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The conditions of stress contagion are induced in bystanders without direct experiences of stressful events. This study determined the effects of stress contagion on masseter muscle nociception in mice. Stress contagion was developed in the bystanders after cohabitating with a conspecific mouse subjected to social defeat stress for 10 days. On Day 11, stress contagion increased anxiety‐ and orofacial inflammatory pain‐like behaviors. The c‐Fos and FosB immunoreactivities evoked by masseter muscle stimulation were increased in the upper cervical spinal cord, while c‐Fos expressions were increased in the rostral ventromedial medulla, including the lateral paragigantocellular reticular nucleus and nucleus raphe magnus in stress contagion mice. The level of serotonin in the rostral ventromedial medulla was increased under stress contagion, while the number of serotonin positive cells was increased in the lateral paragigantocellular reticular nucleus. Stress contagion increased c‐Fos and FosB expressions in the anterior cingulate cortex and insular cortex, both of which were positively correlated with orofacial inflammatory pain‐like behaviors. The level of brain‐derived neurotrophic factor was increased in the insular cortex under stress contagion. These results indicate that stress contagion can cause neural changes in the brain, resulting in increased masseter muscle nociception, as seen in social defeat stress mice. [ABSTRACT FROM AUTHOR]

We investigated the effects of social defeat stress (SDS) and treadmill running on masseter muscle nociception, which was quantified by the orofacial formalin test and c‐Fos and FosB immunoreactivities in the upper cervical spinal cord (C1/C2) region in male mice. After daily SDS or non‐SDS conditioning for 10 days, SDS‐conditioned mice were categorized into SDS‐susceptible versus resilient mice. Several mice, including non‐SDS‐conditioned, SDS‐susceptible, and resilient mice, were selected to assess masseter muscle nociception on Day 11. SDS conditioning for 10 days increased masseter muscle‐evoked nocifensive behaviors and c‐Fos and FosB expression in SDS‐susceptible compared to non‐SDS and resilient mice. The remaining SDS‐susceptible and non‐SDS mice were subjected to an additional 10 days of SDS plus treadmill running or sedentary sessions before assessing masseter muscle nociception on Day 21. Daily treadmill running sessions reduced enhanced masseter muscle nociception in SDS‐susceptible mice but not in non‐SDS mice. The preventive effects of daily treadmill running immediately after each SDS conditioning for 10 days on orofacial nocifensive behaviors were assessed on Day 11. Treadmill running conducted immediately after daily SDS inhibited enhanced orofacial nocifensive behaviors. These findings indicate that repeated SDS increases masseter muscle nociception, which could be prevented by daily treadmill running exercise. [ABSTRACT FROM AUTHOR]

Occlusal support may influence muscular function during complex motor tasks. This study evaluated the duration and sequence of muscular activation of masticatory (temporal, masseter), postural head/neck (sternocleidomastoid, trapezius), postural trunk (rectus abdominis, paravertebrals), and low extremity strength (rectus femoris, gastrocnemius) muscles during simulation of activities of daily living (ADL) in edentulous women wearing complete dentures (n = 10) and in dentate women (n = 10). Electromyographic activity was recorded during tests of stand‐up/sit down in the Chair, sit up/lie down in the Bed and lift/lower Bags. Occlusal support (dentures) had a significant effect on duration of muscular activation in the Chair Test: the masseter muscle activated longer with dentures during the standing movement. The masseter and sternocleidomastoid muscles showed significant alteration in their order of activation in non‐denture‐wearing women. For the Bed Test, dentures had significant effect for the gastrocnemius during the sitting‐up phase and for the rectus abdominis during the lying‐down movement. For the Bag Test, head/neck muscles were activated in a different order as a function of occlusal support. Anticipation of activation of the paravertebral muscles, rectus abdominis, and gastrocnemius was observed in dentate women compared with denture wearers. These findings suggest that occlusal support influences electromyographic activity of some muscles during simulation of ADL. [ABSTRACT FROM AUTHOR]

