌第一磨牙[牙合]力动态变化曲线，分析最大[牙合]力峰值和到达峰值的时间。结果：叩齿运动中下颌第一磨牙殆力峰值平均为24～27kg，到达峰值的时间为100～110ms；紧咬运动的[牙合]力峰值平均为27～30kg，到达峰值的时间为800ms。主要功能侧的最大[牙合]力值大于非主要功能侧，到达峰值的时间短。结论：通过PVDF动态[牙合]力仪的测试，明确了叩齿、紧咬运动中正常牙的[牙合]力动态变化特征，功能刺激、运动频率对殆力值具有一定影响。     

叩齿、紧咬运动中正常牙动态(牙合)力变化特点的研究

目的:研究正常牙紧咬、叩齿运动中牙合力的动态变化特征。方法:选择25名咬合正常的受试者,用PVDF动态牙合力仪测量节律性叩齿和紧咬运动中下颌第一磨牙牙合力动态变化曲线,分析最大牙合力峰值和到达峰值的时间。结果:叩齿运动中下颌第一磨牙牙合力峰值平均为24~27 kg,到达峰值的时间为100~110 m s;紧咬运动的牙合力峰值平均为27~30 kg,到达峰值的时间为800 m s。主要功能侧的最大牙合力值大于非主要功能侧,到达峰值的时间短。结论:通过PVDF动态牙合力仪的测试,明确了叩齿、紧咬运动中正常牙的牙合力动态变化特征,功能刺激、运动频率对牙合力值具有一定影响。     

Bite force and handgrip force in patients with molecular diagnosis of myotonic dystrophy

Myotonic dystrophy is a multisystemic disease with varying symptomatology. The aim of this study was to compare the maximal bite force and handgrip force in patients with molecular diagnosis of myotonic dystrophy with those in a group of healthy individuals. It was hypothesized that these forces were reduced in the patients in comparison with the control subjects. The bite and handgrip forces of 37 patients with molecular diagnosis of myotonic dystrophy and 37 control subjects matched regarding age and gender were measured using an electronic dynamometer. The bite and handgrip forces were significantly lower in the myotonic dystrophy patient group when compared with the healthy controls (P < 0.0001). There were no significant force differences between genders, right- or left-hand side of mastication or hands in the myotonic dystrophy patient group whereas such differences were found among the controls. There were moderate but significant correlations between bite and handgrip force in both groups (r = 0.43-0.59; P < 0.01). It was concluded that there were considerable differences between the myotonic dystrophy group and the control subjects regarding both bite force and handgrip force. The weakness of the masticatory and hand muscles may have various negative consequences for oral function and dental health in patients with myotonic dystrophy.     

Maximal contraction force and endurance of human jaw-closing muscles in isometric clenching

Maximal unilateral bite force and endurance times from maximal bite force to the 75% and 50% levels of maximal values were recorded for 13 men and 15 women with bite openings of 10 and 14 mm. Measurements were made both from right and left molar regions and from the incisal region. Bite force values were significantly higher than previously measured in endurance tests by devices with unilateral housings. Men achieved greater bite force than women in the molar region in every trial. The general assumption that women's muscles are superior to those of men in static endurance when both are working at the same percentage of maximal voluntary contraction was not supported by this work for jaw-closing muscles. The endurance times to 75% and 50% levels of maximal bite force were shorter than could be expected from previous reports, and endurance times to the 50% level were even shorter than those reported for muscles of limbs.     

Influence on force curve exerted by jaw tapping force

The purpose of this study, which made use of visual biofeedback, was to determine how methods of regulating jaw tapping force differed depending on the strength of the tapping, using the force curve as an index. Nine healthy examinees were asked to make 30-35 jaw tapping movements, reproducing the defined target tapping force as accurately as possible. We measured the duration of the tooth contact phase, the time to peak force, the first time derivative of force (peak dF/dt), and the time to peak dF/dt. The results indicated that the duration of the tooth contact phase and the time to peak force increased with the target value (P < 0.01). As the target rose, the peak dF/dt increased significantly (P < 0.01), but the time to peak dF/dt was not significant (P=0.134). We found that the higher the target value, the greater the degree of dependency on feedback information. We also found that both the peak dF/dt and the time to peak dF/dt were determined for each examinee prior to movement.     

