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Effects of experimental muscle pain on mechanical properties of single motor units in human masseter
Clinical Neurophysiology 115 (2004) 76–84 www.elsevier.com/locate/clinph
Effects of experimental muscle pain on mechanical properties of single motor units in human masseter M.K. Sohna,b, T. Graven-Nielsena, L. Arendt-Nielsen, P. Svenssona,c,* a Center for Sensory-Motor Interaction, Aalborg University, DK-9220 Aalborg, Denmark Department of Rehabilitation Medicine, College of Medicine, Chungnam National University, Daejeon, South Korea c Department of Clinical Oral Physiology, Dental School, University of Aarhus, Vennelyst Boulevard 9, DK-8000 Aarhus C, Denmark b
Accepted 5 September 2003
Abstract Objective: Muscle pain is known to influence muscle activity but the details of its effects on the mechanical properties of single motor units (SMU) have not been described. We have recently reported a decreased firing rate of SMU in the human masseter muscle during painful contractions with a constant force output. Force output can be modulated by the SMU discharge rate in relation to the contractile properties of SMU. Therefore, the objective of the present study was to measure the mechanical properties of SMU in the masseter to clarify the mechanism which underlies the decrease in SMU firing rate during jaw-muscle pain. Methods: A spike-triggered averaging (STA) technique was used to determine the mechanical properties of low-threshold SMU in the masseter muscle recorded with fine wire electrodes during a voluntary isometric contraction. The twitch amplitude, contraction time, and half-relaxation time were determined from the averaged force records before and during experimental jaw-muscle pain induced by injection of 0.2 ml (100 mg/ml) capsaicin in 8 healthy subjects. Injections of 0.2 ml isotonic saline served as a non-painful control in 11 healthy subjects. Results: The twitch amplitude was significantly increased during capsaicin-evoked muscle pain (P , 0:001) without significant changes of half-relaxation time and contraction time. No significant changes in SMU twitch properties were observed during the control injections. Conclusions: Potentiation of twitch force could be a possible compensatory mechanism to maintain a constant force output during painful isometric contractions when SMU firing decreases. This finding therefore provides new information on the adaptation of motor function by muscle pain. q 2004 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. Keywords: Experimental muscle pain; Twitch force; Spike triggered averaging; Single motor unit; Masseter muscle
1. Introduction Patients with chronic painful conditions in the musculoskeletal system often demonstrate distinct changes in the electromyographic (EMG) activity of the involved muscles during movements. Clinically these EMG changes are associated with slower movements and limited range of motion. However, the motor control strategies in relation to pain are not completely understood. Experimental models have during the last decade made it possible to study the influence of acute muscle pain on the interference pattern during various movements (for a review see Lund et al., * Corresponding author. (address c). Tel.: þ45-8942-4191. E-mail address: [email protected] (P. Svensson).
1991; Stohler, 1999; Graven-Nielsen et al., 2000; Svensson and Graven-Nielsen, 2001). In some types of movements the EMG activity in the antagonist phase increases and the EMG activity in the agonist phase consistently decreases during acute muscle pain (Lund et al., 1991; Graven-Nielsen et al., 1997; Madeleine et al., 1999a,b). For example during painful mastication, a decreased EMG activity of the jawclosing muscles in the agonist phase has been found in addition to a slightly increased EMG activity in the antagonist phase (Schwartz and Lund, 1995; Svensson et al., 1996, 1997, 1998; Turp et al., 2002). These EMG findings are generally in accordance with the pain-adaptation model (Lund et al., 1991). However, there is still a considerable gap in the knowledge of how muscle pain may interfere with the activity of single motor units (SMU).
1388-2457/$30.00 q 2004 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/S1388-2457(03)00318-3
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Stohler et al. (1990) showed in a preliminary report that the discharge frequency of SMU in the human masseter muscle increased and additional SMU were recruited during jaw opening (antagonist activity) in the presence of experimental jaw-muscle pain. Birch et al. (2000) examined SMU in the hand muscles and found no significant effect of experimental muscle pain on the firing characteristics of the SMU at low force levels. In the masseter muscle, we have recently demonstrated that experimental jaw-muscle pain is associated with a decreased firing rate of SMU without changes of recruitment threshold and recruitment order during a low-level isometric contraction (agonist activity) (Sohn et al., 2000). Several possible mechanisms can be suggested to explain the decreased firing rate of masseter SMU in spite of a constant force output. A change in the mechanical properties of SMU could be one likely compensatory mechanism because the muscle force output is modulated, in part, by the frequency of individual SMU discharge in relation to the intrinsic contractile properties of the constitutive fibers of the SMU. Transient changes of mechanical properties especially twitch potentiation have been observed in human limb muscles during fatiguing repeated activation or tetanic stimulation (Bigland-Ritchie et al., 1983; Vandervoort et al., 1983; Sinkjær et al., 1992; Fuglevand et al., 1999; Carpentier et al., 2001) and in masseter during muscle fatigue (Nordstrom and Miles, 1990). In addition, jaw position, internal muscle contraction strategies and muscle co-activation may significantly influence the mechanical properties of SMU in the masseter muscle (McMillan et al., 1990; Turkawski and van Eijden, 2001). However, there are so far no reports on changes of mechanical properties of SMU during muscle pain. Spike-triggered averaging (STA) is one of the most widely used methods for the study of motor unit contractile properties in muscles during voluntary activity and does not need sophisticated stimulation techniques (Milner-Brown et al., 1973a; Yemm, 1977; Nordstrom et al., 1989). The aim of the present study was to describe the mechanical properties of SMU in the human masseter muscle using the STA technique during low-levels of isometric contraction before and during capsaicin-induced experimental jawmuscle pain.
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study. Their history and clinical examination revealed no signs and symptoms of temporomandibular disorders in accordance with standard screening procedures (Dworkin and LeResche, 1992). The study was conducted in accordance with the Helsinki II Declaration, approved by the local Ethics Committee, and written informed consent was obtained from all participants before the experiment. 2.2. Force recordings The subjects were asked to bite on a piezoelectric transducer (Kistler, Type 9211A1, Switzerland) with their incisor teeth. The diameter of the force transducer was 6 mm and the relationship of the teeth to the transducer was kept constant by dental impression material (Blue-Mousse Superfast, Parkell Bio-Materials Division, NY, USA). The signal from the force transducer was amplified and low-pass filtered (300 Hz, Kistler, Type 5011B, Switzerland). The force signal was further high-pass filtered (0.5 Hz, Aalborg University, Denmark) and recorded on magnetic tape (Teac RD-135T, Tokyo, Japan). 2.3. Surface EMG and SMU recordings Bipolar surface EMG recordings were obtained from the bilateral masseter with disposable surface electrodes (72001-K, Medicotest, Ølstykke, Denmark) with a 2 cm inter-electrode distance along the longitudinal axis of the muscle fibers. A ground electrode was placed around the right wrist. Three fine wire electrodes made of Tefloncoated stainless steel (80 mm diameter; Highways International, Baarn, The Netherlands) were inserted into the anterior part of the right masseter muscle with a 23 gauge disposable needle. The needle was withdrawn 2 mm after bony contact and completely removed with the wire electrodes left in the muscle. The position of the wire electrodes was slightly adjusted and two wire electrodes providing the most stable recording condition of a SMU action potential were selected among the 3 inserted wires. The wires were finally secured by tape to the skin. The EMG signals were amplified, bandpass filtered (20 Hz to 5 kHz, CounterPoint MK2, Dantec, Skovlunde, Denmark), and stored on magnetic tape for off-line analysis.
