Jaw-opening Phase Research Articles

Tongue and jaw movement assessed by 3D motion capture during gum chewing.

The tongue plays an important role in mastication, swallowing, and articulation, but it cannot be directly observed because of its location inside the oral cavity. This study aimed to clarify detailed 3D tongue movements during chewing using electromagnetic articulography (EMA). The participants were 10 healthy, young volunteers (average age 26.8 ± 2.1years; 5 males, 5 females). Tongue and jaw movement during gum chewing was measured and recorded using EMA. Four EMA sensors were attached to the anterior, posterior, left, and right surfaces of the tongue, and one sensor was also attached to the mandibular left incisor. The tongue motion during the chewing cycle was spatially and sequentially analyzed based on the motion trajectories of the tongue and mandible. The tongue moved downward and to the masticatory side in a manner similar to the movement of the jaw. The anterior tongue marker moved downward to a greater extent than the other tongue markers. However, the tongue moved forward as the jaw moved backward. The anterior marker reached the most anterior position during the jaw-opening phase and the posterior markers reached the most anterior position during the jaw-closing phase. Just before maximum jaw-opening, all markers on the tongue reached the bottom lowest position. During the jaw-closing phase, the tongue reached the dominant farthest position in the masticatory side. All the markers reached the most posterior position during the occlusal phase. These findings demonstrate the sequence of tongue motion patterns during gum chewing.

Influence of Chewing Cycles on Flavor Recognition Time in Healthy Adults.

Flavor recognition times were measured in 23 young, healthy participants of both sexes using an electromyography-based system. The participants were instructed to chew a gummy candy, which was randomly selected among six commercially available types, and to press a button immediately on flavor recognition. A total of 107 normally distributed, flavor recognition times were analyzed, with an average time of 7.5 seconds (± 2.34 seconds, standard deviation). No significant differences were found among the six types of candies in terms of recognition time. Analysis of the association between flavor recognition and chewing phase showed that 70 (65%) of the analyzed 107 recognition signals occurred between 0.2 seconds before and 0.4 seconds after the end of jaw closing. Recognition signals occurring during the jaw-opening phase prolonged its duration by an average of 21%, whereas those occurring during the jaw-closing phase did not influence it.

Sagittal Plane Kinematics of the Jaw and Hyolingual Apparatus During Swallowing in Macaca mulatta.

Studies of mechanisms of feeding behavior are important in a society where aging- and disease-related feeding disorders are increasingly prevalent. It is important to evaluate the clinical relevance of animal models of the disease and the control. Our present study quantifies macaque hyolingual and jaw kinematics around swallowing cycles to determine the extent to which macaque swallowing resembles that of humans. One female and one male adult Macaca mulatta were trained to feed in a primate chair. Videofluoroscopy was used to record kinematics in a sagittal view during natural feeding on solid food, and the kinematics of the hyoid bone, thyroid cartilage, mandibular jaw, and anterior-, middle-, and posterior-tongue. Jaw gape cycles were defined by consecutive maximum gapes, and the kinematics of the swallow cycles were compared with those of the two consecutive non-swallow cycles preceding and succeeding the swallow cycles. Although there are size differences between macaques and humans, and macaques have shorter durations of jaw gape cycles and hyoid and thyroid upward movements, there are several important similarities between our macaque data and human data reported in the literature: (1) The durations of jaw gape cycles during swallow cycles are longer than those of non-swallow cycles as a result of an increased duration of the jaw-opening phase; (2) Hyoid and thyroid upward movement is linked with a posterior tongue movement and is faster during swallow than non-swallow cycles; (3) Tongue elevation propagates from anterior to posterior during swallow and non-swallow cycles. These findings suggest that macaques can be a useful experimental model for human swallowing studies.

Influence of Breathing and Chewing on Timing of Flavor Detection

This study aimed to analyze frequencies of flavor detection signals occurring in any of the three respiratory phases and either of the two masticatory phases. Flavors of three fruity (grape, orange, and strawberry) gummy candies in the mouth were detected after chewing in 24 young healthy participants. The results revealed that: 1) more detection signals occurred in the expiratory phase than in the inspiratory or pausing phases and 2) more detection signals occurred in the jaw-closing phase than in the jaw-opening phase. Statistical analysis showed that the difference between the frequencies among the three respiratory phases was significant (P < 0.001) but not between the two masticatory phases. Further analysis showed that the frequencies occurring within individual respiratory phases were not significantly biased. The present results suggest that flavor detection during chewing depends more on the expiratory phase rather than the inspiratory and pausing phases, whereas detection is dependent to a lesser extent on either of the masticatory phases.