Task-related electromyographic spectral changes in the human masseter and temporalis muscles

The masticatory muscles differ in their fiber type composition. It can therefore be expected that their electromyographic (EMG) power spectra will differ during the performance of different bite force tasks. In the present study, surface EMG activity was picked up from the masseter and from the anterior and posterior temporalis muscles of nine adult subjects. At a bite force level as low as 25 N, the mean power frequency (MPF) values of the posterior temporalis were significantly lower than those of the masseter and anterior temporalis. The MPF values of the masseter muscles decreased with an increase of bite force magnitude, whereas the MPF values of the anterior and posterior temporalis did not change significantly. The MPF values were significantly influenced by the direction of bite force. The observed changes of MPF are possibly related to the recruitment of different fiber types, and support the concept that the masticatory muscles behave heterogeneously.     

Posterior scissors‐bite: masticatory jaw movement and muscle activity

Scissors‐bite is a malocclusion characterised by buccal inclination or buccoversion of the maxillary posterior tooth and/or linguoclination or linguoversion of the mandibular posterior tooth. This type of malocclusion causes reduced contact of the occlusal surfaces and can cause excessive vertical overlapping of the posterior teeth. This case–control study is the first to evaluate both masticatory jaw movement and masseter and temporalis muscle activity in patients with unilateral posterior scissors‐bite. Jaw movement variables and surface electromyography data were recorded in 30 adult patients with unilateral posterior scissors‐bite malocclusion and 18 subjects with normal occlusion in a case–control study. The chewing pattern on the scissors‐bite side significantly differed from that of the non‐scissors‐bite side in the patients and of the right side in the normal subjects. These differences included a narrower chewing pattern (closing angle, P < 0·01; cycle width, P < 0·01), a longer closing duration (P < 0·05), a slower closing velocity (P < 0·01) and lower activities of both the temporalis (P < 0·05) and the masseter (P < 0·05) muscles on the working side. In 96% of the patients with unilateral posterior scissors‐bite, the preferred chewing side was the non‐scissors‐bite side (P = 0·005). These findings suggest that scissors‐bite malocclusion is associated with the masticatory chewing pattern and muscle activity, involving the choice of the preferred chewing side in patients with unilateral posterior scissors‐bite.     

The effects of masseter muscle pain on biting performance

The present study applied a standardized test food of known hardness to evaluate the biting performance of 20 female patients who had pain mainly in the masseter muscle during palpation. Another 20 women of a similar age group who were pain-free during examination served as controls. Electromyograms (EMG) of the masseter and sternocleidomastoid (SCM) muscles and the jaw position were recorded and measured when the subjects were biting through two types of test foods with known hardness (hard type, 20 kg hardness and extra-hard type, 60 kg hardness). Pressure-pain-threshold (PPT) values of both the patients and the normal subjects were obtained with an algometer. It was found that the PPT of the patients with pain was significantly lower and that the extra-hard food took more masseter muscle activity and more working side jaw movement in both the pain and the normal groups. During both hard and extra-hard food biting, a significantly longer duration of masseter muscle activity was found in pain patients while the total muscle activity was not significantly stronger. Strong correlation existed between SCM and masseter muscle activity during both hard and extra-hard food biting in the patient group, while such correlation was very weak in the normal group. In conclusion, painful masseter muscles required longer masseter and SCM muscle contraction time for breaking through a hard food of 20 kg and more, and co-activation of SCM and masseter muscles existed and was more evident when the food was harder or the pain was more severe.     

The effect of food bolus location on jaw movement smoothness and masticatory efficiency

Masticatory efficiency in individuals with extensive tooth loss has been widely discussed. However, little is known about jaw movement smoothness during chewing and the effect of differences in food bolus location on movement smoothness and masticatory efficiency. The aim of this study was to determine whether experimental differences in food bolus location (anterior versus posterior) had an effect on masticatory efficiency and jaw movement smoothness. Jaw movement smoothness was evaluated by measuring jerk-cost (calculated from acceleration) with an accelerometer that was attached to the skin of the mentum of 10 asymptomatic subjects, and acceleration was recorded during chewing on two-colour chewing gum, which was used to assessed masticatory efficiency. Chewing was performed under two conditions: posterior chewing (chewing on molars and premolars only) and anterior chewing (chewing on canine and first premolar teeth only). Jerk-cost and masticatory efficiency (calculated as the ratio of unmixed azure colour to the total area of gum, the unmixed fraction) were compared between anterior and posterior chewing with the Wilcoxon signed rank test (two-tailed). Subjects chewed significantly less efficiently during anterior chewing than during posterior chewing (P = 0·0051). There was no significant difference in jerk-cost between anterior and posterior conditions in the opening phase (P = 0·25), or closing phase (P = 0·42). This is the first characterisation of the effect of food bolus location on jaw movement smoothness at the same time as recording masticatory efficiency. The data suggest that anterior chewing decreases masticatory efficiency, but does not influence jerk-cost.     

