Effects of experimental pain on jaw muscle activity during goal-directed jaw movements in humans

To study the effects of masseter muscle pain on jaw muscle electromyographic (EMG) activity during goal-directed tasks. Mandibular movement was tracked and EMG activity was recorded from bilateral masseter, and right posterior temporalis, anterior digastric, and inferior head of lateral pterygoid muscles in 22 asymptomatic subjects at postural jaw position, and during three tasks: (a) protrusion, (b) contralateral (left), (c) open jaw movement. Tasks were performed during three conditions: control (no infusion), test 1 [continuous infusion into right masseter of 4.5% hypertonic saline to achieve 30–60 mm pain intensity on 100-mm visual analog scale (VAS)], and test 2 (isotonic saline infusion; in 16 subjects only); the sequence of hypertonic and isotonic saline was randomized. The average EMG root-mean-square values at 0.5 mm increments of mid-incisor-point displacement were analysed using linear mixed effects model statistics (significance: P < 0.05). Right masseter hypertonic saline infusion resulted in significantly (P < 0.0005) more pain (mean ± SD VAS 47.3 ± 14.3 mm) than isotonic infusion (12.2 ± 17.3 mm). Although there was evidence of inter-subject variation, the principal EMG findings were that the significant effects of hypertonic saline-induced pain on EMG activity varied with the task in which the muscle participated irrespective of whether the muscle was an agonist or an antagonist in the tasks. The direction of the hypertonic saline-induced pain effect on EMG activity (i.e., whether the hypertonic saline-induced EMG activity was less than or greater than control EMG activity) could change with the magnitude of jaw displacement. Hypertonic saline infusion had no significant effect on postural EMG activity in any of the recorded jaw muscles. The data suggest that under constrained goal-directed tasks, the pattern of pain-induced changes in jaw muscle EMG activity is not clear cut, but can vary with the task performed, jaw displacement magnitude, and the subject being studied.     

Blink reflexes in patients with atypical odontalgia and matched healthy controls

Atypical odontalgia (AO) is an orofacial pain condition which has been suggested to involve neuropathic pain mechanisms. The aim of this study was to use a brain stem reflex to investigate craniofacial nociceptive mechanisms in AO. In 38 AO patients and 27 matched healthy controls, the R2 component of the blink reflex (BR) was elicited using a “nociceptive-specific” electrode and recorded with surface electromyography electrodes on both orbicularis oculi muscles. The BR was tested by stimulation of both sides of the face of the participants before, during, and after an intraoral pain provocation test with capsaicin. The data were analyzed with three- and four-way mixed-model analyses of variance. The root mean square value of the ipsilateral R2 (R2i) was significantly reduced in patients compared with controls (P=0.046). No differences in R2 between stimulation sides were detected in either group (P>0.757). In all participants, R2 responses and the intensity of the pain evoked by the electrical stimulus were decreased during and after application of capsaicin compared with baseline (P<0.001). In patients, R2i onset latencies were significantly prolonged compared with controls (P=0.031). The present data show disturbances in the central processing of craniofacial information and that endogenous pain inhibitory systems in AO patients and healthy controls were activated to a similar degree by an acute intraoral nociceptive input. Additional clinical research with AO patients will be needed to determine to what extent neuropathic pain mechanisms are involved in this pain condition.This study was supported by The Danish Medical Research Council, The Aarhus University Research Foundation, the Danish Dental Association, the Health Research Council in the South-East of Sweden (FORSS), and the Swedish Research Council.     