2. Materials and methods 2.4. Experimental jaw-muscle pain 2.1. Subjects Ten healthy, non-medicated individuals (9 men and one woman) with natural dentitions and aged between 20 and 29 years (mean age: 24.7 years) participated. EMG data, interspike intervals (ISI) of SMU and twitch properties were analyzed only in 8 subjects because the selected SMU were lost in two subjects after induction of pain. Eleven other subjects (9 men and two women) aged between 21 and 28 years (mean age: 24.1 years) participated in a control
Experimental pain was induced by injection of 0.2 ml 100 mg/ml capsaicin in the posterior part of the right masseter muscle midway between origin and insertion of the muscle and about 2 cm away from the wire electrodes (Sohn et al., 2000). Injections were done manually over 20 s with the use of a 0.5 ml plastic syringe and a 27 gauge disposable needle. Capsaicin was prepared as described previously (Simone et al., 1991) and diluted with Tween-80 (polyoxyethylene sorbitan monolate) dissolved in isotonic saline.
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In the control experiment, the exact same technique was used to inject 0.2 ml isotonic saline instead of capsaicin. The subjects continuously rated the pain intensity on a 10-cm electronic visual analogue scale (VAS) with the lower extreme (VAS ¼ 0) labeled ‘no pain’ and the upper extreme (VAS ¼ 10) labeled ‘most imaginable pain’. The VAS pain data were sampled once every 5 s and stored on a personal computer. Sampling continued until the pain had disappeared completely. 2.5. Experimental design The subjects were comfortably seated in a dental chair with their incisor teeth on the force transducer and given audio and visual feedback related to the SMU firing rate. During the experiment, all subjects was instructed to perform an isometric contraction in order to keep a steady firing rate of the SMU action potential around 7 –10 Hz, which is close to the lowest constant firing rate obtainable in the masseter muscle, i.e. very low force levels. Force and EMG data were recorded in 1 min epochs before injection (pre-pain), and 1 min (pain-1), 4 min (pain-4), 7 min (pain7), 10 min (pain-10), 15 min (pain-15) and 20 min (pain-20) after injection of capsaicin. The same recording protocol was used in the control experiment. In all experiments special care was taken to ensure that the same SMU action potential throughout the entire experiment. This was accomplished by selecting a specific SMU action potential waveform with the template-matching procedure (see Section 2.6) in addition to visual inspection and audio feedback. Data from two subjects in the capsaicin experiment were discarded because it was not possible
with certainty to identify the same SMU action potential throughout the recording period (n ¼ 8). 2.6. Analysis The analysis was performed off-line from the taped records. SMU action potentials from the wire EMG signal were discriminated using a waveform discriminator (SPS-8701, Signal Processing Systems, Prospect, Australia) based on a template-matching algorithm (Nordstrom and Miles, 1990). A template of one SMU action potential was selected and every time the algorithm recognized this waveform, an event-signal from the SPIKE program (SPS-8701) was fed into another computer. The event signal was then used to demarcate where to cut and average the high-gain force signal, the surface EMG, and the wire EMG from the 1 min recordings. Individual SMU action potentials were accepted with an interval of at least 100 ms to minimize the fusion of twitch forces. Thus, only force records obtained when the same SMU was firing at 10 Hz or less were averaged. The averaged force records (n ¼ 200 – 500 sweeps) were plotted, and the peak twitch force (baseline-to-peak, i.e. the maximal force generated during the contraction), contraction time (the time between the initial rise of a force from the baseline and the time point for the peak twitch force) and half-relaxation time (duration between peak twitch force and the time point where the force is decreased to 50% of the peak twitch force) were measured according to previous descriptions (e.g. Nordstrom et al., 1989; Turkawski et al., 1998). The averaged wire and surface EMG records were also plotted (Fig. 1). The tendency for synchronization of motor units
Fig. 1. Typical examples of mechanical properties in masseter single motor units (SMU) from a single subject in the pre-pain and pain condition. Average triggered responses of SMU recorded with intramuscular (i.m.) wires (A), surface electrodes (B), and twitch responses (C). The amplitude of the twitch force during capsaicin-evoked pain increased compared with the pre-pain recording (C).
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was estimated by comparison of the rectified and nonrectified surface EMG signal (Milner-Brown et al., 1973a). The root-mean-square (RMS) value of a 100 ms window around the action potential was calculated from the rectified and non-rectified surface EMG signal before averaging. The mean ISI and the coefficient of variation (CV ¼ 100 £ SD=mean) of the ISIs in the train spikes from the SMU were measured using a built-in function of the SPS-8701 program and analyzed with POSTPROC software. 2.7. Statistics One-way analysis of variance (ANOVA) for repeated measures was used to describe the effect of condition (pre-pain, pain-1, -4, -7, -10, -15, -20) on the VAS pain scores, twitch properties (peak twitch force, contraction time, half-relaxation time), mean ISIs of the SMU and their firing variability (CV), and RMS values of the surface EMG. If the ANOVA was found significant, it was followed by post-hoc comparisons with the use of Student-Newman-Keuls (SNK) tests to compensate for multiple comparisons. The data are presented as mean values and standard errors (SE) in the figures and the range of EMG and force data are presented in the text. Significance was accepted at P , 0:05.
3. Results 3.1. Experimental jaw-muscle pain Injection of capsaicin into the masseter muscle caused a deep, painful sensation in all subjects. The mean onset and offset of pain was 17.0 ^ 2.0 and 880.5 ^ 96.9 s, respectively, after injection of capsaicin. The VAS pain peak occurred 160.0 ^ 53.4 s after the injection with a mean intensity of 5.5 ^ 0.6 cm. The mean VAS pain scores during pain-1, pain-4 and pain-7 were significantly higher than pain-15 and pain-20 (ANOVA: F ¼ 12:96, P , 0:001, SNK: P , 0:05) (Fig. 2). As expected, injection of isotonic saline was not associated with pain during the recording periods (VAS pain scores ¼ 0). 3.2. Surface EMG and SMU recordings The RMS values of unrectified (range 0.9– 9.4 mV) and rectified (range 3.7 –12.5 mV) averaged surface EMG were not affected by the capsaicin-evoked muscle pain (ANOVA: F , 2:0, P . 0:1) or the control injection of isotonic saline (ANOVA: F , 0:8, P . 0:4) (Fig. 3A,B). Furthermore, the ratio between the RMS of rectified and unrectified surface EMG was not affected by the capsaicin-evoked muscle pain (ANOVA: F ¼ 0:74, P ¼ 0:62) or the control injection (ANOVA: F ¼ 0:73, P ¼ 0:63). These findings indicate that synchronization of SMU was not a problem.