Surface EMG of jaw-elevator muscles and chewing pattern in complete denture wearers

The aim of this study was to investigate the adaptation process of masticatory patterns to a new complete denture in edentulous subjects. For this purpose, muscle activity and kinematic parameters of the chewing pattern were simultaneously assessed in seven patients with complete maxillary and mandibular denture. The patients were analysed (i) with the old denture, (ii) with the new denture at the delivery, (iii) after 1 month and (iv) after 3 months from the delivery of the new denture. Surface electromyographic (EMG) signals were recorded from the masseter and temporalis anterior muscles of both sides and jaw movements were tracked measuring the motion of a tiny magnet attached at the lower inter-incisor point. The subjects were asked to chew a bolus on the right and left side. At the delivery of the new denture, peak EMG amplitude of the masseter of the side of the bolus was lower than with the old denture and the masseters of the two sides showed the same intensity of EMG activity, contrary to the case with the old denture. EMG amplitude and asymmetry of the two masseter activities returned as with the old denture in 3 months. The EMG activity in the temporalis anterior was larger with the old denture than in the other conditions. The chewing cycle width and lateral excursion decreased at the delivery of the new denture and recovered after 3 months.     

Measurement of dynamic bite force during mastication

Summary Efficient mastication of different types and size of food depends on fast integration of sensory information from mechanoreceptors and central control mechanisms of jaw movements and applied bite force. The neural basis underlying mastication has been studied for decades but little progress in understanding the dynamics of bite force has been made mainly due to technical limitations of bite force recorders. The aims of this study were to develop a new intraoral bite force recorder which would allow the study of natural mastication without an increase in the occlusal vertical dimension and subsequently to analyze the relation between electromyographic (EMG) activity of jaw‐closing muscles, jaw movements and bite force during mastication of five different types of food. Customized force recorders based on strain gauge sensors were fitted to the upper and lower molar teeth on the preferred chewing side in fourteen healthy and dentate subjects (21–39 years), and recordings were carried out during voluntary mastication of five different kinds of food. Intraoral force recordings were successively obtained from all subjects. anova showed that impulse of bite force as well as integrated EMG was significantly influenced by food (P < 0·05), while time‐related parameters were significantly affected by chewing cycles (P < 0·001). This study demonstrates that intraoral force recordings are feasible and can provide new information on the dynamics of human mastication with direct implications for oral rehabilitation. We also propose that the control of bite force during mastication is achieved by anticipatory adjustment and encoding of bolus characteristics.     

Effect of a brief episode of experimental muscle pain on jaw movement and jaw‐muscle activity during chewing

The aims of this study were to determine whether: (i) the jaw motor system develops a new pattern of jaw movement and/or jaw‐muscle activity after resolution of an acute episode of jaw‐muscle pain; and (ii) if jaw‐muscle activity and jaw‐movement features change progressively with repetition of a chewing sequence. Jaw movement and jaw muscle (masseter, anterior temporalis, and digastric) activity were recorded during free and rate‐standardized chewing in eight asymptomatic participants (pain infusion group), before and at three time blocks up to 45 min after a single 0.2‐ml bolus infusion of 5% hypertonic saline into the right masseter muscle. The same procedure, without infusion, was performed in another eight participants (control group). There were no significant main effects of group on jaw movement and muscle activity, suggesting that there were no persistent post‐pain effects on chewing. Across groups, repetitions of free and unstandardized chewing movements were associated with progressive increases in velocity and amplitude of jaw movement and masseter and temporalis electromyographic (EMG) activity. These findings suggest that factors unrelated to pain, such as practice effects, may be playing a role in the changes in jaw movement and jaw‐muscle activity observed after resolution of an acute episode of jaw‐muscle pain.     

