Effect of topical cheek surface anesthesia on isometric contractions of the human masseter muscle

To study the possibility of interactions between buccal cutaneous sensory receptors and voluntary maximum isometric contractions of the masseter muscles, six adult subjects exercised maximum teeth clenching before and after spraying the right cheek surface with aerosol containing 20% benzocaine. The right cheek and masseter muscle served as the experimental side, the left cheek and masseter muscle as the control side. Isometric motor outputs, on the right and left sides, were monitored by integrated surface electromyography over periods of 10 seconds. Topical surface anesthesia provided no evidence of motor modulation by cutaneous tactile receptors. Before and after anesthesia, the two muscles showed nearly identical and well-coordinated motor innervation patterns. It is suggested that the cortical motor commands of maximum isometric contractions, with recruitment of practically all available motor units, overrule all modulatory inputs except those of fatigue.     

Cumulative electromyography of the human masseter muscle during fatiguing isometric contractions

Six adult males performed maximum voluntary tooth clenching (MVC) for 10, 20, 30, 40 s and, after 15 min, for 40, 30, 20, 10 s. During the isometric exercises the electrical currents of the masseter muscle were sampled by integrated and cumulative surface electromyography. Subjective masseter fatigue was present after 30 and 40 s of MVC clenching, accompanied by changes in myoelectrical activity. Strength testing of the masseter muscle, before and after endurance testing, showed that the strength increased by a significant 16% following two endurance tests. This observation was explained by a post-tetanic potentiation and/or a differentiated use of motor units in the fatigued muscle. It is concluded that brief MVC isometric activity, or strength testing, is not a reliable measure of fatigue in the masseter muscle when cumulative electromyography is used. Prolonged MVC isometric activity, or progressive endurance testing, monitors reliably the onset and progression of masseter fatigue.     

Effect of ibuprofen on isometric contractions and associated pain of the human masseter muscle

A randomized, single-blind crossover trial tested the effect of 1,000 mg of ibuprofen on jaw muscle pain induced by maximum voluntary teeth clenching (MVC). Subjects exercised MVC until there was onset of pain in the masseter muscles (pain latency in seconds), and until pain and exhaustion of the masseter and anterior temporalis muscles could no longer be endured (pain tolerance in seconds). Pain intensity was quantified by visual analogue scores, and pain sensitivity by the pain sensitivity range and the pain sensitivity ratio. During MVC the mean voltage of the left masseter muscle was recorded by cumulative surface electromyography. Ibuprofen had no significant effect on the pain latency and the pain tolerance. Neither did ibuprofen significantly decrease the pain intensity nor significantly affect the pain sensitivity range and the pain sensitivity ratio. After intake of ibuprofen, the number of electromyograms with a decrease in mean voltage was significantly increased--credibly, an expression of increased central fatigue with voluntary decruitment of motor units, and possibly the result of increased contraction times because of an undisclosed effect of ibuprofen. There was no circumstantial evidence of impaired motor activity that could be attributed to biosynthesis of prostaglandins.     

Electromyographic power-spectrum changes during repeated fatiguing contractions of the human masseter muscle

During 7 sessions at weekly intervals, 10 healthy males performed the same endurance test three times by isometrically contracting the masseter muscles at 20, 30, 40, 50, 60, 75 or 90 per cent of maximal electromyographic activity. The interval between the tests in any one session was 30 s. Changes in the surface electromyogram were monitored by calculating the changes in mean power frequency (MPF) of the signals. With the higher contraction levels, endurance times were shorter, the MPF shifted to lower frequencies more rapidly and the MPF value was lower at the end of the test. At these levels, the subjects reported lack of power as the sole reason for stopping the test but at low levels, pain was the important reason. The endurance time and the rate of MPF shift for the second and third test of each experiment were the same. For the first test, the endurance time was longer (p less than 0.025) and the rate of MPF shift was lower (p less than 0.05). Thus the analysis of specific electromyographic (MPF) indicators provides information about the development of muscle fatigue.     

