Regional 31P magnetic resonance spectroscopy of exercising human masseter muscle

Molecular imaging in masseter muscle observed by muscle function magnetic resonance imaging and 31 P-magnetic resonance spectroscopy in patients with a jaw deformity

Background

Skeletal mandibular protrusion would influence to the muscle fatigue of the masticatory muscles. Establishing a diagnostic procedures combining physiological and biochemical information is necessary for quantitative evaluation of masticatory muscle fatigue.

Objective

The transverse relaxation time (T2 time) of muscle functional magnetic resonance imaging (mfMRI), and ³¹P‐magnetic resonance spectroscopy (MRS) were used to investigate the reliability as parameters for measuring the masseter muscle in patients with skeletal mandibular prognathism.

Method

The subjects were 19 patients diagnosed as skeletal mandibular protrusions and 19 healthy subjects as a control group. Transverse relaxation time (T2 value) determined by mfMRI along with creatine phosphate (PCr) and inorganic phosphorus (Pi) determined by ³¹P‐MRS before, during, and after clenching were used for molecular imaging of muscle fatigue.

Results

The average T2 value of the patient group was significantly higher than that of the healthy control group at rest. Furthermore, the average T2 value transiently increased in both groups during experimental clenching. The PCr and Pi showed a tendency toward a transient decrease and increases, respectively. The pH in the masseter muscle showed a transient decrease in both groups prior to and following experimental clenching. The pH in the masseter muscle of the patient group was significantly lower than that in the healthy control group at rest and recovery.

Conclusion

We showed mfMRI and ³¹P‐MRS are useful for evaluating masseter fatigue during clenching, and the masseter muscle in the prognathic patients showed more severe fatigue than the healthy controls.

Activation of rat masticatory muscle afferent fibres by acidic pH

Previous research findings have suggested an important role for acid sensing ion channels (ASICs) in muscle pain mechanisms. This study was conducted to determine if masticatory muscle afferent fibres express ASICs, if there are sex differences in this expression, and to compare the effects of low pH and hypertonic saline on afferent fibres that innervate the masticatory muscle in vivo. Immunohistochemistry methods were applied to examine the expression of ASICs in trigeminal ganglion neurons, while in vivo electrophysiology techniques were employed to examine changes in masticatory muscle afferent fibre excitability. Both ASIC₁ and ASIC₃ were expressed by predominantly larger masticatory muscle ganglion neurons, but the frequency of ASIC₃ expression (56%) was significantly greater than ASIC₁ (35%). No sex-related differences in expression were identified. Injection of pH 5.8, but not pH 6.8, phosphate buffered saline evoked afferent discharges that were significantly greater than those evoked by pH 7.4 buffer (control). Since ASIC₃ channels are not activated until the pH is around 6, these results indicate that activation of both channels contributes to excitation of masticatory muscle afferent fibres. The results further show that many masticatory muscle afferent fibres, which respond to low pH, are low threshold mechanoreceptors. These findings may explain why injection of low pH solutions into the masticatory muscles of healthy humans is not associated with significant muscle pain.

Experimental low-level jaw clenching inhibits temporal summation evoked by electrical stimulation in healthy human volunteers

OBJECTIVE

To examine the effect of low-level jaw clenching on temporal summation in healthy volunteers.

DESIGN

In 18 healthy volunteers, the pain intensities evoked at the masseter muscle and the hand palm by the first and last stimuli in a train of repeated electrical stimuli (0.3 or 2.0 Hz) were rated using 0-100mm visual analogue scales (VAS), in order to evaluate temporal summation before and after three types of jaw-muscle tasks: low-level jaw clenching, repetitive gum chewing and mandibular rest position. A set of concentric surface electrodes with different diameters (small and large) was used for the electrical stimulation.

RESULTS

The temporal summation evoked by the large diameter electrode with 2.0 Hz stimulation decreased significantly both on the masseter and the hand after low-level clenching (P ≤ 0.03), but did not show any significant change after the other tasks (P > 0.23). The VAS score of the first stimulation did not show any significant changes after low-level clenching (P > 0.57).

