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PubMed1983

Masseter muscle fatigue in man objectively quantified by analysis of myoelectric signals.

Localized muscle fatigue in the masseter muscle was studied with a method based on power spectrum analysis of myoelectric signals. Under the influence of fatiguing contractions, a gradual shift of the spectral curve occurred; the rate of change was taken as a measure of the development of fatigue. The fatigue was dependent on the bite force. The existence of a threshold value of force, below which significant myoelectric fatigue changes do not develop, was shown.

Adult↗

PubMed1983

Changes in the pattern of SDH and PhR staining in fibres of rat masseter muscle following long-term functional stretch.

Histochemical changes in the three basic fibre types of the deep masseter muscle of the rat were studied following functional stretch due to increased vertical dimension of the occlusion. A large number of muscle fibres showed changing histochemical profiles for succinic dehydrogenase (SDH) and phosphorylase (PhR) during degeneration and subsequent regeneration of muscle fibres. On the other hand, a small number of muscle fibres with well defined sarcolemmas presented a normal pattern (i.e. low SDH and high or intermediate PhR activities) during this experiment. The ratio between the three basic fibre types remained unchanged after recovery of the muscle fibres. These results suggest that fast-twitch-oxidative-glycolytic and slow-twitch-oxidative fibres underwent degeneration and regeneration, while almost all the fast-twitch-glycolytic fibres retained normal SDH and PhR staining following functional stretch.

Animals↗

PubMed1984

The influence of clenching level on the post-stimulus EMG complex, including silent periods, of the masseter muscles in man.

Standardized mechanical taps were delivered on an upper central incisor, while the nine subjects investigated maintained constant clenching levels of 5, 10, 20, 40 and 60 per cent of their maximal EMG outputs. Using statistical criteria, a computer program enabled an objective determination of the interaction between the clenching level and the sequence of upward and downward going waves following the stimulus in full-wave rectified and superimposed EMG, called a post-stimulus EMG complex (PSEC). The morphology of the PSEC, including one or two silent periods, was subject-specific and reproducible over one year. The surfaces of the waves were greatly influenced by the clenching level. In general, their total surface decreased as a function of the clenching level. However, the individual waves decreased selectively. The results suggest that the sequence of waves results from overlapping of inhibitory and excitatory inputs. The inhibitory waves, constituting the silent periods, are largely cancelled by the excitatory reflexes of the PSEC.

Adult↗

PubMed1984

Short and long-latency responses in human masseter muscle evoked by chin taps.

Ten healthy subjects received downward, backward and upward chin taps with standardized force and direction. Taps in all three directions elicited short (M1) and long (M2 and M3)-latency excitatory EMG responses in the masseter muscle. M1 (mean latency 8.9 ms, amplitude 150 microV) occurred more frequently and with shorter latency during clenching than during relaxation or opening. Downward taps were more efficient in evoking jaw-jerks (25 per cent) than upward (14 per cent) and backward ones (16 per cent). M2 was a weak biphasic deflection occurring during the silent period (mean latency 42 ms, amplitude 87 microV) and M3 was an EMG burst following the silent period (mean latency 69.3 ms, amplitude 169 microV). No long-latency evoked responses were obtained from the relaxed masseter. Thus, muscle stretch increases M1 response frequency but is not a necessary prerequisite. The hypothesis that vibratory transmission plays an important role was confirmed. Voluntary activation of the stimulated muscle (clenching) increases M1-response frequency and shortens response latency.

Adult↗

PubMed1984

Correlation between surface electromyograms and the susceptibility to fatigue of the human masseter muscle.

Seventeen healthy male subjects were asked to clench as long as possible in the intercuspal position while keeping the electromyographic activity (EMG) of the masseter muscle at 50 per cent of the maximum activity. The endurance time was used as an indicator for the muscular resistance to fatigue. At the 95 per cent probability level, there was a statistically-significant correlation between the endurance time and the root mean square value (RMS) of the EMG signal, the mean power frequency value (MPF) of the signal and the parameter MPF 100 per cent/MPF10 per cent used as an indicator for the contraction-level dependency of the mean power frequency. There was a strong correlation (p less than 0.001) between the endurance time and the rate of EMG frequency shift to lower frequencies measured during the first 30 s.