Experimental skin pain and muscle pain induce distinct changes in human trigeminal motoneuronal excitability

Seeking information on the physiological properties of the trigeminal motoneuronal pool we investigated changes in the excitability of trigeminal motor system induced by two types of experimental pain (muscle and skin). In one session, we studied the effect of muscle pain induced by hypertonic saline infusion into the masseter muscle on the recovery cycle of the heteronymous H-reflex in the temporalis muscle and the homonymous silent period (SP) in the masseter muscle, both elicited by stimulation of the masseteric nerve in ten-healthy subjects. In another session, we studied the effect of laser stimuli applied to the perioral region, at conditioning intervals from 20 to 160 ms, on the temporalis H-reflex and masseter SP in nine healthy subjects. Whereas laser-induced skin pain significantly inhibited the temporalis H-reflex and facilitated the masseter SP (P < 0.01), muscle pain left the time course of the temporalis H-reflex and masseter SP unchanged (P > 0.05). The timing of temporalis H-reflex suppression and masseter-SP enhancement induced by laser stimuli indicates that facial skin nociceptors inhibit trigeminal motoneurones via multysynaptic reflex pathways. Hypertonic saline, a stimulus that predominantly activates group III and IV afferents, left both variables reflecting trigeminal motoneuron excitability unchanged. Due to the differences between the two experimental models, we cannot conclude that such inhibitory reflex pathway does not exist from muscle nociceptors to trigeminal motoneurones.     

Late neural adaptations to electrostimulation resistance training of the plantar flexor muscles

The present study aimed to examine early and late neural adaptations to short-term electrostimulation training of the plantar flexor muscles. Changes in triceps surae muscle activation (twitch interpolation), maximal electromyographic (EMG) activity, H-reflex amplitudes and antagonist coactivation were investigated after electrostimulation training (4 weeks) and after 4 weeks of detraining in a group of ten young healthy men. Maximal voluntary contraction torque was significantly higher (P < 0.01) after training (+19.4%) and detraining (+17.2%) with respect to baseline. Activation level, soleus and lateral gastrocnemius EMG normalized to the maximal M-wave significantly increased as a result of training (P < 0.05), and these gains were preserved after detraining, excepted for soleus EMG. Maximal H reflex to maximal M wave ratio increased significantly between baseline and detraining for both soleus and lateral gastrocnemius muscles (P < 0.05). Tibialis anterior coactivation was unchanged after training but significantly decreased after the detraining period (P < 0.01). Short-term electrostimulation resistance training was accompanied by early (increased muscle activation and EMG activity) and late neural adaptations (increased spinal reflex amplitude and decreased coactivation), likely explaining the increase and then the preservation of the maximal voluntary strength. These effects may help in conceiving and programming effective electrostimulation therapy programs for both healthy and immobilized plantar flexor muscles.     

Paraspinal muscle responses during sudden upper limb loading

The paraspinal muscle responses for unexpected and expected upper limb loading were investigated by surface EMG of 20 healthy volunteers. The simultaneous trunk and hand accelerations with paraspinal, biceps brachii and soleus muscles EMG were measured in four subjects. A short-latency response of ~50 ms was observed in paraspinal muscles. The latency was ~3 ms shorter (P=0.017) during "expected" trials on average and the latency shortened during the first three expected trials (P=0.02). Anticipation also decreased the magnitude of the response (P<0.05). Trunk movement initiated ~35 ms and ~50 ms after the impact of the load at T6 and T12 levels, respectively. In conclusion, visual expectation shortens the latency and decreases the magnitude of the paraspinal muscle response to sudden upper limb loading. Also, the trial repetition has an effect on reflex latency if visual information is available. These results indicate that anticipation modulates the reflex control of paraspinal muscles, which may be significant in understanding spinal function. Electronic Publication     

The effect of the Schwartz-Jampel syndrome on masticatory and facial musculatures--an electromyographic analysis