Fig. 2. Mean values ^ SE pain intensity ratings on a visual analogue scale (VAS: 0–10 cm) for 60 s at 1 min (pain-1), 4 min (pain-4), 7 min (pain-7), 10 min (pain-10), 15 min (pain-15) and 20 min (pain-20) produced by injection of capsaicin into the masseter muscle of 8 subjects. * indicates significantly different from pain-15 and pain-20 conditions (SNK: P , 0:05). Control injection of isotonic saline did not evoke any pain.
The mean interspike intervals (ISI) (range 96– 131 ms) during the first part of the painful condition tended to be longer than during the pre-pain condition, however, this was not significant (ANOVA: F ¼ 1:92, P ¼ 0:10) (Fig. 3C). The coefficient of variation (CV) of the ISIs of the SMU did not show any significant difference in the painful condition (ANOVA: F ¼ 0:70, P ¼ 0:65) or in the control condition (ANOVA: F ¼ 0:26, P ¼ 0:94). During both the capsaicin and control experiments the masseter SMU, which were included in the analysis, were stable and demonstrated only minor variability as indicated in Fig. 4. The characteristic waveform strongly suggested that the same SMU was followed during the entire session. Furthermore, the magnitude of the RMS values (mean 5 –6 mV) (Fig. 3B) represents less than 1% of the maximal voluntary surface EMG activity and clearly indicates that low-threshold SMU were studied. 3.3. Mechanical properties of SMU The peak twitch force (range 5– 125 mN) in masseter SMU was significantly influenced by the capsaicin-evoked muscle pain (ANOVA: F ¼ 7:64, P , 0:001) with significantly higher amplitudes during pain-1 and pain-4 compared with the pre-pain and pain-20 conditions after injection of capsaicin (SNK: P , 0:05) (Fig. 3D). However, there was no significant change of the peak twitch force after the non-painful control injection (ANOVA: F ¼ 0:10, P ¼ 0:99) (Fig. 3D). The contraction time (range 26 – 71 ms) and halfrelaxation time (range 25– 61 ms) of the masseter SMU were not significantly influenced by capsaicin-evoked muscle pain (ANOVA: F , 1:0, P . 0:4) or non-painful control injection (ANOVA: F , 1:4, P . 0:2) (Fig. 3E,F).
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Fig. 3. Effects of capsaicin-evoked muscle pain (n ¼ 8) and non-painful injection of isotonic saline (n ¼ 11) on the surface EMG (A,B), interspike intervals (ISIs, C) and twitch profiles (D –F) of single motor units (SMU) in the masseter muscle. * indicates a significant difference between pre-pain and pain-20 conditions after capsaicin injection (SNK: P , 0:05). Data are represented as the mean ^ SE.
4. Discussion The present study has identified significant changes in the mechanical properties of human masseter SMU, which were associated with the capsaicin-evoked pain condition but not the non-painful control condition. Within the constraints of the experimental set-up, this finding points to a pain-related compensatory mechanism analogous to twitch force potentiation observed during fatigue with the purpose to maintain a constant force output.
faster twitch contraction time (34.7 ^ 10.4 ms) under more stringent constriction of the firing pattern parameters. The range of peak twitch forces in the present study (5 –125 mN) was also similar to other studies on low-threshold masseter SMU (Yemm, 1977; McMillan et al., 1990; Nordstrom and
4.1. Mechanical properties of masseter SMU The range of contraction time in the present study (26 –71 ms) was similar to previous reports of twitch force properties in the human masseter muscle (Goldberg and Derfler, 1977; Yemm, 1977; McMillan et al., 1990). However, Nordstrom and Miles (1990) reported slightly
Fig. 4. Examples of waveforms of single motor units (SMU) action potentials during the session with injection of capsaicin and during the control experiment. Note that there is only a minor change in the waveform, which was secured by the template-matching procedure (see Section 2.6).
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Miles, 1990). In comparison, Goldberg and Derfler (1977) reported considerable higher peak twitch forces (11 – 205 g) in a study on high-threshold masseter SMU. Generally, twitch force and contraction times of SMU in the masseter muscle may differ substantially between human studies using the STA technique and animal studies with electrical stimulation (Kwa et al., 1995; Turkawski et al., 1998). In the present study there was an approximately 25-fold difference in the size of the twitch force although low-threshold SMU were studied. This finding is in accordance with data on the variability of mechanical properties of SMU in different parts of the rabbit masseter muscle (Turkawski and van Eijden, 2001). It has subsequently been suggested that the large variability of SMU properties in the masseter muscle may provide a mechanism for a fine gradation of force for example during biting (Turkawski and van Eijden, 2001). 4.2. Spike-triggered averaging Nordstrom and Miles (1990) emphasized that the masseter muscle is particularly well suited for the investigation of twitch forces using the STA technique because it is a short, wide muscle that inserts into bone without a long elastic tendon. In addition, the connection to the force transducer is via the teeth, which are rigidly embedded in the bone and can be reliably fixed in a position so that the point and direction of application of force to the transducer remains constant. These criteria were also fulfilled in the present study and may have eliminated many of the sources of distortion commonly encountered with twitch force estimation in other muscles. In the STA technique, the force signal is normally averaged in response to 250 – 500 action potentials triggered by an identified SMU action potential. This may impose some methodological limitations. Motor unit twitches may be reduced or distorted by partial fusion between twitch responses because SMU can rarely sustain firing below 8 – 12 Hz (Monster and Chan, 1977; Andreassen and Bar-On, 1983; Calancie and Bawa, 1986; Nordstrom et al., 1989). However, recent studies have under well-controlled conditions demonstrated that the values of STA twitch force are not reduced, compared with twitch force obtained by low rate intraneural motor-axon stimulation (Thomas et al., 1990a,b) and intramuscular microstimulation (Kossev et al., 1994). The time parameters of twitch forces should also be considered. A reduction of contraction time and halfrelaxation time with the STA technique compared with stimulation technique has been described in several studies (Monster and Chan, 1977; Nordstrom et al., 1989; Kossev et al., 1994). On the other hand, Thomas et al. (1990b) found that the STA twitch contraction time was longer than that of motor-axon stimulation. Milner-Brown et al. (1973b) did not find a significant difference in the twitch force and contraction time if the twitch responses were averaged when the units fired at mean frequencies of 7 versus 10 Hz in
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the limb muscle. In the masseter muscle, Nordstrom et al. (1989) have shown that a relatively unfused twitch response may be obtained with the STA technique provided that the interspike intervals before and after the selected SMU action potential are carefully selected. This could be due to a sufficiently fast contraction time of masseter SMU to avoid distortion and fusion of the twitch responses. Therefore, the mean firing rate of SMU when averaged twitches were obtained in the present study was approximately 10 Hz or less (Fig. 3C). Another factor that may cause errors in the averaging procedure is synchronization of motor unit activity in a voluntarily active muscle, which may overestimate the twitch forces or prolong the contraction time (Nordstrom et al., 1989; Thomas et al., 1990b). However, Nordstrom et al. (1990) demonstrated that synchronization of masseter SMU is very weak. In the present study, an estimate of the tendency for synchronization of motor units was tested by the comparison of the rectified and unrectified surface EMG activity (Milner-Brown et al., 1973a). There were no indications of synchronization of the masseter SMU in accordance with previous reports (Goldberg and Derfler, 1977; Yemm, 1977).