EMG, bite force, and elongation of the masseter muscle under isometric voluntary contractions and variations of vertical dimension

The relation EMG activity, bite force, and muscular elongation was studied in eight subjects with complete natural dentition during isometric contractions of the masseter muscle, measured from 7 mm to almost maximum jaw opening. EMG was registered with superficial electrodes and bite force with a gnathodynamometer. In series 1, recordings of EMG activity maintaining bite force constant (10 and 20 kg) show that EMG is high when the bite opening is 7 mm, decreases from 15 to 20 mm, and then increases again as jaw opening approaches maximum opening. In series 2, recordings of bite force maintaining EMG constant show that bite force increases up to a certain range of jaw opening (around 15 to 20 mm) and then decreases as we approach maximum jaw opening. Results show that there is for each experimental subject a physiologically optimum muscular elongation of major efficiency where the masseter develops highest muscular force with least EMG activity.     

Reperfusion response changes induced by repeated,sustained contractions in normal human masseter muscle

The purpose was to evaluate the intramuscular reperfusion response characteristics associated with repeated isometric contractions in normal human masseter. Intramuscular blood volume was quantified with a near-infrared spectroscopic device that measured the total haemoglobin (Hb) concentration in the muscle. Electromyographic (EMG) activity from the masseter and total bite forces were also recorded. Sixteen healthy volunteers, eight females and eight males, without masticatory muscle pain participated. They were asked first to clench their teeth for as long as possible at 50% of their maximum voluntary contraction (MVC). This was followed by a 60s rest and then immediately by a standard clenching task (50% MVC for 30s) and a 60s recovery period, immediately after which they were asked to repeat exactly the same procedure, with a final 5 min recovery period after the second 30s contraction. Bite force, EMG and Hb concentration were measured continuously and the duration of the two endurance tasks and the amplitudes of all recorded signals were compared (first trial versus second trial). Specifically, the difference between the lowest Hb (trough) seen during the standardised 30s contractions and the highest (peak) seen just after them was assessed. The trough-to-peak difference in Hb concentration of the second standard contraction task was significantly smaller than that of the first standard task (P<0.05, paired t-test). These data show that with sustained effort the post-contraction vasodilatory reperfusion responses of the human masseter are diminished, suggesting a progressive desensitisation of the vasodilatory system.     

Effects of local serotonin administration on pain and microcirculation in the human masseter muscle

AIMS: To investigate whether exogenously administered 5-hydroxytryptamine (5-HT) at high or low concentration influences pain and microcirculation in the human masseter muscle. METHODS: In 12 healthy female subjects, 5-HT in 2 concentrations (0.1 micromol/L and 1,000 micromol/L) and isotonic saline were injected into the masseter muscles in a randomized and balanced double-blind manner. The pain intensity after injections was recorded with Borg's rating scale, and intramuscular blood flow was monitored continuously during the experiment with a laser-Doppler technique. Nonparametric statistics were used for analyses. RESULTS: Administration of 5-HT at 1,000 micromol/L induced significantly more pain than saline (Wilcoxon: P < .05), while there was no difference between 5-HT at 0.1 micromol/L and saline. The blood flow did not change significantly after injection of 5-HT at either concentration compared to saline. However, changes in pain intensity and blood flow were positively correlated after injection of 5-HT at 1,000 micromol/L (Spearman: P < .05). CONCLUSION: Intramuscular administration of 5-HT at 1,000 micromol/L into the human masseter muscle induced pain, but 5-HT did not have any effect on local blood flow at either concentration.     

Motor-unit territory in the human masseter muscle

Motor-unit territories in human masseter are reportedly focal and related to putative subvolumes of muscle. However, in the absence of a reliable method of locating needle-electrode recording sites within the muscle in three dimensions and due to inherent weaknesses in electromyographic recording techniques, the limits of motor-unit territory in the masseter may have been underestimated. Single motor-unit responses were recorded as time-locked events from 32 paired-needle recording sites throughout the masseter muscles of three subjects. Recording sites were located stereotactically with an optical system, magnetic resonance imaging, and a common reference, then displayed graphically in three dimensions. The mean linear separation of the paired recording sites was 8.8 +/- 3.4 mm. The putative territories had a preferred orientation in the antero-posterior axis. Motor-unit territories were larger than described previously and appeared to be related to anatomical compartments. The restriction of these territories to discrete regions of the muscle provides an anatomical substrate for selective regional motor control of the human masseter muscle.     