Fast eating has been shown to increase the risk of overweight in both children and adults. The objectives of the present study were to investigate the correlation between chewing rate and the number of chews per mouthful and to evaluate if they were associated with the weight of meal intake. Thirty healthy subjects, aged 18–24 yr, ate a test lunch at their habitual speed until they felt satiated. The activities of masseter and suprahyoid muscles were recorded to determine the number of chews and the moment of swallowing. The weight of meal intake was recorded along with body mass index (BMI), chewing rate, number of chews per mouthful, meal duration, ingestion rate, hunger, and food preference levels. The mean weight (±SD) of meal intake, chewing rate, and number of chews per mouthful were 261.4 ± 78.9 g, 94.4 ± 13.5 chews min−1, 19.2 ± 6.4 chews per mouthful, respectively. Chewing rate was not correlated with the number of chews per mouthful. The multivariable linear regression showed that meal intake was significantly positively associated with chewing rate, meal duration, and BMI, but inversely associated with the number of chews per mouthful (adjusted R2 = 0.42). It was concluded that the number of chews was not associated with chewing rate but meal intake was explained by both reduced number of chews and increased chewing rate. [ABSTRACT FROM AUTHOR]

The aims were to test the hypotheses that experimental masseter muscle pain leads to recruitment and/or derecruitment of motor units at different sites within the masseter and that the patterns of change in motor unit activity differ between sites. Single motor unit (SMU) activity was recorded at two sites within the right masseter [superior/anterior, inferior/posterior (IP)] during isometric biting tasks (ramp, step level) on an intraoral force transducer in 17 participants during three experimental blocks comprising no infusion (baseline), 5% hypertonic saline infusion (pain), or isotonic saline infusion (control). A visual analog scale (VAS) was used to score pain intensity. The VAS scores were statistically significantly greater during infusion of hypertonic saline than during infusion of isotonic saline. No significant differences in force levels and rates of force change were found between experimental blocks. In comparison with isotonic saline infusion, SMUs could be recruited and derecruited at both sites during hypertonic saline infusion. The frequency of recruitment or derecruitment, in comparison with no change, was statistically significantly greater at the IP site than at the superior/anterior site. Experimental noxious masseter stimulation results in a reorganization of motor unit activity throughout the muscle, and the pattern of reorganization may be different in different regions of the muscle. [ABSTRACT FROM AUTHOR]

This study aimed to investigate the effect of glutamate-evoked masseter muscle pain on intramuscular oxygenation during rest and sustained elevated muscle activity ( SEMA). Seventeen healthy individuals participated in two sessions in which they were injected with glutamate and saline in random order. Each session was divided into three, 10-min periods. During the first (period 1) and the last (period 3) 10-min periods, participants performed five intercalated 1-min bouts of masseter SEMA with 1-min periods of 'rest'. At onset of the second 10-min period, glutamate (0.5 ml, 1 M; Ajinomoto, Tokyo, Japan) or isotonic saline (0.5 ml; 0.9%) was injected into the masseter muscle and the participants kept the muscle relaxed in a resting position for 10 min (period 2). The hemodynamic characteristics of the masseter muscle were recorded simultaneously during the experiment by a laser blood-oxygenation monitor. The results demonstrated that glutamate injections caused significant levels of self-reported pain in the masseter muscle; however, this nociceptive input did not have robust effects on intramuscular oxygenation during rest or SEMA tasks. Interestingly, these findings suggest an uncoupling between acute nociceptive activity and hemodynamic parameters in both resting and low-level active jaw muscles. Further studies are needed to explore the pathophysiological significance of blood-flow changes for persistent jaw-muscle pain conditions. [ABSTRACT FROM AUTHOR]

Neurons in the trigeminal mesencephalic nucleus (Vme) have an axon that branches peripherally to innervate the orofacial region and projects centrally to the trigeminal motor nucleus (Vmo). They function as the primary neurons conveying proprioceptive messages. The present study aimed to demonstrate the presence of a periodontal-Vme-Vmo circuit and to provide evidence for its involvement in an experimental unilateral anterior crossbite ( UAC) model, which can induce osteoarthritis in the temporomandibular joint. Cholera toxin B subunit ( CTb) was injected into the inferior alveolar nerve of rats to help identify the central axon terminals of Vme neurons in the Vmo. The levels of vesicular glutamate transporter 1 ( VGLUT1) expressed in the periodontal region, Vme, Vmo, and masseter, and the level of acetylcholinesterase ( AChE) expressed in the masseter, were assessed in UAC rats and controls. In CTb-treated rats, many CTb-labeled cell bodies and endings were identified in the Vme and in the Vmo, respectively. In UAC rats, VGLUT1 was expressed at a statistically significantly higher level in the periodontal ligament, Vme, Vmo, and masseter than it was in control rats. The level of AChE protein was 1.97 times higher in UAC rat masseter compared with control rat masseter. These findings reveal a trigeminal mechanism underlying masseter hyperactivity induced by an altered occlusion. [ABSTRACT FROM AUTHOR]