4.3. Other methodological considerations Fatigue can influence the mechanical properties of SMU. Studies on twitch force of human masseter SMU have demonstrated a broad spectrum of fatigability during a 15 min continuous isometric contraction, i.e. SMU can be associated with both an increase and a decrease in twitch force (Nordstrom and Miles, 1990). In the present study, the isometric contraction was maintained for only 1 min and was followed by at least 2 min rest before the next recording was obtained. It seems unlikely that fatigue may have influenced the mechanical properties of the masseter SMU in the present study because of the short duration and the very low levels of isometric contraction (, 1% of maximal voluntary EMG). Furthermore, in the control experiment with the exact same recording protocol, there was no indication of changes in the mechanical properties of the SMU (Fig. 3D). The level of isometric contraction was determined by the appearance of one SMU action potential, which could clearly be separated from the background noise and other more distant SMU. The template-matching algorithm allowed a small variation in the waveform of the SMU action potential during the discrimination; however, the included SMU changed very little during constant firing rates (Fig. 4). Thus, we do not believe that the present findings in masseter SMU with an increased twitch force during capsaicin-evoked pain can be explained by methodological problems. Various physiological explanations may therefore be considered.
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4.4. Pain-related changes in mechanical properties of SMU A multipennate muscle like the masseter muscle has the potential for multiple lines of action (McMillan and Hannam, 1992) and SMU of the masseter in rabbits demonstrate a large variation in the direction and peak twitch forces in relation to different parts of the muscle (Turkawski et al., 1998; Turkawski and van Eijden, 2000, 2001). Changes of motor unit twitch force by different tasks or direction of effort have also been reported in the human masseter (McMillan et al., 1990). Slightly different internal muscle contraction strategies may be used to carry out the same biting task during painful stimulation. In the present study, the alteration of the vertical twitch force during capsaicin-evoked pain could be related to a subtle shift in the direction of the biting even with the same firing rate of the SMU. Further research with 3-dimensional recordings of twitch forces during painful stimulation may be needed to reveal potential shifts in the direction of the biting. A modification of the interaction between agonist and antagonist EMG activity of the jaw muscles could also conceivably alter the amplitude of twitch forces recorded remotely from the unit, since this would modify the forces recorded at the bite points (McMillan et al., 1990). Increased antagonist EMG activity for muscle pairs during painful contraction has been demonstrated (Graven-Nielsen et al., 1997; Svensson et al., 1997). However, this co-contraction of the antagonist muscles would tend to decrease the twitch forces in the masseter SMU. Therefore, a co-activation of the antagonist jaw-opening muscles in a state of jaw-muscle pain is not a likely mechanism to explain the observed increase in twitch force. Twitch force potentiation has been considered the most probable mechanism for maintaining a constant force output if the firing rate of SMU is decreased (DeLuca et al., 1996). In a recent study, the SMU firing rate in the masseter was decreased without significant change of recruitment threshold during painful contraction in a state of constant force output (Sohn et al., 2000). Therefore, the mechanism of enhancement of the twitch force may be functionally important in order to maintain a stable isometric contraction in conditions with a pain-related decrease in firing rate of the SMU. This mechanism might apply to low-force levels (like in the present study), but may be superseded by other mechanisms at higher force levels (see below). With respect to the observation of post-tetanic twitch force potentiation, the mechanisms are not yet fully understood but could be related to Ca2þ-regulatory mechanisms (Close and Hoh, 1968; Dawson et al., 1980; Gollnick et al., 1991; Grange et al., 1993; MacIntosh and Willis, 2000). However, twitch force potentiation of SMU caused by fatigue has mainly been seen after high-intensity contractions (Bigland-Ritchie et al., 1983; Vandervoort et al., 1983) and is therefore not the most likely explanation for the present findings with increased twitch forces during low-level and short-duration contractions. A recent study described
different mechanical properties of low-threshold and highthreshold SMU in the first dorsal interosseus muscle during fatiguing contractions (Carpentier et al., 2001). The lowthreshold SMU were associated with an increase in twitch force whereas the high-threshold SMU were associated with a decrease in twitch force. Thus, it was suggested on the basis of the different behaviors that motoneuron adaptation and afferent feedback from the muscle during the fatiguing contraction could be involved. In a similar way, nociceptive feedback from the muscle during the isometric contraction could be an important factor for the mechanical properties of masseter SMU. This proposal would fit with the finding that, in particular, the low-threshold SMU may be associated with an increase in twitch force in other conditions. Although the present study demonstrated an increase in twitch force at low-force levels, there is good evidence in the clinical literature that jaw-muscle EMG activity and force records during maximal biting are significantly lower in patients with painful temporomandibular disorders compared to control subjects (e.g. Molin, 1972; Sheikholeslam et al., 1980; Clark et al., 1984; for a review see Svensson and Graven-Nielsen, 2001). This is most likely a protective mechanism to avoid further damage to the tissue and to promote healing and restitution. Human experimental studies have suggested that the reduced capacity to produce a maximum effort is due to the acute effect of pain (Svensson et al., 1998; Wang et al., 2000). Thus, activity in nociceptive muscle afferents has predominately inhibitory effects on the alpha-motoneuron pool of the homonymous muscle during static contractions (Lund and Stohler, 1994). Interestingly, there are also signs of faster neuromuscular fatigue in patients with painful jaw muscles although the muscles do not appear to be in a constant state of fatigue (Gay et al., 1994). Further studies will be necessary to study the interaction between jaw-muscle pain and jaw-muscle fatigue on motor unit behavior at different force levels. In conclusion, the present study has demonstrated a significant increase in the amplitude of the twitch force in low-threshold masseter SMU during painful, gentle biting. This finding could represent a mechanism to compensate for lower discharge rates of SMU during painful contractions at low-force levels and adds new information to the painadaptation model. Acknowledgements This study was conducted at Center for Sensory Motor Interaction, Aalborg University, and supported by The Danish National Research Foundation.