CONCLUSIONS

Experimental low-level jaw clenching can inhibit pain sensitivity, especially temporal summation. Low-level jaw clenching can modify pain sensitivity, most likely through the central nervous system. The findings suggest that potential harmful low-level jaw clenching or tooth contacting could continue despite painful symptoms, e.g., temporomandibular disorders.

Acidic saline-induced pain as a model for experimental masseter myalgia in healthy subjects

BACKGROUND

Repeated injection of acidic saline into skeletal muscles of the leg in rodents induces a prolonged bilateral mechanical hyperalgesia that persists for up to 30 days and may be useful to model widespread muscle pain conditions. In this study, repeated injection of acidic (pH 3.3) saline solution into the masseter muscle of healthy human subjects was undertaken to determine if these injections are painful and whether they would induce a prolonged period of muscle sensitization to artificial and/or natural mechanical stimulation of the masseter and temporalis muscles.

METHODS

Eighteen subjects (10 male, 8 female) participated in the study. Subjects received two injections of 0.5 mL acidic or regular isotonic saline 2 days apart, in a randomized, double blind, crossover design.

RESULTS

There was no significant difference in pain intensity ratings when acidic saline injections were compared with regular saline injections. Pain area drawings were, however, significantly larger in response to the first injection of acidic saline than to the second injection of acidic saline or to either the first or second injection of regular saline. Repeated injection of acidic saline did not significantly alter pressure pain thresholds from the masseter or temporalis muscles on either the injected side or the opposite side over the 10-day post injection monitoring period. There was also no effect of injections on chewing.

CONCLUSION

These findings indicate that, unlike in some rodent models, repeated injection of low pH solutions into jaw muscles of humans fails to induce a period of prolonged muscle hyperalgesia.

A ³¹P-magnet resonance spectroscopy study on the metabolism of human masseter in individuals with different vertical facial pattern

OBJECTIVE

The objective of this study was to investigate differences in masseter metabolism by (31)P-Chemical Shift Imaging (CSI) in adult individuals with different vertical facial patterns. The clinical study should be supported by functional findings at the mRNA level after orthognathic surgery.

STUDY DESIGN

Twenty-two male volunteers (mean age 24.6) were divided into a deep-bite (NL/ML 11.8 ± 2.3°) and open-bite group (NL/ML 34.1 ± 2.6°). Vertical jaw relationship, gonial angle, and masseter volume were defined and compared with the phosphate values obtained from the (31)P spectra. Student t test and regression analysis were used.

RESULTS

Phosphocreatine related strongly to muscle volume (P < .001), gonial angle (P < .001), and ML/NL angle (P < .01). Pi was found to be related to gonial angle (P < .05). Muscle volume was found to be inversely related to ML/NL (P < .01) and to the gonial angle (P < .01).

CONCLUSIONS

A difference in masseter muscle metabolism between long- and short-faced subjects was confirmed at rest position.

Effect of low-level clenching and subsequent muscle pain on exteroceptive suppression and resting muscle activity in human jaw muscles

OBJECTIVE

To investigate the effects of muscle fatigue induced by low-level isometric jaw-clenching and subsequent glutamate-evoked muscle pain on the exteroceptive suppression (ES) response and resting electromyographic (EMG) activities in human jaw muscles.

METHODS

The resting EMG activity and the ESs were recorded before (baseline), after low-level jaw-clenching (Post1), after subsequent glutamate or isotonic saline injections into the left masseter (Post2), and 60 min after the clenching (Post3) in 23 healthy volunteers.

RESULTS

The late ES (ES2) showed more inhibition at Post1 compared with baseline (P<0.05). It was less inhibited after both types of injections (Post2), and increased at Post3 again (P<0.05) with no significant difference between the glutamate and isotonic saline sessions. The resting EMG activity increased at Post1 and Post2 (P<0.05). The glutamate injection further increased the resting EMG activity in the injected muscle (P<0.01).

CONCLUSIONS

Muscle fatigue influences inhibitory reflex pathways in jaw-closing muscles and subsequent acute muscle pain potentiates the local increase in the resting EMG activity of the painful muscle.