Normal jaw opening-closing involves simultaneous jaw and head-neck movements. We previously showed that, in men, integrated jaw-neck movements during jaw function are altered by induced masseter muscle pain. The aim of this study was to investigate possible sex-related differences in integrated jaw-neck movements following experimental masseter muscle pain. We evaluated head-neck and jaw movements in 22 healthy women and 16 healthy men in a jaw opening-closing task. The participants performed one control trial and one trial with masseter muscle pain induced by injection of hypertonic saline. Jaw and head movements were registered using a three-dimensional optoelectronic recording system. There were no significant sex-related differences in jaw and head movement amplitudes. Head movement amplitudes were significantly greater in the pain trials for both men and women. The proportional involvement of the neck motor system during jaw movements increased in pain trials for 13 of 16 men and for 18 of 22 women. Thus, acute pain may alter integrated jaw-neck movements, although, given the similarities between men and women, this interaction between acute pain and motor behaviour does not explain sex differences in musculoskeletal pain in the jaw and neck regions. [ABSTRACT FROM AUTHOR]

The aim of this study was to investigate the association between metabolic syndrome (MetS) and the metabolic indicators of masticatory muscles in an animal model. A total of 16 male Wistar rats were used. To induce MetS, 10 rats were fed with standard rat chow and 32% sucrose solution ad libitum for 16 wk. Six rats fed with standard rat chow and water ad libitum formed the control group. All rats were killed at week 16, and the right superficial masseter muscles were harvested. Metabolic indicators of masticatory muscle metabolism, including antioxidant enzyme activities, ion transport ATPase activities, and the levels of macro and trace elements, were determined in the muscles. Superoxide dismutase, catalase, glutathione peroxidase, and glutathione reductase activities were significantly decreased by 32%, 26%, 33%, and 16%, respectively, in the MetS group. Na+/K+- ATPase activity was significantly decreased in the MetS group by 54% compared with the control group. The levels of chromium and selenium were significantly decreased, and the level of copper was increased, in the MetS group compared with the control group. These results show that significant alterations occurred in antioxidant defense mechanisms, ion transport mechanisms, and trace element levels of masseter muscles in MetS. [ABSTRACT FROM AUTHOR]

A decrease in arterial carbon dioxide tension induces an increase in masseter muscle blood flow and a decrease in mandibular bone marrow blood flow during general anesthesia. In addition, dexmedetomidine infusion reduces oral tissue blood flow. In this study we investigated how end-tidal carbon dioxide tension (ET-CO2) changes influence on oral tissue blood flow during continuous dexmedetomidine infusion in rabbits. Eleven male Japan White rabbits were anesthetized with sevoflurane. Then, ET- CO2 was set at 30 mmHg and adjusted to 40 and 60 mmHg, and heart rate, systolic blood pressure, diastolic blood pressure, mean arterial pressure, common carotid artery blood flow, mandibular bone marrow blood flow, masseter muscle blood flow, and blood flow in other oral tissues were measured. Following this, the ET- CO2 was returned to 30 mmHg and dexmedetomidine was infused over 60 min. The measurements were repeated. Most parameters increased, regardless of whether or not dexmedetomidine was present, and heart rate and masseter muscle blood flow decreased in an ET- CO2-dependent manner. Dexmedetomidine infusion suppressed ET-CO2-dependent masseter muscle blood flow change. Masseter muscle blood flow during ET- CO2 at 30 mmHg with dexmedetomidine was the same as that during ET- CO2 at 40 mmHg without dexmedetomidine. Our findings suggest that dexmedetomidine infusion and slight hypocapnia under general anesthesia suppress an increase in masseter muscle blood flow as well as reducing mandibular bone marrow blood flow. These results may be of significance for decreasing bleeding during oral and maxillofacial surgery. [ABSTRACT FROM AUTHOR]