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motor-axon stimulation and spike-triggered averaging. J Neurophysiol 1990b;64:1347–51. Turkawski SJ, van Eijden TM. EMG power spectrum and motor unit characteristics in the masseter muscle of the rabbit. J Dent Res 2000;79: 950– 6. Turkawski SJ, van Eijden TM. Mechanical properties of single motor units in the rabbit masseter muscle as a function of jaw position. Exp Brain Res 2001;138:153–62. Turkawski SJJ, Van Eijden TMGJ, Weijs WA. Force vectors of single motor units in a multipennate muscle. J Dent Res 1998;77: 1823–31.
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Effects of experimental muscle pain on mechanical properties of single motor units in human masseter M.K. Sohna,b, T. Graven-Nielsena, L. Arendt-Nielsen, P. Svenssona,c,* a Center for Sensory-Motor Interaction, Aalborg University, DK-9220 Aalborg, Denmark Department of Rehabilitation Medicine, College of Medicine, Chungnam National University, Daejeon, South Korea c Department of Clinical Oral Physiology, Dental School, University of Aarhus, Vennelyst Boulevard 9, DK-8000 Aarhus C, Denmark b
Accepted 5 September 2003
Abstract Objective: Muscle pain is known to influence muscle activity but the details of its effects on the mechanical properties of single motor units (SMU) have not been described. We have recently reported a decreased firing rate of SMU in the human masseter muscle during painful contractions with a constant force output. Force output can be modulated by the SMU discharge rate in relation to the contractile properties of SMU. Therefore, the objective of the present study was to measure the mechanical properties of SMU in the masseter to clarify the mechanism which underlies the decrease in SMU firing rate during jaw-muscle pain. Methods: A spike-triggered averaging (STA) technique was used to determine the mechanical properties of low-threshold SMU in the masseter muscle recorded with fine wire electrodes during a voluntary isometric contraction. The twitch amplitude, contraction time, and half-relaxation time were determined from the averaged force records before and during experimental jaw-muscle pain induced by injection of 0.2 ml (100 mg/ml) capsaicin in 8 healthy subjects. Injections of 0.2 ml isotonic saline served as a non-painful control in 11 healthy subjects. Results: The twitch amplitude was significantly increased during capsaicin-evoked muscle pain (P , 0:001) without significant changes of half-relaxation time and contraction time. No significant changes in SMU twitch properties were observed during the control injections. Conclusions: Potentiation of twitch force could be a possible compensatory mechanism to maintain a constant force output during painful isometric contractions when SMU firing decreases. This finding therefore provides new information on the adaptation of motor function by muscle pain. q 2004 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. Keywords: Experimental muscle pain; Twitch force; Spike triggered averaging; Single motor unit; Masseter muscle
1. Introduction Patients with chronic painful conditions in the musculoskeletal system often demonstrate distinct changes in the electromyographic (EMG) activity of the involved muscles during movements. Clinically these EMG changes are associated with slower movements and limited range of motion. However, the motor control strategies in relation to pain are not completely understood. Experimental models have during the last decade made it possible to study the influence of acute muscle pain on the interference pattern during various movements (for a review see Lund et al., * Corresponding author. (address c). Tel.: þ45-8942-4191. E-mail address: [email protected] (P. Svensson).
1991; Stohler, 1999; Graven-Nielsen et al., 2000; Svensson and Graven-Nielsen, 2001). In some types of movements the EMG activity in the antagonist phase increases and the EMG activity in the agonist phase consistently decreases during acute muscle pain (Lund et al., 1991; Graven-Nielsen et al., 1997; Madeleine et al., 1999a,b). For example during painful mastication, a decreased EMG activity of the jawclosing muscles in the agonist phase has been found in addition to a slightly increased EMG activity in the antagonist phase (Schwartz and Lund, 1995; Svensson et al., 1996, 1997, 1998; Turp et al., 2002). These EMG findings are generally in accordance with the pain-adaptation model (Lund et al., 1991). However, there is still a considerable gap in the knowledge of how muscle pain may interfere with the activity of single motor units (SMU).
1388-2457/$30.00 q 2004 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/S1388-2457(03)00318-3
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Stohler et al. (1990) showed in a preliminary report that the discharge frequency of SMU in the human masseter muscle increased and additional SMU were recruited during jaw opening (antagonist activity) in the presence of experimental jaw-muscle pain. Birch et al. (2000) examined SMU in the hand muscles and found no significant effect of experimental muscle pain on the firing characteristics of the SMU at low force levels. In the masseter muscle, we have recently demonstrated that experimental jaw-muscle pain is associated with a decreased firing rate of SMU without changes of recruitment threshold and recruitment order during a low-level isometric contraction (agonist activity) (Sohn et al., 2000). Several possible mechanisms can be suggested to explain the decreased firing rate of masseter SMU in spite of a constant force output. A change in the mechanical properties of SMU could be one likely compensatory mechanism because the muscle force output is modulated, in part, by the frequency of individual SMU discharge in relation to the intrinsic contractile properties of the constitutive fibers of the SMU. Transient changes of mechanical properties especially twitch potentiation have been observed in human limb muscles during fatiguing repeated activation or tetanic stimulation (Bigland-Ritchie et al., 1983; Vandervoort et al., 1983; Sinkjær et al., 1992; Fuglevand et al., 1999; Carpentier et al., 2001) and in masseter during muscle fatigue (Nordstrom and Miles, 1990). In addition, jaw position, internal muscle contraction strategies and muscle co-activation may significantly influence the mechanical properties of SMU in the masseter muscle (McMillan et al., 1990; Turkawski and van Eijden, 2001). However, there are so far no reports on changes of mechanical properties of SMU during muscle pain. Spike-triggered averaging (STA) is one of the most widely used methods for the study of motor unit contractile properties in muscles during voluntary activity and does not need sophisticated stimulation techniques (Milner-Brown et al., 1973a; Yemm, 1977; Nordstrom et al., 1989). The aim of the present study was to describe the mechanical properties of SMU in the human masseter muscle using the STA technique during low-levels of isometric contraction before and during capsaicin-induced experimental jawmuscle pain.
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study. Their history and clinical examination revealed no signs and symptoms of temporomandibular disorders in accordance with standard screening procedures (Dworkin and LeResche, 1992). The study was conducted in accordance with the Helsinki II Declaration, approved by the local Ethics Committee, and written informed consent was obtained from all participants before the experiment. 2.2. Force recordings The subjects were asked to bite on a piezoelectric transducer (Kistler, Type 9211A1, Switzerland) with their incisor teeth. The diameter of the force transducer was 6 mm and the relationship of the teeth to the transducer was kept constant by dental impression material (Blue-Mousse Superfast, Parkell Bio-Materials Division, NY, USA). The signal from the force transducer was amplified and low-pass filtered (300 Hz, Kistler, Type 5011B, Switzerland). The force signal was further high-pass filtered (0.5 Hz, Aalborg University, Denmark) and recorded on magnetic tape (Teac RD-135T, Tokyo, Japan). 2.3. Surface EMG and SMU recordings Bipolar surface EMG recordings were obtained from the bilateral masseter with disposable surface electrodes (72001-K, Medicotest, Ølstykke, Denmark) with a 2 cm inter-electrode distance along the longitudinal axis of the muscle fibers. A ground electrode was placed around the right wrist. Three fine wire electrodes made of Tefloncoated stainless steel (80 mm diameter; Highways International, Baarn, The Netherlands) were inserted into the anterior part of the right masseter muscle with a 23 gauge disposable needle. The needle was withdrawn 2 mm after bony contact and completely removed with the wire electrodes left in the muscle. The position of the wire electrodes was slightly adjusted and two wire electrodes providing the most stable recording condition of a SMU action potential were selected among the 3 inserted wires. The wires were finally secured by tape to the skin. The EMG signals were amplified, bandpass filtered (20 Hz to 5 kHz, CounterPoint MK2, Dantec, Skovlunde, Denmark), and stored on magnetic tape for off-line analysis.