This study had the goal to perform an electromyography evaluation of the orbicularis oris, orbicularis oculi, masseter, and temporal muscles of two siblings with Schwartz-Jampel syndrome (SJS), in different clinical activities, comparing them to healthy controls (C). The Schwartz-Jampel syndrome is a rare genetic disorder (71 cases reported in worldwide literature), in which myotonia may be observed in the facial muscles, determining a standard facie that shows an appearance of someone who is sad and weeping For the electromyography (EMG), a Myotronics--K61 electromyographer, was used, with superficial and disposable silver chloride electrodes. By means of the analysis of the obtained results, we observed significant statistical differences for the masseter muscle and for orbicularis oculi muscles (p < 0.01) among the studied groups, in which the individuals with SJS presented greater muscular activity than the normal ones, used as healthy controls. The statistical difference between the two groups was not significant for the temporal muscle, as well as for the orbicularis oris muscle, although the EMG averages were much greater in patients who were bringers of the syndrome. This high muscular activity may be related to the facial osseous alterations, evidenced in both patients with SJS, such as the mandibular migrognathia, with an atrophy of the mandibular cortex and a consequent approximation between the inferior dental root apices and the mandible base, as well as the presence of hypoplastic condyles in terms of size and height.     

Electromyographic activity and thickness of masticatory muscles in children with unilateral posterior crossbite

The aim of this study was to assess the electromyographic (EMG) activity and thickness of the masseter and anterior temporalis muscles in children with unilateral posterior crossbite (PCB). Thirty‐six children (22 boys, 14 girls, and mean age of 8.8 ± 1.1 years) were divided into the following groups: The case group with 20 PCB patients (10 on the left side, 10 on the right side); the control group with 16 normal occlusion (NOccl) subjects. EMG activity was recorded with bipolar surface electrodes at rest and during maximal clenching. The muscle thickness was measured with real‐time ultrasound. Data were compared between groups and between sides. The correlation between EMG activity and muscle thickness was also evaluated. The data were analyzed using the Shapiro‐Wilks test, Pearson's correlation and Spearman as appropriate, paired and unpaired t‐ test, and Mann‐Whitney test. The results revealed that the masseter of the crossbite side was more active than that of the non‐crossbite side in PCB group during maximal clenching. The comparisons of EMG activity between PCB and NOccl groups revealed some variability in the results, depending on the crossbite side. The ultrasonographic evaluation did not show statistically significant differences between groups, nor between sides in the PCB and NOccl groups. Significant correlation between EMG activity and thickness was observed only in the left masseter in the NOccl group. In conclusion, these findings showed that asymmetric muscle activity of the masticatory muscles was not related to the thickness of these muscles in children with PCB. Clin. Anat. 22:200–206, 2009. © 2008 Wiley‐Liss, Inc.     

EMG power spectra of cervical muscles in lateral flexion and comparison with sagittal and oblique plane activities

The purpose of this study was to calculate electromyographic (EMG) power spectra of the sternocleidomastoid (SCM), the splenius capitis (SPL) and the trapezius (TRP) muscles from both sides of the body in bilateral lateral flexion and compare them with the EMG spectral characteristics of the same muscles in the sagittal and oblique planes recorded from the same subjects in the same experimental session but reported elsewhere in the literature. Forty normal and healthy young adults (21 males, 19 females) volunteered for the study. The subjects were prepared and positioned appropriately to exert linearly ramping isometric exertion aided by visual feedback against a rigid and load cell instrumental device. Exertions were carried out in a random order. The maximal voluntary contraction was reached within a 5-s test period. The torque and EMG from the SCM, SPL and TRP muscles were sampled bilaterally at a rate of 1 kHz. The EMG data were subjected to fast Fourier transform analysis. During lateral flexion, the torque generated by females was significantly lower than males (P<0.01) and represented at 75% of male torque. Contrary to flexion/extension and oblique plane activities, the ipsilateral SCM was most active with highest power and up to four dominant frequencies demonstrating four distinct peaks. The bandwidth of this muscle progressively increased with the grade of contraction up to 400 Hz, similar to flexion/extension with median frequency (MF) spread between 21 and 109 Hz for males and 78 and 99 Hz for females. The ipsilateral SPL was the second most active muscle, which maintained a narrow bandwidth (200 Hz) and one dominant frequency, rising in power with increasing grade of contraction. Flexion and extension caused symmetrical activity bilaterally. The results of MANOVA revealed a significant main effect of gender, activity direction, grade of contraction and individual muscles (P<0.001). There was a significant interaction between muscle and activity direction (P<0.001) implying significantly different MF and mean power frequency if the direction of exertion was changed. Male and female subjects responded to direction differently (P<0.001). Electronic Publication     