The aim of this study was to characterize the kinematics and masseter muscle activation in unilateral posterior crossbite. Eighty-two children (8.6 ± 1.3 yr of age) with unilateral posterior crossbite and 12 children (8.9 ± 0.6 yr of age) with normal occlusion were selected for the study. Electromyography (EMG) and kinematics were concurrently recorded during mastication of a soft bolus and a hard bolus. The percentage of reverse cycles in the group of patients was 59.0 ± 33.1% (soft bolus) and 69.7 ± 29.7% (hard bolus) when chewing on the crossbite side. When chewing on the non-affected side, the number of reverse cycles was 16.7 ± 24.5% (soft bolus) and 16.7 ± 22.3% (hard bolus). The reverse cycles on the crossbite side were narrower with respect to the cycles on the non-affected side. Although both types of cycles in patients resulted in lower EMG activity of the masseter of the crossbite side than of the contralateral masseter, the activity of the non-affected side was larger for reverse than for non-reverse cycles. It was concluded that when chewing on the crossbite side, the masseter activity is reduced on the mastication side (crossbite) and is unaltered (non-reverse cycles) or increased (reverse) on the non-affected side. [ABSTRACT FROM AUTHOR]

In humans, inhibitory jaw reflexes can be depressed by painful stimulation of remote parts of the body. The underlying mechanisms may involve diffuse noxious inhibitory controls (DNIC). Animal experiments have shown that the neurons which may mediate DNIC show spatial encoding (i.e. their responses vary in relation to the size of the body area being stimulated). The aim of this study was to investigate whether the modulation of an inhibitory jaw reflex shows similar spatial dependency. Electromyographic recordings were made in 9 subjects, from a masseter muscle that was activated to a level equivalent to 10% of that obtained during a maximum voluntary contraction. Reflex inhibitions were evoked by electrical stimuli to the upper lip, either alone (controls) or during the application of conditioning stimuli (47°C water) to the fingers, the hand, the half forearm or the whole forearm. Conditioning stimuli applied to the larger but not to the smaller areas resulted in significant modulations of the reflex. There was a significant correlation between stimulus area and reflex magnitude. These results demonstrate a spatial dependency for the modulation of an inhibitory jaw reflex by painful stimuli – a further parallel with DNIC as studied on single neurons in animals. [ABSTRACT FROM AUTHOR]

The jaw muscle activity of rats has been investigated for specific tasks. However, the daily jaw muscle use remains unclear. The purpose of the present study was to examine daily jaw muscle activity, and its variability over time, in the rat ( n = 12) by the use of radio-telemetry. A telemetric device was implanted for the continuous recording of masseter muscle and digastric muscle activity. Daily muscle use was characterized by calculating the total time that each muscle was active (duty time), the number of bursts, and the average length of bursts. All parameters were estimated for activities exceeding various levels (5–90%) of the day's peak activity. Daily muscle use remained constant for 4 wk. At the low-activity level, the duty time and burst number of the digastric muscle were significantly ( P < 0.01) higher than those of the masseter muscle, whereas the opposite was true at the high-activity level ( P < 0.05). No significant intermuscular correlation was observed between the number of bursts of the masseter and digastric muscles, but the interindividual variation of both muscles changed, depending on the level of activation. These findings suggest that the masseter muscle and the digastric muscle show a differential active pattern, depending on the activity level. [ABSTRACT FROM AUTHOR]

It has been suggested that occlusal interferences may lead to pain and tenderness of the masticatory muscles. Tender jaw muscles are more sensitive to pressure pain, as assessed by means of pressure algometry. We tested the effects of occlusal interferences on the pressure pain threshold of the jaw muscles by means of a double-blind randomized crossover experiment carried out on 11 young healthy females. Golden strips were glued either to an occlusal contact area (active interference) or to the vestibular surface of the same tooth (dummy interference) and left for 8 d each. Pressure pain thresholds of the masseter and anterior temporalis muscles were assessed under interference-free, dummy-interference and active-interference conditions. The results indicated that the application of an active occlusal interference, as used in this study, did not influence significantly the pressure pain thresholds of these muscles in healthy individuals. [ABSTRACT FROM AUTHOR]