2. Materials and methods 2.4. Experimental jaw-muscle pain 2.1. Subjects Ten healthy, non-medicated individuals (9 men and one woman) with natural dentitions and aged between 20 and 29 years (mean age: 24.7 years) participated. EMG data, interspike intervals (ISI) of SMU and twitch properties were analyzed only in 8 subjects because the selected SMU were lost in two subjects after induction of pain. Eleven other subjects (9 men and two women) aged between 21 and 28 years (mean age: 24.1 years) participated in a control
Experimental pain was induced by injection of 0.2 ml 100 mg/ml capsaicin in the posterior part of the right masseter muscle midway between origin and insertion of the muscle and about 2 cm away from the wire electrodes (Sohn et al., 2000). Injections were done manually over 20 s with the use of a 0.5 ml plastic syringe and a 27 gauge disposable needle. Capsaicin was prepared as described previously (Simone et al., 1991) and diluted with Tween-80 (polyoxyethylene sorbitan monolate) dissolved in isotonic saline.
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In the control experiment, the exact same technique was used to inject 0.2 ml isotonic saline instead of capsaicin. The subjects continuously rated the pain intensity on a 10-cm electronic visual analogue scale (VAS) with the lower extreme (VAS ¼ 0) labeled ‘no pain’ and the upper extreme (VAS ¼ 10) labeled ‘most imaginable pain’. The VAS pain data were sampled once every 5 s and stored on a personal computer. Sampling continued until the pain had disappeared completely. 2.5. Experimental design The subjects were comfortably seated in a dental chair with their incisor teeth on the force transducer and given audio and visual feedback related to the SMU firing rate. During the experiment, all subjects was instructed to perform an isometric contraction in order to keep a steady firing rate of the SMU action potential around 7 –10 Hz, which is close to the lowest constant firing rate obtainable in the masseter muscle, i.e. very low force levels. Force and EMG data were recorded in 1 min epochs before injection (pre-pain), and 1 min (pain-1), 4 min (pain-4), 7 min (pain7), 10 min (pain-10), 15 min (pain-15) and 20 min (pain-20) after injection of capsaicin. The same recording protocol was used in the control experiment. In all experiments special care was taken to ensure that the same SMU action potential throughout the entire experiment. This was accomplished by selecting a specific SMU action potential waveform with the template-matching procedure (see Section 2.6) in addition to visual inspection and audio feedback. Data from two subjects in the capsaicin experiment were discarded because it was not possible
with certainty to identify the same SMU action potential throughout the recording period (n ¼ 8). 2.6. Analysis The analysis was performed off-line from the taped records. SMU action potentials from the wire EMG signal were discriminated using a waveform discriminator (SPS-8701, Signal Processing Systems, Prospect, Australia) based on a template-matching algorithm (Nordstrom and Miles, 1990). A template of one SMU action potential was selected and every time the algorithm recognized this waveform, an event-signal from the SPIKE program (SPS-8701) was fed into another computer. The event signal was then used to demarcate where to cut and average the high-gain force signal, the surface EMG, and the wire EMG from the 1 min recordings. Individual SMU action potentials were accepted with an interval of at least 100 ms to minimize the fusion of twitch forces. Thus, only force records obtained when the same SMU was firing at 10 Hz or less were averaged. The averaged force records (n ¼ 200 – 500 sweeps) were plotted, and the peak twitch force (baseline-to-peak, i.e. the maximal force generated during the contraction), contraction time (the time between the initial rise of a force from the baseline and the time point for the peak twitch force) and half-relaxation time (duration between peak twitch force and the time point where the force is decreased to 50% of the peak twitch force) were measured according to previous descriptions (e.g. Nordstrom et al., 1989; Turkawski et al., 1998). The averaged wire and surface EMG records were also plotted (Fig. 1). The tendency for synchronization of motor units
Fig. 1. Typical examples of mechanical properties in masseter single motor units (SMU) from a single subject in the pre-pain and pain condition. Average triggered responses of SMU recorded with intramuscular (i.m.) wires (A), surface electrodes (B), and twitch responses (C). The amplitude of the twitch force during capsaicin-evoked pain increased compared with the pre-pain recording (C).
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was estimated by comparison of the rectified and nonrectified surface EMG signal (Milner-Brown et al., 1973a). The root-mean-square (RMS) value of a 100 ms window around the action potential was calculated from the rectified and non-rectified surface EMG signal before averaging. The mean ISI and the coefficient of variation (CV ¼ 100 £ SD=mean) of the ISIs in the train spikes from the SMU were measured using a built-in function of the SPS-8701 program and analyzed with POSTPROC software. 2.7. Statistics One-way analysis of variance (ANOVA) for repeated measures was used to describe the effect of condition (pre-pain, pain-1, -4, -7, -10, -15, -20) on the VAS pain scores, twitch properties (peak twitch force, contraction time, half-relaxation time), mean ISIs of the SMU and their firing variability (CV), and RMS values of the surface EMG. If the ANOVA was found significant, it was followed by post-hoc comparisons with the use of Student-Newman-Keuls (SNK) tests to compensate for multiple comparisons. The data are presented as mean values and standard errors (SE) in the figures and the range of EMG and force data are presented in the text. Significance was accepted at P , 0:05.
3. Results 3.1. Experimental jaw-muscle pain Injection of capsaicin into the masseter muscle caused a deep, painful sensation in all subjects. The mean onset and offset of pain was 17.0 ^ 2.0 and 880.5 ^ 96.9 s, respectively, after injection of capsaicin. The VAS pain peak occurred 160.0 ^ 53.4 s after the injection with a mean intensity of 5.5 ^ 0.6 cm. The mean VAS pain scores during pain-1, pain-4 and pain-7 were significantly higher than pain-15 and pain-20 (ANOVA: F ¼ 12:96, P , 0:001, SNK: P , 0:05) (Fig. 2). As expected, injection of isotonic saline was not associated with pain during the recording periods (VAS pain scores ¼ 0). 3.2. Surface EMG and SMU recordings The RMS values of unrectified (range 0.9– 9.4 mV) and rectified (range 3.7 –12.5 mV) averaged surface EMG were not affected by the capsaicin-evoked muscle pain (ANOVA: F , 2:0, P . 0:1) or the control injection of isotonic saline (ANOVA: F , 0:8, P . 0:4) (Fig. 3A,B). Furthermore, the ratio between the RMS of rectified and unrectified surface EMG was not affected by the capsaicin-evoked muscle pain (ANOVA: F ¼ 0:74, P ¼ 0:62) or the control injection (ANOVA: F ¼ 0:73, P ¼ 0:63). These findings indicate that synchronization of SMU was not a problem.