SIGNIFICANCE

Muscle fatigue which can be observed in patients with oral dysfunctions may interact with nociceptive regulation and influence the clinical presentation of jaw symptoms and function.

Change of mRNA amount of myosin heavy chain in masseter muscle after orthognathic surgery of patients with malocclusion

INTRODUCTION

Surgical correction of malocclusion changes the force to moment ratio of masticatory muscles inserting at the mandible caused by shortening, lengthening and rotation of the bone following osteotomy. During muscle adaptation the expression of mRNA for the myosin heavy chain (MyHC) of type I and type II fibres may be changed.

MATERIAL AND METHODS

The adaptation of the masseter muscle was investigated at the mRNA level in 10 patients 6 months after orthognathic surgery in the mandible. The competitive polymerase chain reaction (cPCR) is a suitable method for quantification of MyHC mRNA. For application of this minimal invasive method an amount of 35 mg muscle tissue was sufficient.

RESULTS

6 month postoperatively there was a deficiency of about 87% of MyHC mRNA for fibre type I and II in both groups of patients. The deficiency in patients with mesial position of the mandible was higher but not significant different to patients with distal malocclusion.

CONCLUSION

Patients should use the postoperative interval for training their masticatory muscles. This improves the stability of treatment result and prevents relapse.

Effects of muscle fatigue induced by low-level clenching on experimental muscle pain and resting jaw muscle activity: gender differences

The purpose of this study was to investigate the effects of jaw-muscle fatigue evoked by low-level tooth-clenching followed by the induction of experimental muscle pain by injection of glutamate on the perception of fatigue and pain and on the resting electromyographic (EMG) activity. In addition, the role of gender on these interactions was studied. The EMG activities of bilateral masseter (MAL, MAR) and temporalis (TAL, TAR) muscles in 11 healthy young women and 12 men were measured before (Baseline) and after tooth-clenching for 30 min at 10% of maximal force (Post1), after subsequent glutamate (Glu) or isotonic saline (Iso) injection into the MAL following the tooth-clenching (Post2) and 60 min after tooth-clenching (Post3). The intensities of fatigue, fatigue-related muscle pain and headache-like symptoms were scored on 0-10 cm visual analog scales (VAS). The glutamate-evoked pain was continuously scored on an electronic VAS. Sustained low-level tooth-clenching consistently produced fatigue sensation, fatigue-related muscle pain and headache-like symptoms in both genders with significantly higher fatigue VAS scores in men than in women, while the accompanying increase in the resting EMG activity appears higher in women than in men in the masseter muscles. In this study no gender differences were found for the perceived amount of experimental pain induced by glutamate injection. Additional increases of the resting EMG activity after injections occurred only in men in the injected masseter muscle and non-injected temporalis muscles. The present findings provide new information on the complex influence of gender on sensory-motor integration in the trigeminal system which may contribute to differences in susceptibility to develop musculoskeletal pain problems.

The influence of tissue blood flow volume on energy metabolism in masseter muscles

This study investigated the energy metabolism of masseter muscles by 31P-Magnetic Resonance Spectroscopy (MRS) during increased blood flow induced by hot pack application to clarify the influence of changes in blood flow on muscle fatigue. Twelve healthy subjects with no history of muscle pain in the masticatory system participated in this study. The 31P-MRS measurements were performed before and after hot pack application and the ratio of phosphocreatine (PCr) acting as the energy source to reproduce ATP to beta-ATP, the PCr/beta-ATP ratio, was analyzed. Results showed that PCr/beta-ATP ratios increased significantly by an average of 22.4% after the hot pack application. The results suggest that changes in blood flow volume influence the energy metabolism in masseter muscles and that blood flow increases due to the hot pack cause higher energy levels in masseter muscles and offer an advantageous condition for preventing and relieving muscle fatigue.