Evaluation of jaw and neck muscle activities while chewing using EMG-EMG transfer function and EMG-EMG coherence function analyses in healthy subjects

This study aims to quantitatively clarify the physiological features in rhythmically coordinated jaw and neck muscle EMG activities while chewing gum using EMG-EMG transfer function and EMG-EMG coherence function analyses in 20 healthy subjects. The chewing side masseter muscle EMG signal was used as the reference signal, while the other jaw (non-chewing side masseter muscle, bilateral anterior temporal muscles, and bilateral anterior digastric muscles) and neck muscle (bilateral sternocleidomastoid muscles) EMG signals were used as the examined signals in EMG-EMG transfer function and EMG-EMG coherence function analyses. Chewing-related jaw and neck muscle activities were aggregated in the first peak of the power spectrum in rhythmic chewing. The gain in the peak frequency represented the power relationships between jaw and neck muscle activities during rhythmic chewing. The phase in the peak frequency represented the temporal relationships between the jaw and neck muscle activities, while the non-chewing side neck muscle presented a broad range of distributions across jaw closing and opening phases. Coherence in the peak frequency represented the synergistic features in bilateral jaw closing muscles and chewing side neck muscle activities. The coherence and phase in non-chewing side neck muscle activities exhibited a significant negative correlation. From above, the bilateral coordination between the jaw and neck muscle activities is estimated while chewing when the non-chewing side neck muscle is synchronously activated with the jaw closing muscles, while the unilateral coordination is estimated when the non-chewing side neck muscle is irregularly activated in the jaw opening phase. Thus, the occurrence of bilateral or unilateral coordinated features in the jaw and neck muscle activities may correspond to the phase characteristics in the non-chewing side neck muscle activities during rhythmical chewing. Considering these novel findings in healthy subjects, EMG-EMG transfer function and EMG-EMG coherence function analyses may also be useful to diagnose the pathologically in-coordinated features in jaw and neck muscle activities in temporomandibular disorders and whiplash-associated disorders during critical chewing performance.

Effects of short-term training on behavioral learning and skill acquisition during intraoral fine motor task

Sensory information from the orofacial mechanoreceptors are used by the nervous system to optimize the positioning of food, determine the force levels, and force vectors involved in biting of food morsels. Moreover, practice resulting from repetition could be a key to learning and acquiring a motor skill. Hence, the aim of the experiment was to test the hypothesis that repeated splitting of a food morsel during a short-term training with an oral fine motor task would result in increased performance and optimization of jaw movements, in terms of reduction in duration of various phases of the jaw movements. Thirty healthy volunteers were asked to intraorally manipulate and split a chocolate candy, into two equal halves. The participants performed three series (with 10 trials) of the task before and after a short-term (approximately 30min) training. The accuracy of the split and vertical jaw movement during the task were recorded. The precision of task performance improved significantly after training (22% mean deviation from ideal split after vs. 31% before; P<0.001). There was a significant decrease in the total duration of jaw movements during the task after the training (1.21 s total duration after vs. 1.56 s before; P<0.001). Further, when the jaw movements were divided into different phases, the jaw opening phase and contact phase were significantly shorter after training than before training (P=0.001, P=0.002). The results indicate that short-term training of an oral fine motor task induces behavior learning, skill acquisition and optimization of jaw movements in terms of better performance and reduction in the duration of jaw movements, during the task. The finding of the present study provides insights into how humans learn oral motor behaviors or the kind of adaptation that takes place after a successful prosthetic rehabilitation.

Neuronal activities of the vestibular nuclear complex during mechanically induced rhythmic jaw movements in rats

We studied the neuronal activities of the vestibular nuclear complex (VN) neurons during rhythmic jaw movements in rats anesthetized with urethane. Rhythmic jaw movements were induced by mechanical stimulation of the palate mucosa. The firing rate of approximately 25% of VN neurons increased significantly, and that of 10% of VN neurons decreased significantly, during these rhythmic jaw movements. There was no correlation between the change in the firing rate and the phase of the rhythmic jaw movements (jaw-opening and jaw-closing phases). The neurons that were affected were intermingled in the VN. These results suggest that the VN neurons are involved in controlling jaw movements.