Fig. 2. Mean values ^ SE pain intensity ratings on a visual analogue scale (VAS: 0–10 cm) for 60 s at 1 min (pain-1), 4 min (pain-4), 7 min (pain-7), 10 min (pain-10), 15 min (pain-15) and 20 min (pain-20) produced by injection of capsaicin into the masseter muscle of 8 subjects. * indicates significantly different from pain-15 and pain-20 conditions (SNK: P , 0:05). Control injection of isotonic saline did not evoke any pain.
The mean interspike intervals (ISI) (range 96– 131 ms) during the first part of the painful condition tended to be longer than during the pre-pain condition, however, this was not significant (ANOVA: F ¼ 1:92, P ¼ 0:10) (Fig. 3C). The coefficient of variation (CV) of the ISIs of the SMU did not show any significant difference in the painful condition (ANOVA: F ¼ 0:70, P ¼ 0:65) or in the control condition (ANOVA: F ¼ 0:26, P ¼ 0:94). During both the capsaicin and control experiments the masseter SMU, which were included in the analysis, were stable and demonstrated only minor variability as indicated in Fig. 4. The characteristic waveform strongly suggested that the same SMU was followed during the entire session. Furthermore, the magnitude of the RMS values (mean 5 –6 mV) (Fig. 3B) represents less than 1% of the maximal voluntary surface EMG activity and clearly indicates that low-threshold SMU were studied. 3.3. Mechanical properties of SMU The peak twitch force (range 5– 125 mN) in masseter SMU was significantly influenced by the capsaicin-evoked muscle pain (ANOVA: F ¼ 7:64, P , 0:001) with significantly higher amplitudes during pain-1 and pain-4 compared with the pre-pain and pain-20 conditions after injection of capsaicin (SNK: P , 0:05) (Fig. 3D). However, there was no significant change of the peak twitch force after the non-painful control injection (ANOVA: F ¼ 0:10, P ¼ 0:99) (Fig. 3D). The contraction time (range 26 – 71 ms) and halfrelaxation time (range 25– 61 ms) of the masseter SMU were not significantly influenced by capsaicin-evoked muscle pain (ANOVA: F , 1:0, P . 0:4) or non-painful control injection (ANOVA: F , 1:4, P . 0:2) (Fig. 3E,F).
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Fig. 3. Effects of capsaicin-evoked muscle pain (n ¼ 8) and non-painful injection of isotonic saline (n ¼ 11) on the surface EMG (A,B), interspike intervals (ISIs, C) and twitch profiles (D –F) of single motor units (SMU) in the masseter muscle. * indicates a significant difference between pre-pain and pain-20 conditions after capsaicin injection (SNK: P , 0:05). Data are represented as the mean ^ SE.
4. Discussion The present study has identified significant changes in the mechanical properties of human masseter SMU, which were associated with the capsaicin-evoked pain condition but not the non-painful control condition. Within the constraints of the experimental set-up, this finding points to a pain-related compensatory mechanism analogous to twitch force potentiation observed during fatigue with the purpose to maintain a constant force output.
faster twitch contraction time (34.7 ^ 10.4 ms) under more stringent constriction of the firing pattern parameters. The range of peak twitch forces in the present study (5 –125 mN) was also similar to other studies on low-threshold masseter SMU (Yemm, 1977; McMillan et al., 1990; Nordstrom and
4.1. Mechanical properties of masseter SMU The range of contraction time in the present study (26 –71 ms) was similar to previous reports of twitch force properties in the human masseter muscle (Goldberg and Derfler, 1977; Yemm, 1977; McMillan et al., 1990). However, Nordstrom and Miles (1990) reported slightly
Fig. 4. Examples of waveforms of single motor units (SMU) action potentials during the session with injection of capsaicin and during the control experiment. Note that there is only a minor change in the waveform, which was secured by the template-matching procedure (see Section 2.6).
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Miles, 1990). In comparison, Goldberg and Derfler (1977) reported considerable higher peak twitch forces (11 – 205 g) in a study on high-threshold masseter SMU. Generally, twitch force and contraction times of SMU in the masseter muscle may differ substantially between human studies using the STA technique and animal studies with electrical stimulation (Kwa et al., 1995; Turkawski et al., 1998). In the present study there was an approximately 25-fold difference in the size of the twitch force although low-threshold SMU were studied. This finding is in accordance with data on the variability of mechanical properties of SMU in different parts of the rabbit masseter muscle (Turkawski and van Eijden, 2001). It has subsequently been suggested that the large variability of SMU properties in the masseter muscle may provide a mechanism for a fine gradation of force for example during biting (Turkawski and van Eijden, 2001). 4.2. Spike-triggered averaging Nordstrom and Miles (1990) emphasized that the masseter muscle is particularly well suited for the investigation of twitch forces using the STA technique because it is a short, wide muscle that inserts into bone without a long elastic tendon. In addition, the connection to the force transducer is via the teeth, which are rigidly embedded in the bone and can be reliably fixed in a position so that the point and direction of application of force to the transducer remains constant. These criteria were also fulfilled in the present study and may have eliminated many of the sources of distortion commonly encountered with twitch force estimation in other muscles. In the STA technique, the force signal is normally averaged in response to 250 – 500 action potentials triggered by an identified SMU action potential. This may impose some methodological limitations. Motor unit twitches may be reduced or distorted by partial fusion between twitch responses because SMU can rarely sustain firing below 8 – 12 Hz (Monster and Chan, 1977; Andreassen and Bar-On, 1983; Calancie and Bawa, 1986; Nordstrom et al., 1989). However, recent studies have under well-controlled conditions demonstrated that the values of STA twitch force are not reduced, compared with twitch force obtained by low rate intraneural motor-axon stimulation (Thomas et al., 1990a,b) and intramuscular microstimulation (Kossev et al., 1994). The time parameters of twitch forces should also be considered. A reduction of contraction time and halfrelaxation time with the STA technique compared with stimulation technique has been described in several studies (Monster and Chan, 1977; Nordstrom et al., 1989; Kossev et al., 1994). On the other hand, Thomas et al. (1990b) found that the STA twitch contraction time was longer than that of motor-axon stimulation. Milner-Brown et al. (1973b) did not find a significant difference in the twitch force and contraction time if the twitch responses were averaged when the units fired at mean frequencies of 7 versus 10 Hz in
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the limb muscle. In the masseter muscle, Nordstrom et al. (1989) have shown that a relatively unfused twitch response may be obtained with the STA technique provided that the interspike intervals before and after the selected SMU action potential are carefully selected. This could be due to a sufficiently fast contraction time of masseter SMU to avoid distortion and fusion of the twitch responses. Therefore, the mean firing rate of SMU when averaged twitches were obtained in the present study was approximately 10 Hz or less (Fig. 3C). Another factor that may cause errors in the averaging procedure is synchronization of motor unit activity in a voluntarily active muscle, which may overestimate the twitch forces or prolong the contraction time (Nordstrom et al., 1989; Thomas et al., 1990b). However, Nordstrom et al. (1990) demonstrated that synchronization of masseter SMU is very weak. In the present study, an estimate of the tendency for synchronization of motor units was tested by the comparison of the rectified and unrectified surface EMG activity (Milner-Brown et al., 1973a). There were no indications of synchronization of the masseter SMU in accordance with previous reports (Goldberg and Derfler, 1977; Yemm, 1977).