Analyses of muscular activity, energy metabolism, and muscle fiber type composition in a patient with bilateral masseteric hypertrophy

Hyperwork of the masseter muscles due to habitual parafunction is thought to induce masseteric hypertrophy (so called work hypertrophy). However, the causes underlying this disease are not yet fully understood. Recently, we had a patient with bilateral masseteric hypertrophy, and we performed a partial excision of the masseter muscles. In this patient's case, we examined muscular activity, energy metabolism, and fiber type composition of the masseter muscles using electromyograms (EMG), 31P-magnetic resonance spectroscopy (MRS), and enzyme-histochemistry. The EMG showed no hyperactivity, and the 31P-MRS showed normal energy spectral patterns and PCr contents of the masseter muscles. The fiber type composition, however, in the muscles in this case was very different from that in muscles with "work hypertrophy" and also that in normal masseter muscles: 1. Loss of type IIB fibers; 2. Increases in type IIA and in type IM & IIC fibers; and 3. Decrease in type I fibers. The findings suggest that this is not a case of work hypertrophy but a case of compensatory hypertrophy possibly due to a lack of high-tetanus-tension type IIB fibers.

Magnetic resonance spectroscopy of the masseter muscle in different facial morphological patterns

The aims of this study were (1) to develop a reliable noninvasive method to evaluate the masseter muscle metabolism, by using (31)P-magnetic resonance spectroscopy, and (2) to evaluate the metabolic profile of the masseter muscle in subjects with various facial patterns. The maxillary-mandibular relationship, which varied from hypodivergent to hyperdivergent, was measured on lateral cephalograms of 20 adults, 22 to 35 years of age. (31)P-spectra were acquired from the masseter muscle at rest with a custom-made, single-turn, double-tuned, 3 x 5-cm oblong surface coil. The inorganic phosphate to phosphocreatine (Pi/PCr) ratios were measured and compared in relation to vertical and sagittal cephalometric measurements. A statistically significant (R(2) = 0.65, r = 0.81, P = .001) relationship was found between Pi/PCr ratio and the palatal-to-mandibular plane angle. As the maxillary-to-mandibular divergence increased, the Pi/PCr ratio decreased. This correlation suggests that muscles with a higher Pi/PCr ratio have a higher resting metabolic activity than those with a lower Pi/PCr ratio. Consequently, these muscles may keep bone under more tension and influence its growth in a more horizontal direction. Another possible explanation of the results is that the fiber type composition of the masseter muscle varies with facial morphology.

Mapping energy metabolism in jaw and tongue muscles during chewing

Investigators have used positron emission tomography with 18F-fluoro-D-deoxyglucose to obtain information not only for the diagnosis of cancers, but also for researching physiology in skeletal muscles. The aim of this study was to evaluate the activities of the jaw and tongue muscles during gum-chewing. Five volunteers aged 32-61 years were studied by positron emission tomography. They were requested to chew two pieces of chewing gum for 30 min after intravenous injection of 18F-fluoro-D-deoxyglucose. 18F-fluoro-D-deoxyglucose uptake in the intrinsic tongue muscle was significantly (p < 0.05) higher than that in the masseter, temporal, and medial pterygoid muscles. Heterogeneous uptake of 18F-fluoro-D-deoxyglucose was observed in the masticatory muscles. In addition, the tongue exhibited higher activity than the masticatory muscles. In conclusion, positron emission tomography with 18F-fluoro-D-deoxyglucose appeared to be a useful technique for investigating the physiologic activities of the skeletal muscles, which have been difficult to examine by conventional methods.

Effects of 28-day mechanical and chewing stress on content of bound and diffusible ions in muscles of mastication

Type I and type II muscle fibres have different ion concentrations. Muscles adapt to chronic stress by changing of fibre types and remodelling of the myosin heavy chains in the muscle fibres. The present investigation on ionic change during muscular contraction was carried out on 10-week-old pigs (6 treated animals, 6 controls) over a 28-day period. Six pigs received acrylic build-ups to induce mechanical advancement of the lower jaw and chronic chewing stress. Muscle tissue was taken from the masseter (M1, M2, M3), temporal (TP1, TP2), medial pterygoid (PM) and geniohyoid (GH) muscles by a standardized method. Eighty-four muscle samples were used for histological fibre differentiation with mATPase. Energy-dispersive X-ray microanalysis of muscles was carried out in an environmental scanning electron microscope. Endurance stress in the stressed muscles was seen as an increase of type I fibres (P < 0.001). This histological change and ionic alterations were measured in the anterior region of the masseter (M1 and M2) and in the posterior region of the temporal muscle (TP2). Smaller changes were found in the medial pterygoid muscle. We measured in this muscles increases in potassium, sulphur, chloride (P < 0.05) and even larger increases in phosphate (up to 1.5 mmol/g to 2.3 mmol/g, P < 0.001) and sodium (3-fold, P < 0.001). The results reveal the effects of chronic stress on muscle fibres and ion concentration in the muscle. Chronic stress resulted in an increase of type I fibres and increased ion concentration in the same muscle region. These are considered to be indicators of more efficient contraction. The changes in ion concentration are an important factor in muscle contraction.