In 10 normal subjects, the electromyographic (EMG) activities of the masseter, anterior and posterior temporalis, medial pterygoid, inferior lateral pterygoid, as well as the anterior digastric, were bilaterally recorded during clenching with various resulting force vectors. The intraoral force transfer was achieved with a three-component force transducer. The direction and magnitude of the force vectors were controlled by visual feedback. The goal of the study was to investigate how various clenching directions at constant magnitude of force influence the EMG activity in the masticatory muscles during gradual increase of the horizontal force component. Depending on the force direction and the individual muscle, an increase or decrease of activity was observed during clenching with increasing horizontal force components at constant magnitude of force. The inferior lateral pterygoid exhibited the highest activation (about 80% of maximal voluntary clenching) of all masticatory muscles. The medial pterygoid showed the greatest range of variation in activation behavior, and it was the most active muscle in relation to all clenching directions. In conclusion, the results show that with growing horizontal force components at constant magnitude of force, all muscles demonstrated an increase or decrease of activity in several clenching directions. [ABSTRACT FROM AUTHOR]

Little is known about the habitual daily masticatory activity of subjects with different vertical craniofacial morphology. The purpose of the present study was to compare the daily long-term muscle activity of short-face subjects with that of long-face subjects as assessed in their natural environment. Digital photographs of the facial profile were obtained from a sample of 300 subjects and the ratio between anterior total and anterior lower facial height was assessed (Vertical Facial Index: VFI). Fourteen long-face and 16 short-face subjects were selected from the opposite tails of the frequency distribution of VFI. Long-term masseter activity was monitored for 8 h d−1 in the natural environment by means of portable one-channel electromyograph recorders over three working days. Assessments included calculation of the number of activity periods (APs) per h ( N/h), their mean amplitude (A mean), and their mean duration (Dur). The maximal electromyogram activity did not differ significantly between the short-face and the long-face subjects. There was no significant difference between the two groups investigated in N/h, in A mean and Dur. The findings suggest that habitual activity of masseter muscle in the natural environment was not influenced by the vertical craniofacial morphology as assessed in the present study. [ABSTRACT FROM AUTHOR]

The aim of the study was to compare neuromuscular features of the masseter muscle in subjects with different vertical craniofacial morphology. Fifteen short-faced (mandibular plane-Frankfurt plane angle < 15°) and 15 normal- to long-faced (mandibular plane–Frankfurt plane angle ≥ 23°) male students participated. The thickness of the masseter was assessed by ultrasonography. Onset and endurance of exercise pain were recorded during sustained biting at a level of 15% of maximum voluntary contraction and 30 µV electromyographic activity. Pain and fatigue was measured on visual analog scales before and after the biting, as well as before and after 10 min chewing. Statistical comparison showed that the masseter muscle was significantly thicker (+15%) in the short-faced than the normal- to long-faced subjects. The pain onset time and endurance time were also consistently shorter in short-faced subjects, whereas the intensity of pain and fatigue did not differ significantly between the two groups. Multiple stepwise regression showed positive influence from the mandibular plane inclination and the masseter thickness on the pain onset time and endurance time. The present findings support the concept that subjects with different craniofacial morphology show neuromuscular differences. [ABSTRACT FROM AUTHOR]

The plasticity of the masseter muscle was studied by comparing two groups of rabbits that were fed soft- and hard-diet for 87 d. Incisors of the soft-diet group were cut back to minimize the bite forces. Muscle fibres were immunohistochemically defined as fast- or slow-contracting fibres and their cross-sectional area was measured. The muscles of animals fed with the hard-diet were composed of fibres with larger cross-sectional areas than the soft-diet group. The relative difference was larger in slow-contracting fibres than in fast-contracting fibres. The results were similar for the different regions of the muscle. No changes in fibre composition were found. In conclusion, the difference in food consistency, as induced in this study, caused changes in the muscle fibre cross-sectional area that can be recognized from the altered necessary occlusal forces, which result from the modified forces developed by the masseter muscle. [ABSTRACT FROM AUTHOR]

The aim of this randomized, controlled, double‐blind study was to examine how the activation pattern of the masseter muscle changes during natural function when experimental pain is induced in a discrete anterior area of the muscle. In 20 subjects, three bipolar surface electrodes and three intramuscular fine‐wire electrodes (antero‐posterior mapping) were simultaneously attached above and in the right masseter muscle to record the electromyographic (EMG) activity during unilateral chewing before and after infusion of a 0.9% isotonic and 5% hypertonic saline bolus in the anterior area of the muscle. The activity of the contralateral masseter muscle was registered by surface electrodes. In addition, the development of pain intensity was quantitatively measured with a numerical rating scale (NRS). While both saline concentrations caused pain, the hypertonic solution evoked stronger pain. The experiments also provided evidence of a significant although differential activity reduction of the ipsilateral masseter muscle in the antero‐posterior direction. The activity reduction decreased with increasing distance from the location of the infusion. The results support the idea that the strategy of differential activation protects the injured muscle while simultaneously maintaining optimal function. [ABSTRACT FROM AUTHOR]