Effect of clenching levels on heteronymous H-reflex in human temporalis muscle

 Selective stimulation of the masseteric nerve has been shown to elicit a heteronymous H-reflex in the ipsilateral temporalis muscle during voluntary clenching. However, the relation between the electromyographic (EMG) activity of the temporalis muscle and the amplitude of the H-reflex has not been previously described. In the present study, the hypothesis was tested that there would be a positive relationship between the level of EMG activity and the amplitude of the H-reflex. The direct motor response (M-response) in the masseter muscle and the heteronymous H-reflex in the anterior temporalis muscle were successfully elicited by electrical stimulation of the masseteric nerve in 12 of 13 subjects. A new automatic system was used to control the on-line EMG activity and to trigger the stimulus. In a random order, two series of 20 stimuli were delivered at each of four clenching levels (0, 25, 50, and 75% of maximal voluntary contraction). The analysis showed that both the masseteric M-response and the temporalis H-reflex were reproducible within and between series. The amplitude of the temporalis H-reflex increased significantly at higher clenching levels (ANOVA: P=0.003). Clenching at 50% and 75% of the maximal voluntary contraction caused significantly larger amplitudes of the H-reflex than clenching at 25% of the maximal voluntary contraction; at rest, no H-reflex could be recorded. There was a significant correlation between the background EMG activity in the ipsilateral temporalis muscle and the amplitude of the H-reflex (Pearson: r=0.313, P=0.008). These data indicate that the heteronymous H-reflex can be reliably elicited by means of an automatic system for stimulus delivery and that the amplitude of the H-reflex is dependent on the preceding activity of the motoneuron pool. Received: 27 November 1998 / Accepted: 16 February 1999     

Maximal force,force/time and activation/coactivation characteristics of the neck muscles in extension and flexion in healthy men and women at different ages

This study examined the force production characteristics, activation/coactivation and endurance capacity of the neck extension and flexion muscles in healthy men (n=29) and women (n=28) divided into three age groups (18–26 years, 30–37 years and 45–55 years). Force and electromyography (EMG) measurements were performed during the maximal voluntary isometric extension and flexion actions. This was followed by an endurance test (ET; 60% force level of maximal voluntary contraction sustained until exhaustion), after which the force and EMG recordings were repeated. Men were both stronger and had higher values (P<0.001) for explosive force (rate of force development, RFD) than women in both actions. Younger subjects of both genders exhibited larger (P<0.05 in women) RFD values than older subjects in extension. The coactivation of the antagonist muscles during the maximal extension or flexion did not differ significantly between men and women, but the coactivation of the antagonists was larger (P<0.05) in the older age groups than in the youngest group. Women maintained the 60% force level longer than men in both actions (extension, P<0.001; flexion, not significant). The fatiguing loading led to significant decreases in maximal isometric force (P<0.001) and RFD (P<0.01–0.001), but these relative decreases did not differ between the groups. In conclusion, large gender differences in the voluntary extension and flexion force production characteristics of the neck muscles did exist, as reported earlier for other muscles of the body. No age-related differences were observed in maximal force of the extension and flexion actions within the age ranges of the subject groups studied here, but the older subjects exhibited greater coactivation and produced lower force values in the early portions of the force/time curve of the extension than the youngest group. The data indicate that explosive force production may be sensitive to aging earlier than maximal strength in the case of the neck extensor muscles. Electronic Publication     

Strength,power output and symmetry of leg muscles: effect of age and history of falling