Adult↗

PubMed1984

Effects of topical cooling on isometric contractions of the human masseter muscle.

Six adult males performed tooth clenching for 10 and 80 s at maximum voluntary contraction strength (MVC). Motor activity in the right and left masseter muscles was monitored by surface electromyography. Local temperatures of both cheeks were monitored by a thermocouple. MVC activity for 80 s induced pain and fatigue in both muscles; 10 s of clenching caused no muscle discomfort. Ice, subsequently applied to the right cheek for 30 min, lowered the temperature of the cheek and masseter muscle. The isometric MVC exercises were then repeated. During 10 s of isometric contraction, MVC motor activity in the cooled masseter muscle was significantly increased by 29 per cent; that of the non-cooled contralateral muscle was insignificantly reduced by 12 per cent. During 80 s of clenching, MVC motor activity in the cooled muscle was significantly increased by 30 per cent; that of the non-cooled muscle was insignificantly reduced by 4 per cent. MVC activity for 80 s by the cooled muscle caused no pain and fatigue, but the discomforts continued in the non-cooled muscle. Increased MVC alpha motor activity in the cooled masseter muscle might have resulted from motor facilitation that was mediated by cutaneous, muscular or mucosal cold receptors, or it might have been due to the absence of pain and fatigue.

Adult↗

PubMed1984

Mechanical properties of glycerinated guinea-pig temporal and masseter muscles.

At a muscle length, L0 (just taut), isometric tension and tension transients in response to rapid step stretches in length (less than 1 per cent of L0 within 2 ms) were measured at constant levels of Ca2+ activation of various magnitudes in glycerinated temporal and masseter muscles (1.7-2.5 mm long, 48-96 micron in diameter) from guinea pigs. The experiments were at 20 and 30 degrees C, and pH 6.8. Steady isometric tension increased in a sigmoid fashion as Ca2+ varied from about pCa 6.6 to 4.4 in both muscles. However, the maximum tension of temporal muscle was about 4.4 times greater than that of masseter muscle at 20 degrees C. The tension transients showed an immediate tension increase coincident with the stretch (the first phase) and an exponential-like tension decrease (the second phase). The time constant of the second phase was about 87.5 ms in temporalis and 3.7 ms in masseter at 30 degrees C. Decreasing temperature from 30 to 20 degrees C markedly increased the time constant of the tension response in the second phase (Q10 of about 3.4 in temporalis and 2.2 in masseter). Although there was a difference in the sarcomere length between the two muscles at L0 (2.14 micron in temporalis and 1.84 micron in masseter), the mechanical characteristics were almost independent of the muscle length. Thus the cross-bridges in temporalis appear to cycle more slowly and produce more isometric tension than those in masseter muscle.

Animals↗

PubMed1984

Flexibility in motor-unit firing pattern in the human temporal and masseter muscles related to type of activation and location.

Morphological studies have shown that each human jaw muscle and its sub-portions have a characteristic fibre-type pattern, suggesting special functional tasks. The firing pattern of 68 low-threshold motor units in different portions of the temporal and masseter muscles in 5 healthy subjects were determined by single-fibre EMG during voluntary isometric muscle contraction. The findings showed that each unit may have different activity patterns for different types of activation, which is not so for limb muscles. Thus, a given motor unit discharged with a different regularity in firing rate during retraction of the mandible than during clenching of the jaws in the intercuspal position, probably related to a changed central and peripheral influence on the motoneurone pool. No activity of single units was detected in the mandibular rest position. The results underline the complexity of the human stomatognathic system and the importance of combined morphological and physiological studies.

Adolescent↗

PubMed1985

Bite force and masseter muscle electromyographic activity during onset of an isometric clench in man.

The surface electromyogram (EMG) of the masseter muscle and interocclusal force were recorded in six human subjects during fast and slow clenches. The simple linear force/EMG relation that applies for steady bite-force levels was inadequate to explain their relationship under conditions of changing force: a second component of force, its rate of change, was necessary. EMG/force equations for fast and slow clenches were broadly similar. The rate of change factor may account for the apparent EMG/force lag observed in earlier studies.