Endurance to physical strain in patients with temporomandibular disorders: a case-control study

Objective. The hypothesis tested was whether patients with temporomandibular disorders (TMD) have an impaired endurance to motor tasks, under physical strain. Material and methods. The study population included 81 cases with TMD and 75 controls. After giving their informed consent, the participants filled out a questionnaire and performed five endurance trials. Each trial was measured in seconds from start to termination. The tests included: (a) holding the arm in a straight out position with a 2 kg weight in the hand, (b) lifting and lowering the arm with a 2 kg weight in the hand, (c) opening and closing the mandible repeatedly against a resistance of 1.6 kg during the jaw-opening phase, (d) protruding and retracting the mandible repeatedly against a resistance of 1.6 kg during the protruding phase and (e) unilateral chewing of three pieces of chewing gum. Analysis of differences between cases and controls for endurance were evaluated with Mann-Whitney U-test. A p-value < 0.05 was considered statistically significant. Results. The cases had statistically significantly (p < 0.0001) lower endurance time than controls for all tests. Conclusions. Patients with TMD compared to those without signs and symptoms of TMD have an impaired capacity to endure motor tasks that involve physical demand of the jaw muscles and shoulder girdle muscles.

Effects of neck muscle activities during rhythmic jaw movements by stimulation of the medial vestibular nucleus in rats

This study first examines whether there is rhythmic activity of the neck muscles during cortically induced rhythmic jaw movements in rats anesthetized by urethane. Rhythmic jaw movements were induced by repetitive electrical stimulation of the orofacial motor cortex. An electromyogram in the splenius muscles (spEMG) showed rhythmic bursts during the jaw-opening phase, or during the transition from the jaw-opening phase to the jaw-closing phase. In the sternomastoid (stEMG), however, the electromyogram did not show any bursts during rhythmic jaw movements. A further study then examines whether stimulation of the medial vestibular nucleus (MVN) modulates the rhythmic activity of the neck muscles. Stimuli applied in the jaw-closing phase induced a transient burst in the stEMG, and the duration of activity in the spEMG was increased. Stimuli applied in the jaw-opening phase induced a transient burst in the stEMG and an inhibitory period in the spEMG. These results imply that the MVN is involved in the modulation of neck muscle activities during rhythmic jaw movements induced by stimulating the orofacial motor cortex.

Sources of Variance in Temporal and Spatial Aspects of Jaw Kinematics in Two Species of Primates Feeding on Foods of Different Properties

Chewing kinematics reflects interactions between centrally generated motor signals and peripheral sensory feedback from the constantly changing oral environment. Chewing is a strongly modulated behavior that responds to differences in material properties among different type of foods and to changes in the external physical properties of the food as the bolus gets processed. Feeding, as any complex biological behavior, presents variation at multiple hierarchical levels, from among species or higher-order levels to variation among chewing cycles within a single feeding sequence. Thus, to understand the mechanics and evolution of feeding systems requires estimation of how this variation is distributed across each of these hierarchical levels, which in turn requires large sample sizes. The development of affordable, high-resolution, three-dimensional kinematic recording systems has increased our ability to collect large amounts of data on complete or near-complete feeding sequences that can be used to shed light on the mechanisms of control in vertebrate feeding. In this study, we present data on the nature and sources of variation (from species to chewing cycle levels) in kinematics of chewing in two species of primates, Cebus and Macaca, while they feed on foods of known material properties. Variation in chewing kinematics was not evenly distributed among hierarchical levels. Most of the variation was observed among chewing cycles, most likely in response to changes in the external properties of the food bolus throughout the feeding sequence. Species differences were found in duration and vertical displacement during slow-close phase suggesting that each species exhibits different power stroke dynamics. Cebus exhibited more variable gape cycles than did Macaca, in particular when eating low-toughness foods. This increased ability to temporally and spatially modulate the gape cycle may reflect increased efficiency in processing food because Cebus monkeys use fewer, but longer cycles, than does Macaca when feeding on low-toughness foods. This is due to an increase in duration of the jaw-opening phases of the gape cycle, when the tongue repositions the food bolus in the oral cavity.