Regional differences of metabolism in human masseter muscle by two-dimensional 31P-chemical shift imaging

Many reports have demonstrated significant region-dependent differences in the fiber-type composition of the human masseter muscle. Therefore, it is considered that there is intramuscular heterogeneity of metabolic activity in the muscle. The present study was carried out, with two-dimensional Chemical Shift Imaging, to detect differences between the deep and superficial parts of the human masseter muscle at rest. Masseter muscle from 11 volunteers, from 20 to 27 years old, was examined, and characteristic spectra of the inorganic phosphate (Pi), phosphocreatine (PCr), and alpha-, beta-, and gamma-adenosine triphosphate (ATP) from each part of the muscles were obtained. In this study, the deep and superficial parts of the masseter muscle were distinguished by the existence of aponeurosis. The Pi/PCr, PCr/beta-ATP, and Pi/beta-ATP ratios as well as the pH in the deep and superficial parts were calculated from the peak spectra. Compared with the deep part, the Pi/PCr of the superficial part was lower (p < 0.05) and the PCr/beta-ATP was higher (p < 0.01). The Pi/beta-ATP and pH showed no significant differences between the two parts. The results indicate that the superficial part of the masseter muscle contains more PCr than the deep part, and this may be related to functional differences between these two parts. In future examinations of the metabolic activity of the human masseter muscle, the deep and superficial parts must be measured separately.

Comparison of magnetic resonance imaging and gross findings regarding masseter muscle aponeuroses in cadavers

OBJECTIVES

The main objective of this study was to compare the actual distribution and thickness of aponeuroses in cadavers with the distribution and thickness as determined by means of magnetic resonance imaging for the sake of evaluating magnetic resonance imaging as a diagnostic modality for assessing masseter muscle aponeuroses.

STUDY DESIGN

The aponeuroses of 26 masseter muscles from 13 intact cadavers were examined by magnetic resonance imaging.

RESULTS

The ratio of concordance between gross findings and magnetic resonance imaging findings was 99.0%, although depiction of thin parts of the aponeuroses on magnetic resonance imaging was poor.

CONCLUSIONS

Magnetic resonance imaging was useful as a diagnostic modality in the assessment of masseter muscle aponeuroses. Aponeuroses were distributed throughout almost the entire masseter muscle, although almost no aponeuroses were seen below the lower half of the anterior margin. This was thought to be a characteristic finding of masseter aponeuroses.

Bilateral masseteric contractile activity in unilateral gum chewing: differential calculus

Twelve healthy subjects performed 10 s, 15 s, 20 s, and 25 s of right-sided and, subsequently, left-sided gum chewing. The contractile activities of the ipsilateral (chewing side) and contralateral (non-chewing side) masseter muscles, mainly the concentric contractions of the phase of jaw closing and the isometric contractions of the phase of dental occlusion, were recorded through cumulative surface electromyography (EMG). A linear function (y = ax + b) described the association between an increase in the duration (x) of unilateral gum chewing and the cumulative EMG (y) of both the ipsilateral and the contralateral masseter muscle, and because of different slopes (a) of the two straight lines a geometric function (y = aqx) described the progressively larger differences between the paired and straight lines. When differential calculus was applied to the exponential functions, it became evident that the chewing forces generated by the ipsilateral masseter muscle continually exceeded those generated by the contralateral masseter muscle, and that the positive work (force x distance) produced by the concentric contractions of the ipsilateral masseter muscle continually exceeded that produced by the concentric contractions of the contralateral masseter muscle. It was inferred that mechano-physical work plays a major role if clinical muscle fatigue develops during prolonged unilateral gum chewing.