4.3. Other methodological considerations Fatigue can influence the mechanical properties of SMU. Studies on twitch force of human masseter SMU have demonstrated a broad spectrum of fatigability during a 15 min continuous isometric contraction, i.e. SMU can be associated with both an increase and a decrease in twitch force (Nordstrom and Miles, 1990). In the present study, the isometric contraction was maintained for only 1 min and was followed by at least 2 min rest before the next recording was obtained. It seems unlikely that fatigue may have influenced the mechanical properties of the masseter SMU in the present study because of the short duration and the very low levels of isometric contraction (, 1% of maximal voluntary EMG). Furthermore, in the control experiment with the exact same recording protocol, there was no indication of changes in the mechanical properties of the SMU (Fig. 3D). The level of isometric contraction was determined by the appearance of one SMU action potential, which could clearly be separated from the background noise and other more distant SMU. The template-matching algorithm allowed a small variation in the waveform of the SMU action potential during the discrimination; however, the included SMU changed very little during constant firing rates (Fig. 4). Thus, we do not believe that the present findings in masseter SMU with an increased twitch force during capsaicin-evoked pain can be explained by methodological problems. Various physiological explanations may therefore be considered.
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4.4. Pain-related changes in mechanical properties of SMU A multipennate muscle like the masseter muscle has the potential for multiple lines of action (McMillan and Hannam, 1992) and SMU of the masseter in rabbits demonstrate a large variation in the direction and peak twitch forces in relation to different parts of the muscle (Turkawski et al., 1998; Turkawski and van Eijden, 2000, 2001). Changes of motor unit twitch force by different tasks or direction of effort have also been reported in the human masseter (McMillan et al., 1990). Slightly different internal muscle contraction strategies may be used to carry out the same biting task during painful stimulation. In the present study, the alteration of the vertical twitch force during capsaicin-evoked pain could be related to a subtle shift in the direction of the biting even with the same firing rate of the SMU. Further research with 3-dimensional recordings of twitch forces during painful stimulation may be needed to reveal potential shifts in the direction of the biting. A modification of the interaction between agonist and antagonist EMG activity of the jaw muscles could also conceivably alter the amplitude of twitch forces recorded remotely from the unit, since this would modify the forces recorded at the bite points (McMillan et al., 1990). Increased antagonist EMG activity for muscle pairs during painful contraction has been demonstrated (Graven-Nielsen et al., 1997; Svensson et al., 1997). However, this co-contraction of the antagonist muscles would tend to decrease the twitch forces in the masseter SMU. Therefore, a co-activation of the antagonist jaw-opening muscles in a state of jaw-muscle pain is not a likely mechanism to explain the observed increase in twitch force. Twitch force potentiation has been considered the most probable mechanism for maintaining a constant force output if the firing rate of SMU is decreased (DeLuca et al., 1996). In a recent study, the SMU firing rate in the masseter was decreased without significant change of recruitment threshold during painful contraction in a state of constant force output (Sohn et al., 2000). Therefore, the mechanism of enhancement of the twitch force may be functionally important in order to maintain a stable isometric contraction in conditions with a pain-related decrease in firing rate of the SMU. This mechanism might apply to low-force levels (like in the present study), but may be superseded by other mechanisms at higher force levels (see below). With respect to the observation of post-tetanic twitch force potentiation, the mechanisms are not yet fully understood but could be related to Ca2þ-regulatory mechanisms (Close and Hoh, 1968; Dawson et al., 1980; Gollnick et al., 1991; Grange et al., 1993; MacIntosh and Willis, 2000). However, twitch force potentiation of SMU caused by fatigue has mainly been seen after high-intensity contractions (Bigland-Ritchie et al., 1983; Vandervoort et al., 1983) and is therefore not the most likely explanation for the present findings with increased twitch forces during low-level and short-duration contractions. A recent study described
different mechanical properties of low-threshold and highthreshold SMU in the first dorsal interosseus muscle during fatiguing contractions (Carpentier et al., 2001). The lowthreshold SMU were associated with an increase in twitch force whereas the high-threshold SMU were associated with a decrease in twitch force. Thus, it was suggested on the basis of the different behaviors that motoneuron adaptation and afferent feedback from the muscle during the fatiguing contraction could be involved. In a similar way, nociceptive feedback from the muscle during the isometric contraction could be an important factor for the mechanical properties of masseter SMU. This proposal would fit with the finding that, in particular, the low-threshold SMU may be associated with an increase in twitch force in other conditions. Although the present study demonstrated an increase in twitch force at low-force levels, there is good evidence in the clinical literature that jaw-muscle EMG activity and force records during maximal biting are significantly lower in patients with painful temporomandibular disorders compared to control subjects (e.g. Molin, 1972; Sheikholeslam et al., 1980; Clark et al., 1984; for a review see Svensson and Graven-Nielsen, 2001). This is most likely a protective mechanism to avoid further damage to the tissue and to promote healing and restitution. Human experimental studies have suggested that the reduced capacity to produce a maximum effort is due to the acute effect of pain (Svensson et al., 1998; Wang et al., 2000). Thus, activity in nociceptive muscle afferents has predominately inhibitory effects on the alpha-motoneuron pool of the homonymous muscle during static contractions (Lund and Stohler, 1994). Interestingly, there are also signs of faster neuromuscular fatigue in patients with painful jaw muscles although the muscles do not appear to be in a constant state of fatigue (Gay et al., 1994). Further studies will be necessary to study the interaction between jaw-muscle pain and jaw-muscle fatigue on motor unit behavior at different force levels. In conclusion, the present study has demonstrated a significant increase in the amplitude of the twitch force in low-threshold masseter SMU during painful, gentle biting. This finding could represent a mechanism to compensate for lower discharge rates of SMU during painful contractions at low-force levels and adds new information to the painadaptation model. Acknowledgements This study was conducted at Center for Sensory Motor Interaction, Aalborg University, and supported by The Danish National Research Foundation.
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