Changes in masseter muscle blood flow during voluntary isometric contraction in humans

The effect of jaw clenching on local blood flow in the masseter muscle was measured using the hydrogen clearance method in 13 healthy subjects. Sustained isometric masseter-muscle contraction levels of 25 and 50% of maximum voluntary contraction (MVC) were investigated. The blood flow at 25% MVC before contraction, during contraction and after contraction was 12.3 +/- 10.9, 19.2 +/- 12.1 and 78.8 +/- 63.9 mL min(-1) (100 g)(-1) (mean +/- s.d.), respectively. At 50% MVC, it was 14.2 +/- 12.9, 18.6 +/- 10.0 and 80.1 +/- 61.8, respectively. The volume of blood flow was significantly greater after contraction as compared with before contraction at both levels (P < 0.0001) and there was no significant difference between before and during contraction periods (P = 0.17: 25% MVC; P = 0.38: 50% MVC). At 50% MVC blood flow before contraction and the difference in blood flow before and during contraction showed significant negative correlation (r = -0.636, P < 0.02). When the volume of blood flow was low before contraction it tended to increase during contraction and decreased when it was high before contraction. These findings indicate that blood flow in the masseter muscle during sustained isometric contraction is affected by the condition of contraction and may be influenced by the muscle region. It was also indicated that the blood flow during high level contraction was influenced by the volume of blood flow before contraction. Clinically, our findings may help to understand pathological changes which may lead to chronic masticatory muscle pain.     

Electromyographic heterogeneity in the human masseter muscle.

The complex, pennate architecture of the human masseter muscle points to a functional division into more than the commonly distinguished deep and superficial parts. In this study, the possible existence of regional differences in activation was examined. EMG activity was registered in three deep and three superficial regions with the use of bipolar fine-wire electrodes. Recordings were made during different static bite tasks, in specified directions, and with a specified bite-force magnitude. A linear bite-force/EMG relationship was observed. Furthermore, it appeared that muscle regions showed a different pattern of change in activity as a function of bite-force direction. Heterogeneity was nearly absent in anteriorly-, anteriomedially-, and medially-directed bites, but became increasingly obvious in the other bite-force directions. The posterior deep region showed the most aberrant activation pattern, which was almost opposite that from the other regions. This part was fully active in posterolaterally-directed bites. The posterior superficial region showed the largest variability in activity as a function of bite-force direction. The results point to a functional partition of the masseter muscle in at least three parts: anterior deep, posterior deep, and superficial. A further subdivision of the superficial portion might be present, but was not as obvious as the division of the deep masseter.     

Effects of experimental nasal obstruction on human masseter and suprahyoid muscle activities during sleep

The effect of nasal obstruction on nocturnal masseter and suprahyoid muscle activities using a newly developed portable electromygram (EMG)-recording unit was examined. Ten healthy Japanese males participated in this study. EMG activities of both the right masseter and bilateral suprahyoid muscles were recorded with a portable EMG-recording unit. At midnight, the subject was asked to lie on a bed after complete preparation with surface electrodes. After maximal clenching and jaw-opening effort (100% maximum voluntary contribution), the subject was allowed to fall asleep. In the first half of the night, EMG activities were recorded for about three hours of sleep without nasal obstruction. In the second half of the night, EMG activities were recorded for about three hours of sleep with nasal obstruction. In both muscles, there were no significant changes in either the maximal EMG activities or the number of events beyond 40% MVC with nasal obstruction. But in an evaluation based on the distribution of muscle activities, the EMG activity of the masseter muscle tended to decrease (P = .07) and that of the suprahyoid muscles increased significantly (P = .02) with nasal obstruction. These results suggest that nasal obstruction could modulate the activities of the masseter and suprahyoid muscles during sleep.     