Gum chewing and jaw muscle fatigue and pains

To study possible associations between gum chewing and fatigue and pains in the jaw muscles, eight healthy adults performed prolonged idling, prolonged unilateral chewing of gum, and brief vigorous clenching of the teeth (MVC). Through surface electromyography (EMG), the authors monitored the cumulative (microV.s) as well as the average rates (microV.s-1) of contractile activities in the right and left masseter muscles. During 10 min of idling there was an absence of muscle fatigue and muscle pains when the EMG rates of the right and left masseter muscles were 2% and 3%, respectively, of those required to elicit isometric muscle pains through MVC. During 10 min of right-sided gum chewing at a rate of 1.2 Hz, the majority of subjects (75%) experienced weak jaw muscle fatigue-not jaw muscle pains-when the EMG rates of the right and left masseter muscles were 38% and 19%, respectively, of those required to elicit isometric pains through MVC. In comparison with 10 min of idling, the weak muscle fatigue of 10 min of unilateral gum chewing appeared when the total contractile activities of the right and left masseter muscles were increased by 1664% and 519%, respectively. It seemed as if prolonged unilateral gum chewing and previous pain-releasing MVC caused some sensitization of muscle nociceptors which, in turn, aggravated subsequent isometric jaw muscle pains elicited through MVC. Even though the right masseter muscle was the most frequent site of clinical fatigue and pains, the authors found no evidence supporting the theoretical foundation of the myofascial pain/dysfunction syndrome.

Effect of maturation on 31P magnetic resonance spectroscopy of the rabbit masseter muscle

This work studies the dynamic metabolic changes of the rabbit masseter muscle during post-natal development. The composition and proportion of oxidative and glycolytic muscle fibers alter during maturation. The masseter muscle, as most muscles of the craniofacial region, exhibits unusual development in composition of isoforms of myosin. The effect of this unusual composition on the dynamic metabolic properties of the masseter muscle have not been assessed. The metabolism of the rabbit masseter muscle was studied by means of 31P-nuclear magnetic resonance (NMR) spectroscopy. Contraction was elicited by electrical stimulation of the muscle in the anesthetized animal. Five animals were studied at 8 weeks and 24 weeks so that both the juvenile and adult stages could be evaluated. The dynamic biochemical changes in the masseter muscle were studied by the analysis of NMR spectra. A single-turn surface coil (copper) was used, and the original signal was treated with Fourier transforms to obtain 31P spectra. The low signal-to-noise ratio required averaging 16 acquisitions (acquisition time = 400 msec, repetition rate = 1.8 sec) in 30 sec and then obtaining continuous spectra for 27 min. Each averaged spectrum demonstrated five peaks: inorganic phosphate (Pi), creatine phosphate (PCr), and three peaks related to adenosine triphosphate (ATP). The protocol involved recording an initial three-minute rest period, stimulating the muscle at 5 Hz for 3 min twice, separated by three-minute rest periods, and stimulating the muscle at 50 Hz twice for 3 min separated by rest periods. The Pi/PCr ratio increased significantly in the adult masseter during both 5-Hz stimulations, evoking twitching, and the first 50-Hz stimulation, evoking tetany (repeated ANOVA, P < 0.05). The resting pH (6.96 +/- 0.13) was significantly lowered during both twitching (6.85 +/- 0.10; P < 0.0038) and tetany (6.55 +/- 0.13; P < 0.0001), but only in the adult masseter muscle. These finding suggest that the adult masseter muscle possesses more glycolytic fibers as it modifies its metabolism during postnatal development.