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. 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 massetcr muscles decreased with an increase of bite force magnitude, whereas thc MPF values of thc anterior and posterior temporalis did not change significantly. Thc MPF values were significantly influenced by the direction of bite force. Thc observed changes of MPF arc possibly related to the recruitment of different fiber types, and support the concept that thc masticatory muscles behave heterogeneously. [ABSTRACT FROM AUTHOR]

The aim of this study was to analyze the effects of prolonged muscular elongation induced by bite-raising on the length of the muscle belly, sarcomeres and aponeurosis of the anterior deep masseter in the growing rat. Another aim was to determine the role of different functional conditions of this muscle in the adaptation process. Ninety-six young male rats were split into two groups: one was fed a hard diet and the other a soft diet to develop different functional capacities in the masticatory muscles. After 2 wk, half of the rats in both groups were fitted with an appliance that raised the bite by 2 mm. The measurements on the muscles were performed in situ. The insertion of the appliance stretched the anterior masseter muscle. After 4 wk, the vertical dentoskeletal dimension, the muscle belly, and the sarcomeres showed no difference in length among the groups. However, the aponeurosis was longer in the rats wearing the appliance compared to the controls, and among the bite block groups it was longer in the rats fed a hard diet. Length adaptation occurred in the aponeurosis. Clinically this may imply a need for reactivation of functional appliances to increase their efficiency, at a rate possibly depending on masseter muscles functional condition. [ABSTRACT FROM AUTHOR]

The metabolism of the human masseter muscle was investigated using phosphorus ([sup31]P) magnetic resonance spectroscopy (MRS) during long periods of exercise and recovery. Eleven subjects aged 19 to 28 yr were examined by [sup31]P MRS during four consecutive periods of 13 min each: rest, exercise, recovery 1 and 2. For each subject, a biting force equal to 20% of maximum voluntary biting force was applied and controlled during the exercise period to produce maximum fatigue. [sup31]P MR spectra were localized from a 24 cm³ volume of interest using an image selected in vivo spectroscopy (ISIS) sequence and a 6 cm diameter surface coil placed on the left masseter. Compared to the resting level, the phosphocreatine(PCr) content decreased by 26% during exercise, while the inorganic phosphate(Pi)concentration increased by 65%. During the two recovery periods, the Pi content remained decreased compared with the resting level by 36% and 30% respectively. The Pi/PCr ratio was increased from 0.30±0.04 during exercise at rest to 0.63±0.13 during exercise while the adenosine triphosphate(ATP)/Pi ratio was decreased. The pH decrease from 7.02±0.03 to 6.93±0.04 during exercise and returned to control level (7.09±0.08) only during the second recovery period. These results suggest that the masseter muscle is characterized by high ATP turnover and, therefore, high oxidative phosphorylative activity in agreement with its constitution of predominantly fatigue resistant type 1 fibers. [ABSTRACT FROM AUTHOR]

Pressure pain thresholds (PPTs) of the bilateral masseter and temporalis muscles were assessed over a single day and in between 2 days with the aid of an electronic algometer in 11 males and 11 females. Using a mixed model approach, covariance results were controlled for confounding of gender, day, time of the day and the within-session replications of the measurements. For every jaw muscle, the first PPT measurement of a session was markedly higher than the last one of that same session. Except for the right temporalis muscles, no significant systematic PPT differences could be observed between male and female subjects. The differences between morning and afternoon sessions were not significant for any of the muscles tested. Although the PPTs measured on the 1st day were systematically lower than those on the 2nd day for the left masseter and temporalis muscles, no significant difference between the time of the day could be observed when the values of the different muscles were grouped within a subject. Analysis of variance showed that the variability induced by the day or the tine of the day was similar to the variability of the measurement itself. The inter-individual variability of PPTs was 2.3 to 9 × higher than the variability observed between trials, sessions or days. The results indicate that the PPT measurement will not be systematically influenced by the time of registration (morning/afternoon) or inbetween consecutive days. In addition, the important inter-individual differences favor the use of longitudinal within-subject studies. [ABSTRACT FROM AUTHOR]