Modulation of cortically induced rhythmic jaw movements in rats by stimulation of the vestibular nuclear complex

We study whether stimulation of the vestibular nuclear (VN) complex can modulate rhythmic jaw movements in rats anesthetized by urethane. Rhythmic jaw movements were induced by repetitive electrical stimulation of the orofacial motor cortex. Stimulation of the medial vestibular nucleus (MVN) during the jaw-closing phase increased the amplitude of the jaw-closing movement. (This is not a movement that continues to closure.) Stimulation of the MVN during the jaw-opening phase disturbed the rhythm of jaw movements and induced a small jaw-closing movement. Stimulation of the superior VN (SVN) and the lateral VN (LVN) during the jaw-closing phase did not affect the amplitude of the jaw-closing movement. Stimulation of the SVN and the LVN during the jaw-opening phase increased the amplitude of the jaw-opening movement, however. Stimulation of the inferior VN during the jaw-closing and the jaw-opening phase, respectively decreased the amplitude of the jaw-closing and the jaw-opening movements. Stimulation applied outside the VN did not modulate the amplitude of the jaw movements. These results imply that the VN is involved in the modulation of rhythmic jaw movements induced by stimulation of the orofacial motor cortex.

Measurement of jerk-cost using a triaxial piezoelectric accelerometer for the evaluation of jaw movement smoothness

Jerk-cost as a measure of jaw movement smoothness has been used to evaluate the masticatory function of patients with tooth loss and malocclusion. Jerk-cost measuring systems have thus far been unable to determine the jerk-cost of each chewing phase over time. The purposes of this study were (i) to develop a new method for measuring momentary jerk-cost of the jaw movement using a small triaxial piezoelectric accelerometer and (ii) to test the hypothesis that the lowest smoothness is seen during jaw-opening phase. The accelerometer was attached to the skin of the mentum of each subject. Vertical jaw displacement, acceleration, the jerk, and the time differential of jerk-cost during gum chewing were analysed as a function of time in five normal dentate subjects (n = 5). The system revealed intra-class correlation coefficients of intra-examiner, inter-examiner, and test-retest consistency of substantially high values (0.80-0.88). In all subjects, the highest jerk-cost was observed in the opening phase of each chewing cycle when the gum was parting from the teeth; the lowest jerk-cost was observed in the intercuspal phase throughout the chewing cycle, thus confirming the hypothesis. Significant differences were observed between the opening, closing, and intercuspal chewing phases (N = 5, P = 0.007). The smoothness measurement system used in this study evaluated the momentary smoothness of each chewing phase in the masticatory cycle. The system may serve as a diagnostic tool to evaluate the smoothness of the jaw movement in general dental practice.

Mastication-induced modulation of the jaw-opening reflex during different periods of mastication in awake rabbits

The present study aimed to determine if sensory inputs from the intraoral mechanoreceptors similarly contributed to regulating the activity of the jaw-opening muscles throughout the masticatory sequence. We also aimed to determine if sensory inputs from the chewing and non-chewing sides equally regulated the activity of the jaw-opening muscles. Electromyographic (EMG) activities of jaw muscles (digastric and masseter) and jaw movements were recorded in awake rabbits. The entire masticatory sequence was divided into preparatory, rhythmic-chewing and preswallow periods, based on jaw muscles activity and jaw movements. The jaw-opening reflex (JOR) was evoked by unilateral low-intensity stimulation of the inferior alveolar nerve (IAN) on either the chewing or non-chewing side. Amplitude of the JOR was assessed by measuring peak-to-peak EMG activity in the digastric muscle, and was compared among the masticatory periods and between the chewing and non-chewing sides. The JOR was strongly suppressed during the jaw-closing phase in the rhythmic-chewing and preswallow periods, but this effect was transiently attenuated during the late part of the jaw-opening phase in these periods. However, modulation of the JOR varied from strong suppression to weak facilitation during the preparatory period. The patterns of JOR modulation were similar on the chewing and non-chewing sides in all masticatory periods. The results suggest that the sensory inputs from the intraoral mechanoreceptors regulate the activity of the jaw-opening muscles differently during the preparatory period compared with the other masticatory periods. Sensory inputs from both the chewing and non-chewing sides similarly regulate the activity of the jaw-opening muscles.

Movement of the mandibular condyle and activity of the masseter and lateral pterygoid muscles during masticatory-like jaw movements induced by electrical stimulation of the cortical masticatory area of rabbits