Plasticity of craniomandibular muscle function: 31P magnetic resonance spectroscopy of the rabbit masseter muscle

The masseter muscle was studied during postnatal development of the rabbit from the juvenile to adult stage in which the oral function was altered during maturation by modifying the diet to soft food. The muscle was assessed using phosphate magnetic resonance (31P NMR) spectroscopy with a single-turn copper surface coil to study potential changes in phosphate metabolism. The 31P NMR spectra consisted of five peaks related to unbound forms of inorganic phosphate (Pi), creatine phosphate (PCr), and three peaks related to the adenosine triphosphate (ATP). The masseter was assessed in one group of five rabbits at 8 weeks postnatally (juvenile) and after 4 months of this experimental group masticating on soft food. They were compared with a control group of five rabbits raised on a normal hard diet. The Pi/PCr ratio increased in the adult masseter much higher during twitching, tetany, and periodic contraction than in the juvenile regardless as to whether the adult animal had been raised from the juvenile period on soft or hard diet. There were relatively few differences between the experimental adult animals raised on a soft diet and the normal adult animals despite the soft diet animals demonstrating a significantly lower weight and smaller muscle mass. These findings suggest that chronic underuse of the masseter muscle by decreasing the masticatory loads has a minimal effect on the phosphate metabolism of the maturing masseter.

31P-magnetic resonance spectroscopy of the rabbit masseter muscle

Dynamic biochemical changes in the masseter muscle were studied in 14 New Zealand adult male rabbits by 31P-nuclear magnetic resonance (NMR) spectroscopy. NMR spectra were obtained during rest and electrical stimulation of the muscle in the anaesthetized animal at 33 recording sessions. Electrical stimulation was applied by a pair of copper wires placed separately with hypodermic needles into the muscle. NMR spectra were acquired with a 2 x 3 cm, double-turn, copper transmit/receive coil. Sixteen spectra were averaged over 30 s to obtain averaged spectra continuously during a 30-min recording. The spectra were processed automatically using a non-linear 'least-squares' fitting program on the spectrometer. A Lorentzian line shape was assumed for the peaks, and values of peak height, area and chemical shifts were generated. Each averaged spectrum consisted of five peaks: inorganic phosphate (Pi), creatine phosphate (PCr), and three peaks related to ATP. Data were analysed as to absolute changes in Pi and PCr, in the ratio of Pi/PCr, and the shift of Pi to PCr to estimate pH. Several protocols were used in which ranges of frequency, intensity and duration of electrical stimulation were tested. The protocol for detailed studies involved stimulating the muscle twice at 5 Hz for 3 min separated by a 3-min rest period, then stimulating twice at 50 Hz for 3 min separated by a rest period. During contraction of the muscle, there was a significant increase in the Pi/PCr ratio (p < 0.05) as compared to the resting level. The ratio reached a plateau over a 3-min contraction using 5-Hz stimulation, then increased significantly more with the 50-Hz stimulation but decayed during the 3 min. Sustained stimulation with 50 Hz for 15-45 min evoked an initial sharp change in Pi/PCr, which then reached a steady plateau that remained over the entire stimulation. These findings indicate that the rabbit masseter muscle is relatively fatigue resistant in maintaining a steady-state equilibrium in the relation of Pi to PCr.

Fatigue development during electrical stimulation in the masseter muscle of rhesus monkeys (Macaca mulatta)

Fatigue development was investigated in five adult female rhesus monkeys, 9-11 yr old (mean weight, 4.6 kg). After sedation and anaesthesia, silver electrodes were implanted in the anterior and posterior parts of the right masseter; the contralateral muscle was used as a control. The bite force was monitored. Muscle biopsies were obtained from the central part of the masseter and were immediately frozen in liquid nitrogen. After freeze-drying a fluorometric analysis using enzymatic methods for measuring levels of glycogen, glucose, lactate, pyruvate, creatine phosphate, creatine, NADH and NAD was made. The masseters were stimulated for 3 min (100 V, 4 Hz and 2 ms). After a 5-min rest period the stimulation was repeated with the same voltage, frequency and duration. The rhesus monkey masseters were easy to fatigue. After the stimulations 25% of the initial bite force remained. A marked substrate depletion was evident. The precontraction values of glycogen, glucose and phosphocreatine were reduced. The NADH concentration increased and the NAD content decreased. An accumulation of waste products was observed; the pyruvate increased by 92% and the lactate increased by a factor of 3. The substantial substrate depletion in combination with a prominent metabolic waste-product accumulation may induce a decrease in bite-force production.