Effects of denervation of the masseter nerve and bite raising on the masseter muscle of developing rats

The effects of masseteric denervation and bite raising on muscle fiber type differentiation were examined in the masseteric muscle of developing rats by histological and histochemical studies. Three-week-old Wistar rats had of their masseteric nerve at right side dissected, and a part of them were bite raised at anterior region one week after denervation for three weeks. The unoperated side and number of sham animals served as control. The animals were killed 3, 7, 10, 14, 21, 28, 35, 49 days later and then their masseter muscles were removed wholly and weighed them. After freezing of superficial and deep masseter muscles serial sections were made and HE staining carried out, ATPase staining and NADH-TR staining. On the photographs of the HE stained specimens, I measured the diameter of the muscle fibers. Gross findings on the denervated group, revealed the lower incisors shifted to unoperated side and on with the bite raising group, it shifted to the operated side. In denervated animals, the wet weight of the masseter muscle had decreased significantly. The masseteric muscle fibers in three-old-week control rats were undifferentiated on ATPase staining, but it became well differentiated on and after four-old-week. The superficial masseter of control mainly composed of type 2B fibers. In the deep masseter, about 10% type 1 and 2C fibers were found in limited area around the muscle spindles, and surrounded with type 2A and 2B fibers. In superficial masseter muscle of the denervated group, the percentage of type 2A fibers increased, and that of type 2B fibers decreased. The type 2C fibers were found from the 21st days after denervation. In deep masseter muscles, the percentage of type 1 and 2A fibers decreased, and that of type 2B and 2C fibers increased. In the denervated bite raising group, the composition of the muscle fiber type approached the control group. These results as above suggested, masseteric denervation causes degeneration of muscle fiber composition, and the possibility that early preventive treatment like bite raising may recover the muscle fiber composition normally.     

The relationship between the craniofacial morphology and the fatigability of the masseter muscle during isometric contraction

The purpose of this study was to evaluate the relationship between craniofacial morphology and the fatigability of the masseter muscle during isometric contraction.Data were obtained from 23 healthy Japanese males (25.7 ± 3.8 years). EMG signals were taken from bilateral masseter muscles. The subjects were instructed to perform maximal clenching for 10 s. The analyses were conducted using 2–4 s sampling (T1) and 8–10 s sampling (T2) of 10 s sampling. The integrated EMG (IEMG) and the median frequency of the power spectrum for EMG (MF) were determined for each sampling. The fatigability index (FI) was determined by calculating the ratio of (T1–T2) to T1 for the MF. Lateral cephalograms were taken to evaluate the craniofacial morphology. Thirteen angular and 10 ratio variables were constructed for the craniofacial measurements. Paired t-tests were used to determine the mean differences for each EMG measurement between T1 and T2. Pearson's correlation coefficients were calculated to determine the correlation between the cephalometric parameters and the FI values.Both the IEMG and the MF for EMG significantly decreased from T1 to T2. The FI values were positively correlated with Y-axis, palatal plane angle, occlusal plane angle, anterior facial height and molar height and also negatively correlated with anterior lower facial height and maxillary anterior tooth height. These results suggested that the fatigability in the masseter muscle might therefore be associated with the craniofacial morphology.     

Effects of facial nerve denervation on masseter muscle of the rats

The effects of unilateral facial nerve denervation on the facial growth were studied by physiological and by histochemical examinations, using four- week- old Wistar rats. Two hundred twenty rats were divided into three groups: a control group, a group of subjects which resection of the left facial nerve, and a sham operation group. The rats were killed 1, 5, 10, 30, 60, 90, 120 days later. The superficial and deep masseter muscle fibers on both sides were classified into type I and type IIA or IIB on the basis of myosin ATPase activity and the diameter of these fibers was measured. The masseter muscles were stained with H&E and NADH-TR. Electromyogram recordings were analyzed during the rest position, stretch reflex, voluntary action and eating of solid foods in half of all groups. (1) The face of rats after denervation deviated to the right side, and the maxillo-facial skeleton was affected from the 30th day. (2) In the denervation groups the right side showed higher action potentials than the left side. (3) In the deep masseter muscles near muscle spindle in the denervated groups, the percentage of type I fibers on the right side increased on the 90th day, and that of type IIA fibers decreased on the 30th day. (4) The diameter of the deep masseter muscle fibers, of type I, IIA and IIB increased on the right side. These results prove that facial nerve denervation led to a functional disorder and a transformation of growing direction.     