Risk factors for medically unexplained falls may include reduced muscle power, strength and asymmetry in the lower limbs. Conflicting reports exist about strength and there is little information about power and symmetry. Forty-four healthy young people (29.3 ± 0.6 years), 44 older non-fallers (75.9 ± 0.6 years), and 34 older fallers (76.4 ± 0.8 years) were studied. Isometric, concentric and eccentric strength of the knee and ankle muscles and leg extension power were measured bilaterally. The younger group was stronger in all muscles and types of contraction than both older groups (P < 0.02–0.0001). Strength differences between the older groups occasionally reached significance in individual muscles and types of contraction but overall the fallers had 85% of the strength and 79% of the power of the non-fallers (P < 0.001). Young subjects generated more power than both older groups (P < 0.0001) and the fallers generated less than the non-fallers (P = 0.03). Strength symmetry showed an inconsistent age effect in some muscles and some contraction types. This was similar overall in the two older groups. Both older groups had greater asymmetry in power than the young (P < 0.02–0.004). Power asymmetry tended to be greater in the fallers than the non-fallers but this did not reach significance. These data do not support the suggestion that asymmetry of strength and power are associated with either increasing age or fall history. Power output showed clear differences between age groups and fall status and appears to be the most relevant measurement of fall risk and highlights the cumulative effects on function of small changes in strength in individual muscle groups.     

Knee extensor and plantar flexor muscle size and function following 90 days of bed rest with or without resistance exercise

Skeletal muscle atrophy and strength loss induced by short-term simulated spaceflight are offset or attenuated by resistance exercise (RE). This study compared the effects of plantar flexor and knee extensor RE on muscle size and function in 17 healthy men (aged 26–41years) subjected to 90 days 6° head-down-tilt bed rest with (BRE; n=8) or without (BR; n=9) RE. The RE program consisted of coupled maximal concentric and eccentric actions in the supine squat (4 sets of 7 repetitions) and calf press (4×14) every third day employing a gravity-independent flywheel ergometer (FW). Prior to, and following bed rest, muscle volume was assessed using magnetic resonance imaging. Similarly, muscle strength and power and surface electromyographic (EMG) activity were determined during maximal actions using FW or isokinetic dynamometry. In BR, knee extensor and plantar flexor muscle volume decreased (P<0.05) 18% and 29%, respectively. Torque or force and power decreased (P<0.05) 31–60% (knee extension) and 37–56% (plantar flexion) while knee extensor and plantar flexor EMG activity decreased 31–38% and 28–35%, respectively following BR. Muscle atrophy in BRE was prevented (P>0.05; knee extensors) or attenuated (–15%; plantar flexors). BRE maintained task-specific force, power and EMG activity. The decrease in non-task-specific torque was less (P<0.05) than in BR. The present data imply that the triceps surae and quadriceps muscles show different responsiveness to long-term bed rest with or without resistance exercise. The results also suggest that designing in-flight resistance exercise protocols for space travellers is complex and must extend beyond preserving muscle only.     

Modulation of H reflexes in the forearm during voluntary teeth clenching in humans

We investigated whether there is any modulation of the H reflex in the forearm during teeth clenching and how any correlation that may be found is modulated. The H reflexes of the flexor carpi radialis (FCR) and the extensor carpi radialis (ECR) muscles were evoked on the right side in five healthy adult volunteers. The H reflexes of the FCR and ECR muscles were facilitated in association with voluntary teeth clenching in a force-dependent manner (r=0.46–0.663, P<0.05). The increase in amplitude of the H reflex of the FCR muscle associated with teeth clenching started before the onset of the EMG activity of the masseter muscle. The results of the present study demonstrate that oral motor activity exerts strong influences on the motor activity of the forearm.A part of this study was presented at the World Congress on Sports Dentistry and Dental Traumatology, 22 June 2001, Boston, Massachusetts, USA     