Adult↗

PubMed1985

Electromyographic study of human jaw-closing muscle endurance, fatigue and recovery at various isometric force levels.

The effect of a sustained isometric clenching, at various force levels or maximum voluntary bite-force levels, was evaluated on normal jaw-function subjects. Surface electromyographic (EMG) recordings of the masseter and temporalis were taken and bite force was measured using an intra-oral force transducer placed unilaterally in the 2nd premolar-1st molar region. Subjects sustained isometric force at the 25, 50, 75 and 100 per cent force level. Maximal bite force and EMG contraction levels were measured before, during and after these sustained isometric tasks. Subjects showed no change in their brief maximal contraction or force levels during or after various fatigue-inducing isometric tasks. The findings suggested a lack of contractile failure in the jaw-closing muscles. Pain intolerance rather than neuromuscular fatigue is the limiting factor of a sustained submaximal or even maximal clenching effort.

Adult↗

PubMed1985

Ca2+ sensitivities and transient tension responses to step-length stretches in feline mechanically-stripped single-fibre jaw-muscle preparations.

At a muscle length, Lo (just taut), isometric tension at constant levels of various Ca2+ activations and transient tension responses to rapid length stretches (less than 1 per cent of Lo within 2 ms) at maximal Ca2+ activation level were measured in temporal, masseter and digastric (anterior belly) muscles (2-3 mm long and 24-48 micron in diameter). Steady isometric tension increased in a sigmoid fashion with increasing Ca2+ concentration from about pCa 7.28 to 4.49 in temporalis, from about pCa 6.18 to 4.40 in masseter and from about pCa 5.82 to 4.40 in digastric. The maximum tension was 75.5 +/- 10.2 g/mm2 in temporalis, 44.7 +/- 14.1 g/mm2 in masseter, and 46.1 +/- 20.1 g/mm2 in digastric. In the resting state, the sarcomere length at Lo was 2.34 +/- 0.06 micron in temporalis, 2.20 +/- 0.08 micron in masseter, and 2.20 +/- 0.00 micron in digastric. When the sarcomere length was stretched from 2.20 to 2.34 micron (the sarcomere length of temporalis at Lo) in the masseter and digastric, the Ca2+ sensitivity increased without significant change of the maximum tension in either muscle. The transient tension responses in all three muscles showed two distinct phases; an immediate tension increase coincident with the length stretch followed by an exponential tension decrease. The mean value of the time constant in the second phase was 58.5 +/- 19.7 ms in temporalis, 58.5 +/- 12.6 ms in masseter, and 362.6 +/- 16.8 ms in digastric. Thus temporalis showed a higher Ca2+ sensitivity at Lo and a greater maximum tension-producing capability than the other muscles and the cross-bridge turnover rate appears to be slower in digastric than in the others.

Animals↗

PubMed1985

The effect of stimulus intensity and gape on electrically-evoked jaw reflexes in man.

The changes in masseter EMG and biting force evoked by electrical stimulation of the lip were measured in normal people. Stimulation at noxious intensities elicited a characteristic, biphasic inhibitory response in the masseter EMG and a transient reduction in the biting force. The pattern of masseter inhibition and force change varied both with stimulus intensity and with inter-incisal separation. In general, the decrease in biting force was greater at intermediate jaw positions than in the open and closed positions. The duration of the EMG inhibitory period increased as the stimulus intensity increased but was independent of the jaw separation. Reflex thresholds could not be correlated with pain thresholds because subjects found it difficult to decide at what level an electrical stimulus became painful. Nevertheless this stimulation of high-threshold afferents may be useful in estimating the rate of relaxation of tension in the jaw muscles.

Adolescent↗

PubMed1985

The masseteric post-stimulus electromyographic-complex in people with dysfunction of the mandibular joint.

In mandibular joint-symptom-free subjects, post-stimulus EMG complexes (PSEC) were derived by standardized mechanical stimulation of an upper central incisor during clenching at a constant level of 5 per cent of maximal masseteric EMG activity. Seventy-two sweeps per subject were processed by means of a computer program, and different morphologies of the PSEC were seen. There appears to be no correlation with the Helkimo-index as a whole or its specific components. There is a strong correlation between tooth-grinding habits and the occurrence of a single inhibitory period. Patients with myofascial pain dysfunction consistently had a single inhibitory wave. When there was one affected side, the end latency of the second inhibitory wave was significantly shorter.