We examined the functional role of the lateral pterygoid muscle (LP) and the masseter muscle (MS) in the movement of the mandibular condyle in masticatory-like jaw movements induced by electrical stimulation of the cortical masticatory area of urethane-anaesthetised rabbits. EMGs of the LP and MS were recorded along with video images of the mandibular condyle movement filmed with a high-speed CCD camera at a time resolution of 8 ms. The time required for the contractile force of the MS and LP to emerge as bite force or jaw movement was determined by direct electrical stimulation to respective muscles: 32.8 ± 1.5 ms for the MS and 34.3 ± 2.9 ms for the LP. The LP on the working side showed biphasic activity not only in the jaw-opening phase but also in the middle occlusal phase. It is assumed that the MS on the working side begins to exert actually the maximum mechanical influence on the bite force 32.8 ms after its EMG peak in the early occlusal phase. Such development of the mechanical effect of the MS during middle occlusal phase is appeared to be involved in stabilization of the condyle in the middle occlusal phase. Approximately 25 ms (25.1 ± 2.5 ms) after the peak of mechanical influence of the MS, the mechanical influence of the working-side LP activity reached maximum in the late stage of the occlusal phase. This LP mechanical influence seems to be functionally associated with an antero-inferior movement of the working side condyle in the late stage of the occlusal phase that is coincident with a movement of the incisal point towards the balancing side across the midline.

EMG study for perioral facial muscles function during mastication

This study aimed to clarify the temporal and quantitative modulation in the orbicularis oris (OO) and buccinator (BUC) muscle activities during mastication. Ten healthy males (26.9 +/- 1.0 years) participated. Electromyograms (EMGs) of the facial muscles were recorded with fine wire electrodes when chewing the chewing gum (one to four sticks) and peanuts (one to five pieces). Surface EMGs of the masseter (MAS) and digastric muscles were recorded simultaneously. EMGs of the OO and BUC showed rhythmic single-peaked bursts corresponding to the jaw-opening phase of chewing cycles. The total cycle lengths were constant regardless of the food amount. Integrated EMGs of the OO changed significantly when the amount of both foods changed (anova: P < 0.05). Those of the BUC changed significantly with the amount of gum changed (P < 0.05), but did not change with the amount of peanuts changed. The burst duration of OO changed significantly when the amount of gum changed during ipsilateral chewing (P < 0.05). When the amount of peanuts changed during ipsilateral chewing, the onset of OO and the peak of BUC based on the onset of MAS activity changed significantly (P < 0.05). However, the onset, peak and offset of the OO and BUC based on the offset of MAS did not change regardless of the amounts chewed. The changes of the OO and BUC activities may derive from chewing-generated sensory inputs in accordance with the physical property of food in part, which would relate to the function of these muscles during mastication.

Modulation of cortically induced rhythmical jaw movements by stimulation of the red nucleus in the rat

We study whether stimulation of the red nucleus (RN) can modulate rhythmical jaw movements in rats anesthetized by urethane. Rhythmical jaw movements were induced by repetitive electrical stimulation of the two cortical masticatory areas (area A: the orofacial motor cortex; area P: the insular cortex). Stimuli applied to the RN did influence rhythmical jaw movements induced by stimulation of the A-area. Stimuli applied in the jaw-closing phase increased the amplitude of the jaw-closing movement. Stimuli applied in the jaw-opening phase disturbed the rhythm of jaw movements and induced a small jaw-closing movement. Stimuli applied to the RN did not influence rhythmical jaw movements induced by stimulation of the P-area. These results indicate that the RN is involved in the modulation of rhythmical jaw movements induced by stimulation of the A-area.

Effects of food consistency on the pattern of extrinsic tongue muscle activities during mastication in freely moving rabbits

The effects of physical characteristics of foods on the coordination of extrinsic tongue muscle activities during natural mastication were evaluated. Electromyograms of tongue-retractor (styloglossus, SG) and tongue-protractor (genioglossus, GG) muscles as well as the jaw-movement trajectories were recorded during raw rice and chow pellet chewing in the freely moving rabbit. Each masticatory cycle included a jaw closing (Cl) phase consisting of a fast-closing (FC) and a slow-closing (SC) phase, and a jaw opening (Op) phase. The duration of the Cl and SC phases was found to be much larger while the duration of the FC phase was much smaller during rice chewing than pellet chewing. The jaw movements during rice chewing had smaller amplitudes of the gape and lateral excursion of the jaw as compared with those during pellet chewing. The SG muscle had a double-peaked burst activity in each masticatory cycle with one peak during the Op phase (the SG1 burst) and the other during the Cl phase (the SG2 burst). They were significantly larger during pellet chewing as compared with rice chewing, but the duration of the SG2 burst was significantly longer during rice chewing than pellet chewing. The offset of the SG2 burst was delayed during rice chewing as compared with that during pellet chewing. There was little difference in the activity pattern of the GG burst between the foods. Our present results suggest that the SG muscle activity could be modified by the sensory feedback possibly to adapt to environmental demands during chewing.