EMG versus force relationship in painful masseter muscles before and after intramuscular anesthetics and saline injections

Abstract – Electromyographic (EMG) activity of masseter and anterior temporal muscles, and bite force were analyzed in 28 young, fully dentate women with painful and tender masseter muscles before and after intramuscular injection of either lidocaine or saline in superficial masseter muscle. EMG-activity of the descending part of the trapezius muscle during increased bite force was also analyzed. As regards the mandibular elevators, the intraindividual relationship between slopes for EMG-force regression at low and high contraction levels before injection were the same as for healthy women investigated previously. Intramuscular injection of lidocaine reduced postural EMG-activity of the masseter muscles and EMG-force regression curve became less steep at low contraction levels. After saline injections no changes in EMG-activity were found. The EMG-activity of the descending part of the trapezius muscle was increased significantly for the total group during the strongest bite.     

Motor behavior of the jaw muscles during different clenching levels

The goals of this study were to investigate whether (i) muscle activities are affected by different feedback strategies, (ii) the balancing behavior of the neuromuscular system is influenced by different force levels, and (iii) axial loading of the posterior teeth is a realistic biomechanical conception. In 10 healthy subjects, all jaw muscles were recorded bilaterally. Intra-oral force transfer and force measurement were achieved by using a measuring device simulating natural maximum intercuspation. Under visual feedback-control, the subjects generated pure vertical and directionally unrestricted force vectors with identical force magnitude at different force levels. The force transmission characteristics under experimental occlusion were investigated by calculating the reduction point (RP) of the resultant bite force. Directionally unrestricted clenching revealed a higher activation of the musculature than pure vertical clenching and was also characterized by a distinct anterior force component. Under both test conditions, the RP moved towards a posterior position with increasing clenching forces. The results indicate an essential recruitment difference of the jaw muscles between the two clenching conditions. Pure axial loading of teeth seems to be impeded by the anterior force component during bilateral clenching. The posterior movement of the RP might prevent overloading of the temporomandibular joints and anterior teeth.     

Anterior and posterior neck muscle activation during a variety of biting tasks

Bruxism may be involved in the aetiology of myofascial neck pain. The objective of this study was to test the hypothesis that anterior and posterior neck muscles co-contract during jaw clenching. Ten test subjects developed different feedback-controlled submaximum bite forces in a variety of bite-force directions by means of bite-force transducers. The electromyographic activity of the sternocleidomastoid and supra/infrahyoidal muscles, and of the semispinalis capitis, semispinalis cervicis, and multifidi muscles was recorded by use of surface electrodes and intramuscular wire electrodes, respectively. For normalization of electromyography data, maximum voluntary contraction tasks of the neck muscles were conducted in eight different loading directions. The results confirmed co-contraction of the neck muscles in the range of 2-14% of the maximum voluntary contraction at a bite force ranging from 50 to 300 N. Significant activity differences were observed as a result of the different force levels and force directions exerted by the jaw muscles. Long-lasting tonic activation of specific neck muscles triggered by the jaw-clenching tasks was also detected. These findings support the assumption of a relationship between jaw clenching and the activity of the neck muscles investigated. The low level of co-contraction activity, however, requires further study to elucidate possible pathophysiological interactions at the level of single motor units.     

Effects of masticatory muscle training on maximum bite force and muscular endurance

Abstract

Objective. This study evaluates the effects of chewing training on strength and endurance of the masticatory muscles. Materials and methods. Of the 49 healthy young adults included in the study, nine served as controls for a baseline measurement of bite force. The 40 participants who actively trained their masticatory muscles were randomly divided into a ‘continuous training group' (CTG) and an ‘intermittent training group' (ITG). The participants performed oral motor training by clenching silicon tubes (Chewy Tubes^?) according to a designed protocol. The muscular strength was studied in terms of maximum bite force. Muscular endurance was evaluated by measuring the duration for which the participants held 50% of their maximum bite force value. Results. Both the maximum bite force and the muscular endurance capacity increased after intensive training for both groups. After 2 months, the ITG stopped training for 1 month. At this point, a significant difference was identified both in the mean bite force values and the mean muscular endurance duration: the ITG exhibited lower values. For both groups, the highest values were attained after 3 months of training. The maximum bite force values and the muscular endurance duration were observed to follow similar patterns. The effects attained decreased rapidly in both groups when the training stopped. Conclusions. For both the continuous and intermittent training groups, 4 months of chewing exercises strengthened masticatory muscles, but such effects diminished gradually for both groups when the exercises stopped.