Adolescent↗

PubMed1986

The influence of jaw position and antagonistic tooth relations on the appearance of a short-latency excitatory reflex in the human masseter muscles following mechanical tooth stimulation.

Post-stimulus electromyographic complexes (PSEC) were elicited by standardized tapping of a central upper incisor in 12 subjects during isometric voluntary biting in two positions, the incisor edge-to-edge (IEE) or the maximal occlusion (MO). In IEE a short latency excitatory peak (P wave) was present in the PSEC; this was absent in MO. By superimposing the rough EMG sweeps without full-wave rectification, it appeared that this P wave was brought about by synchronous masseter motor-unit firing.

Action Potentials↗

PubMed1986

Inhibition of jaw-closing muscle activity by tactile air-jet stimulation of peri- and intra-oral sites in man.

There were two distinct periods of suppression in electromyograms of masseter and temporalis muscles in 8 subjects after light tactile stimulation (a puff of air) of the peri-oral zone; the latencies were 16 and 60 ms, respectively. These effects were similar to those induced by painful electrical stimulation. A single period of suppression with a latency of 50 ms, was evoked by tactile stimulation of the mucosa of the hard palate or of the tongue. Threshold for suppression was lower in mucosa than skin. Low-threshold and fast-adapting receptors are likely to be involved in this reflex inhibition.

Adult↗

PubMed1986

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.

Adult↗

PubMed1986

Histochemical enzyme profile of the masseter, temporal and lateral pterygoid muscles of the European hedgehog (Erinaceus europeaus).

In man, there are large differences in histochemical fibre-type composition, distribution and size between jaw and trunk muscles, probably related to the special functions of the human stomatognathic system. In the hedgehog, the influence of alkaline and acid pre-incubations on the reaction for myofibrillar ATPase was different from that in man, suggesting a different myosin structure; the fibre composition was different also. The masseter, the superficial portion of the temporal and the lateral pterygoid muscles all showed a homogeneous fibre type profile with almost 100 per cent alkali-stable fibres. In two animals, the deep temporal muscles showed an apparent heterogeneous fibre pattern with 81 per cent alkali-stable fibres, 4 per cent alkali-labile fibres and 15 per cent ATPase-intermediate fibres; in one animal 87 per cent alkali-stable fibres and 13 per cent ATPase-intermediate fibres. There was no difference in cross-sectional area between the three fibre types within each muscle, but the fibres of the lateral pterygoid were smaller than the alkali-stable and the alkali-labile fibres of the masseter and temporal muscles. The limb and trunk muscles showed reactions for myofibrillar ATPase similar to the jaw muscles, but had a heterogeneous fibre-type profile. There was no significant difference in cross-sectional fibre area between the jaw and the limb muscles. Thus the jaw and limb muscles of the hedgehog have similar fibre types and about equal fibre size.

Adenosine Triphosphatases↗

PubMed1986

Electrophysiological study of size and fibre distribution of motor units in the human masseter and temporal muscles.

Probably related to their unique function, human jaw muscles have a specialized muscle-fibre composition different from that of limb and trunk muscles. In jaw muscle from healthy young adults, there were large groups of densely-packed fibres of the same histochemical type, a finding which indicates re-innervation when seen in limb muscles. The number of muscle fibres per motor unit, the local distribution of muscle fibres within motor units and the neuromuscular transmission in human masseter and temporal muscles were examined by single-fibre EMG and macro EMG. One hundred and forty-seven low-threshold motor units in four healthy subjects were recorded. The first dorsal interosseus muscle of the hand was similarly measured. In the jaw and hand muscles the low-threshold motor units were slightly smaller, than in large arm and leg muscles. The temporal muscle differed from the masseter in having significantly-higher fibre density, which may be explained by the higher frequency of small type-II fibres in the temporal causing closer packing of the type-I, low-threshold motor units. The fibre concentration within motor units was similar to that of normal limb muscles without any sign of grouping of fibres from the same motor unit. Normal motor end-plate transmission was another argument against re-innervation.

Adult↗