Electromyographic (EMG) recording of masticatory muscle activity was performed in 9 adult men with unilateral condylar fracture immediately after conservative treatment of the condylar fracture (T0) and 4 (T1) and 8 (T2) months after the trauma. From T0 to T2, maximal voluntary contraction of the anterior and posterior temporal muscles and of the masseter muscle opposite the fracture side (contralateral) increased significantly. Activity in the anterior temporal muscles also rose significantly during natural chewing, whereas the masseter muscles remained at the same level, but activity consistently tended to be strongest contralaterally to the condylar fracture. In addition, there was a tendency from T1 to T2 for natural chewing to take place predominantly on the contralateral side. The most obvious overall changes during the follow-up were shorter and stronger contractions in all muscles during chewing, on the side of the impaired joint. The increase of activity in the anterior temporal muscles during maximal bite and natural chewing, and the occurrence of stronger and shorter contractions during ipsilateral chewing, were interpreted as signs of improved function due to healing and relief of pain from the impaired joint. On the other hand, there was a specific rise of maximal activity only in the contralateral masseter, and during natural chewing, activity was constantly stronger in the same muscle in combination with the tendency of increasing predominance of contralateral strokes. These traits specificly related to the masseter muscles were considered an indication of a permanent functional distortion due to reflex suppression on the fracture side as an after-effect of the injury. [ABSTRACT FROM AUTHOR]

The present report aimed at evaluating the within- and between-subject electro-myographic coordination between the masseter (M) and temporalis anterior (T) muscles during the performance of a standardized chewing task. Electromyo-graphic activity of M and T muscles was recorded in 60 young healthy adults (30 men, 30 women) during two 15-s unilateral mastications of gum. Left-right differential potentials (δM=MR-ML. δT=TR-TL) were computed and the √(δM2+δT2) moduli were calculated. The maximum modulus relative to each masticatory cycle was located, and each modulus and differential potential were expressed as a % of the maximum modulus for each subject and chewing trial. For each subject and chewing side, the masticatory frequency was computed, and statistics of the moduli as %s of the maximum were determined by means of bivariate analysis. Within-subject repeatability of the unilateral chewing patterns was good. Mean population values for the modulus position (bivariate analysis), chewing frequency, and maximum modulus of the differential potentials (univariate statistics) were computed. A significant gender difference was found for the masticatory frequency, with larger values in men than in women. Conversely, no gender or side differences were found for the mean values of the maximum modulus or for the mean position of the percentage moduli. The chewing test applied allowed the evaluation of the neuromuscular coordination during the performance of a standardized physiologic activity. In particular, it quantified the within-subject and chewing side repeatability of the muscular pattern. [ABSTRACT FROM AUTHOR]

Skinfold measurements by means of calipers and the use of linear regression functions, representing relationships between maximal EMG activity and skinfold thickness, provide a simple and non-invasive procedure to diminish the influence of the thickness of the subcutaneous tissues overlying jaw-elevator muscles on electromyographic data of the maximal voluntary contraction (MVC) level. This method, which allows an improved comparison between different subject or muscle groups in terms of neuromuscular capacity, has been applied to MVC values from the masseter and the anterior temporal muscles (surface EMG) of 21 healthy males. 14 healthy females, and 14 females suffering from myogenous temporomandibular disorders (TMD). Non-corrected MVC values from both muscle groups were larger for the male controls than for the female controls. As the skinfold thickness was smaller for males than for females, these MVC levels did not differ significantly after correction for skinfold thickness. As the skinfold thickness was very similar for the female TMD patients and the controls. the correction method did not appreciably change the lower MVC values of the TMD patients. [ABSTRACT FROM AUTHOR]

The present report aimed at evaluating the influence of recording conditions on the relative uptake of direct motor responses obtained in the masseter and temporalis muscles after electrical stimulation of their nerves, using a monopolar needle technique. In 10 subjects, various surface electromyographic recording set-ups were compared using supramaximal M-responses. Volume-conducted potentials originating from the masseter muscle could be recorded from the neck. For the masseter muscle, the optimal position for the active recording electrode was over the anterior and inferior part of the muscle belly. The configuration of the masseter M-response recorded from various positions over the muscle belly was highly variable both intra- and interindividually. The configuration of the M-response of the anterior part of the temporalis muscle showed less variability. [ABSTRACT FROM AUTHOR]