Corticospinal activation of internal oblique muscles has a strong ipsilateral component and can be lateralised in man

Trunk muscles receive corticospinal innervation ipsilaterally and contralaterally and here we investigate the degree of ipsilateral innervation and any cortical asymmetry in pairs of trunk muscles and proximal and distal limb muscles. Transcranial magnetic stimulation (TMS) was applied to left and right motor cortices in turn and bilateral electromyographic (EMG) recordings were made from internal oblique (IO; lower abdominal), deltoid (D; shoulder) and first dorsal interosseus (1DI; hand) muscles during voluntary contraction in ten healthy subjects. We used a 7-cm figure-of-eight stimulating coil located 2 cm lateral and 2 cm anterior to the vertex over either cortex. Incidence of ipsilateral motor evoked potentials (MEPs) was 85% in IO, 40% in D and 35% in 1DI. Mean (± S.E.M.) ipsilateral MEP latencies were longer (P<0.05; paired t-test) than contralateral MEP latencies (contralateral vs. ipsilateral; IO: 16.1±0.4 ms vs. 19.0±0.5 ms; D: 9.7±0.3 ms vs. 15.1±1.9 ms; 1DI: 18.3±0.6 ms vs. 23.3±1.4 ms), suggesting that ipsilateral MEPs were not a result of interhemispheric current spread. Where data were available, we calculated a ratio (ipsilateral MEP areas/contralateral MEP areas) for a given muscle (IO: n=16; D: n=8; 1DI: n=7 ratios). Mean values for these ratios were 0.70±0.20 (IO), 0.14±0.05 (D) and 0.08±0.02 (1DI), revealing stronger ipsilateral drive to IO. Comparisons of the sizes of these ratios revealed a bias towards one cortex or the other (four subjects right; three subjects left). The predominant cortex showed a mean ratio of 1.21±0.38 compared with 0.26±0.06 in the other cortex (P<0.05). It appears that the corticospinal control of IO has a strong ipsilateral component relative to the limb muscles and also shows hemispheric asymmetry.     

Isometric axial rotation of the human trunk from pre-rotated postures

The purpose of this investigation was to study the torque and electromyogram (EMG) in axial rotation from pre-rotated postures. A group of 50 young adults (27 men and 23 women) volunteered for the study. These prepared subjects carried out axial rotation with pre-rotated postures in the direction of pre-rotation and away from it. Torque and EMG were recorded bilaterally from latissimus dorsi, erector spinae at L₃ and T₁₀ levels, pectoralis, rectus abdominis, external and internal oblique. In 15° pre-rotated posture the axial rotation in the direction of pre-rotation reduced the torque by between 11% and 17% and away from it increased the torque by 12% to 16%. In 30° pre-rotated posture the decrement in torque in the direction of pre-rotation was 24%–33%, and in the opposite direction the gain was between 21% and 32%. Even with decreased torque with rotation in the pre-rotation direction the EMG increased up to 123%. The EMG magnitude and slopes of EMG in these activities demonstrated significant increases while in the opposite direction slight decreases were observed. The EMG of each muscle was significantly different from all other muscles (P<0.001). A significant (P<0.01) but low correlation between EMG and torque was obtained. Significant linear regressions between torque and EMG of different muscles were obtained (P<0.01; r=up to 0.70). Electronic Publication     

Filters for optimal smoothing of acoustic and electric blink reflex EMG responses to determine blink response magnitude

The functional (i.e., kinematic) aspects of the blink reflex depend on the size of the integrated orbicularis oculi EMG response. Peak amplitude of smoothed rectified EMG is often used as an approximation of integrated EMG. A comparison was made between the outputs of 24 different smoothing filters, correlating peak amplitude of acoustic or electric blink reflexes with integrated EMG. The coefficient of determination (R²) was largest (i.e., ≥.95 for acoustic and ≥.90 for electric blink reflexes) when using either (1) a first-order resistor-capacitor filter with a time constant of 50 or 100 ms, (2) a boxcar filter averaging 51 or 101 data points, or (3) an unequal-weight finite impulse response filter with a cutoff frequency of 5 or 10 Hz. These filters are thus recommended when determining peak amplitude. Applying a baseline correction on peak amplitude and integrated EMG produced slightly smaller values of R² compared with uncorrected measures.     