Acoustic myography in the assessment of human masseter muscle

The feasibility of examining electro-mechanical activity of the human masseter muscles using non-invasive recording techniques was examined in six healthy dentate adults (aged 34-57 years). Electrical activity of the muscle was examined by surface electro-myography (EMG) and the mechanical activity, in the form of muscle sounds, was examined by acoustic myography (AMG).
Bilateral recordings of EMG and AMG were made simultaneously using composite probes which were placed on the skin over the masseter muscles. A standardized pressure was applied to the probes via adjustable rods attached to a safety helmet. Pressures were monitored by strain gauges placed between the ends of the rods and the probes. With the subject seated, recordings of AMG and EMG were obtained during maximal jaw clenching for 4s and the raw signals were stored on a computer. Of three maximal contractions performed, the last two were used in the analysis. The raw amplified signals underwent frequency analysis by fast Fourier Transform. Total activity was also assessed after amplification, full-wave rectification and integration, and repeatability of the results was assessed.
The AMG frequency range was 6-15 Hz and was similar to values for other human skeletal muscles. The integrated values for EMG and AMG were repeatable on both sides of the face (IEMG, right r = 0.99, left r = 0.99; IAMG right r = 0.70, left r = 0.71). Simultaneous recordings of AMG and EMG from the masseter muscles may be useful for assessing electro-mechanical muscle function but further validation studies are required before the technique can be used clinically.     

Effect of cold pressor stimulation (4 degrees C) on human masseter muscle haemodynamics during and after sustained isometric contraction

The effect of cold pressor (CP) stimulation and sustained isometric contraction on the blood volume of the right masseter muscle was examined in seven healthy males, who performed 1 min of isometric jaw clenching at 50% of their maximum voluntary contraction without, with and again without a 4 degrees C CP stimulation. Total haemoglobin was measured in the masseter before, during and after the contraction task using near-infrared spectroscopy. CP stimulation during the isometric contraction diminished the magnitude of the contraction-induced decrease of blood volume when compared to the trials without CP stimulation. However, in the immediate post-contraction period (while the CP stimulation was still in place), no increase in blood volume above the usual post-contraction hyperaemia was evident. Once the CP stimulation had been removed, there was a clear decrease (faster return to baseline) in the vasodilation occurring in the post-contraction period. This diminished period of vasodilation occurred in spite of the fact that the vascular resistance (blood pressure) and heart rate were still substantially elevated by the CP effect during this same period. These data suggest that the strong CP stimulation produced a biphasic response. First, there was an early-onset strong vasodilation (during CP), which was followed by a period of diminished vasodilation, suggesting that an active, but delayed, vasoconstrictive drive may be induced by the CP stimulus.     

Effects of chewing frequency and bolus hardness on human incisor trajectory and masseter muscle activity

Nine adults with no orofacial dysfunctions were instructed to chew a standardized piece of soft or hard gum on the right side in time with a metronome set at 46, 100 or 160 beats/min. Jaw movements were recorded with a Myotronics kinesiograph and masseter electromyograms were detected with surface electrodes. The chewing patterns on either gum were not significantly different in any of their spatial or temporal aspects, in mean or peak opening or closing velocities, or in the timing or level of activity in either masseter at any of the three chewing frequencies. These findings suggest that during metronome-paced chewing the change in sensory feedback resulting from a change in gum hardness exerts little or no effect on either the spatial or temporal aspects of masticatory motor output.     

Mental stress-induced physiological changes in the human masseter muscle

The effect of a long mental stress on the hemodynamics of masticatory muscles has not been investigated to date. We hypothesized some hemodynamic and electromyographic changes in jaw-closure muscles related to sympathetic nervous system activity. While healthy adult female volunteers performed a two-hour mental stress task, electromyographic activity of the temporal and masseteric muscles was recorded, and hemodynamic changes of the masseter muscle were measured non-invasively. Autonomic function was assessed by heart rate spectral analysis. Integrated electromyographic activity of the temporalis muscle, but not the masseter muscle, showed an increase that coincided with the increase in sympathetic nervous activity. In the masseter muscle, despite little change in integrated electromyographic activity, notable changes were found in hemodynamic parameters. These results suggest that hemodynamics of jaw muscles is susceptible to mental stress, implying a potential role in the etiology of jaw muscle dysfunction associated with mental stress.