Coordination of jaw and extrinsic tongue muscle activity during rhythmic jaw movements in anesthetized rabbits

To clarify the jaw-closer and tongue-retractor muscle activity patterns during mastication, electromyographic activity of the styloglossus (SG) as a tongue-retractor and masseter (Mass) as a jaw-closer muscles as well as jaw-movement trajectories were recorded during cortically evoked rhythmic jaw movements (CRJMs) in anesthetized rabbits. The SG and Mass muscles were mainly active during the jaw-closing (Cl) phase. The SG activity was composed of two bursts in one masticatory cycle; one had its peak during the jaw-opening (Op) phase (SG1 burst) and the other during the Cl phase (SG2 burst). The Mass activity during the Cl phase was dominant on the working side (opposite to the stimulating side) while the SG1 and SG2 bursts were not different between the sides. When the wooden stick was inserted between the molar teeth on the working side during CRJMs, the facilitatory effects on the SG1 and SG2 bursts on both sides were noted as well as those on the Mass bursts, but the effects on the SG1 burst seemed to be weak as compared with those on the Mass and SG2 bursts. The difference in the burst timing between the sides was noted only in the SG1 burst. When the trigeminal nerves were blocked, the peak and area of the SG and Mass burst decreased during CRJMs, and the facilitatory effects of the wooden stick application on the muscles were not noted. The results suggest that the jaw and tongue muscle activities may be adjusted to chew the food and make the food bolus.

Neuronal activity patterns in primate primary motor cortex related to trained or semiautomatic jaw and tongue movements.

The present study was undertaken to determine the firing patterns and the mechanoreceptive field (RF) properties of neurons within the face primary motor cortex (face-MI) in relation to chewing and other orofacial movements in the awake monkey. Of a total of 107 face-MI neurons recorded, 73 of 74 tested had activity related to chewing and 47 of 66 neurons tested showed activity related to a trained tongue task. Of the 73 chewing-related neurons, 52 (71.2%) showed clear rhythmic activity during rhythmic chewing. A total of 32 (43.8%) also showed significant alterations in activity in relation to the swallowing of a solid food (apple) bolus. Many of the chewing-related neurons (81.8% of 55 tested) had an orofacial RF, which for most was on the tongue dorsum. Tongue protrusion was evoked by intracortical microstimulation (ICMS) at most (63.6%) of the recording sites where neurons fired during the rhythmic jaw-opening phase, whereas tongue retraction was evoked by ICMS at most (66.7%) sites at which the neurons firing during the rhythmic jaw-closing phase were recorded. Of the 47 task-related neurons, 21 of 22 (95.5%) examined also showed chewing-related activity and 29 (61.7%) demonstrated significant alteration in activity in relation to the swallowing of a juice reward. There were no significant differences in the peak firing frequency among neuronal activities related to chewing, swallowing, or the task. These findings provide further evidence that face-MI may play an important role not only in trained orofacial movements but also in chewing as well as swallowing, including the control of tongue and jaw movements that occur during the masticatory sequence.

Coordination between the masticatory and tongue muscles as seen with different foods in consistency and in reflex activities during natural chewing

To study the coordination between the masticatory and extrinsic tongue muscles during natural chewing, electromyographic activities in the digastric (Dig) as a jaw opener, the masseter (Mas) as a jaw closer, the genioglossus (Gg) as a tongue protruder, and the styloglossus (Sg) as a tongue retractor as well as jaw movement trajectories were recorded while rabbits chewed soft, hard, and very hard foods. The Dig and Gg were active in the jaw-opening phase (OP active group), and the Mas and Sg were active in the jaw-closing phase (CL active group). Food consistency affected differently on the duration of burst activities between the muscle groups, i.e. in the CL active group, the duration was longer for the harder food, while there was no difference in the duration of the OP active group among the foods. During hard food chewing in particular, we confirmed our recent findings that reflexly-induced short but large bursts of activity could be documented in the Dig during the jaw-closing phase. Similar short bursts were also documented in the Gg as with the Dig in this study. Inhibitory periods were often observed in the Mas with the Dig short burst and were also observed in the Sg along with the Gg short burst; however the inhibitory effect in the Sg was less pronounced. These findings suggest that: (1) both masticatory and extrinsic tongue muscles are active in a well-coordinated manner during stable chewing, but that (2) reflex effects on antagonistic muscles (i.e. Dig vs. Mas in the masticatory muscles, Gg vs. Sg in the tongue muscles) evoked by tooth contact during chewing may not be analogous between the two muscle groups.