Myosin heavy chains in skeletal muscles of the common shrew (Sorex araneus): absence of a slow isoform and transitions of fast isoforms with ageing

Myosin heavy chain (MHC) composition in seven skeletal muscles of the common shrew (Sorex araneus) was analysed by gradient SDS-PAGE and immunoblotting with monoclonal antibodies. Characteristic for the studied muscles (diaphragm, lateral gastrocnemius, medial gastrocnemius, masseter, plantaris, soleus and tibialis anterior) was a total absence of the slow isoform, MHCI; all muscles were exclusively composed of two fast isoforms MHCIId and MHCIIb. In young adults the amount of MHCIId varied between 34% (tibialis anterior) and 97% (masseter). In over-wintered senescent individuals MHCIId was clearly the dominant isoform in all muscles studied; the lowest MHCIId content was measured for tibialis anterior (69%), while in diaphragm, masseter and soleus it was practically the sole isoform (over 96%). Ageing associated isoform transition from MHCIIb to MHCIId occurred in all seven muscles studied (P< 0.05).     

Differences in activation patterns in elbow flexor muscles during isometric,concentric and eccentric contractions

Summary To investigate the relative activation of the synergistic muscles during three different types of muscle contraction, the electromyograms (EMG) of two elbow flexor muscles, the biceps brachii (BB) and the brachioradialis (BR), have been compared. To accomplish this eight healthy human subjects performed the following elbow flexions against the same load — concentric, eccentric and isometric contractions. The isometric contractions were performed at three elbow angles: 10, 45 and 90° (0° equal to full expension). The EMG were recorded by bipolar surface electrodes, and the relative activation between the two muscles was evaluated as the quotient of mean EMG activities (BR/BB). For the isotonic elbow flexions, BR/BB were calculated at three angle divisions: 0–30°, 30–60° and 60–90°. Results indicated that the relative activation of the BR during the concentric contractions was higher than that of the eccentric contraction, particularly at the extended elbow angles, i.e. the BR/BB of the concentric contractions for the elbow joint angles ranging from 0–30° and 30–60° were significantly greater (P<0.05) than those of the eccentric contractions. During the isometric and eccentric contractions, the BR/BB at the flexed joint angles tended to be greater than those at the extended angles. In contrast, there were no angle-dependent BR/BB variations during the concentric elbow flexions. Further, changing patterns in the EMG power spectra due to the type of contraction were different between BB and BR. These results indicated that the activation pattern in the two elbow flexor muscles varied with the muscle contraction pattern.     

Parafunctional Clenching, Pain, and Effort in Temporomandibular Disorders

This study tested the hypotheses that (1) parafunctional clenching increases pain and can lead to a diagnosis of temporomandibular disorder (TMD) pain and (2) electromyographic (EMG) activity during parafunctional clenching is significantly and positively correlated with reports of pain. Fourteen individuals without TMD participated in 5 consecutive days of 20-min long EMG biofeedback training sessions of the left and right temporalis and masseter muscles. Subjects were randomly assigned to either a Decrease or Increase group and were instructed to maintain EMG activity below 2 microV or above 10 microV during training, respectively. Two Increase subjects and no Decrease subjects were diagnosed, by a blinded examiner, with TMD pain following training. Self-reported pain posttraining was significantly higher for the Increase group. Masseter EMG activity was strongly correlated with pain. Parafunctional clenching increases pain and can lead to a diagnosis of TMD in otherwise pain-free individuals. Pain reports are positively correlated with the activity of the masseter muscle.     

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.