Abstract
Background and aims
To our knowledge there are no studies that have examined the effects of the experimental pain on muscle fibre excitability as measured by the amplitudes of the potentials evoked by direct muscle stimulation (DMS) in a muscle at rest. We hypothesized that evoked pain can modulate the muscle compound action potential (CMAP) obtained by DMS possibly due to changes in muscle fibre excitability.
Methods
Pain was evoked by intramuscular infusion of hypertonic saline in 50 men. Ten control subjects were infused with isotonic saline. The infusions were given distal to the motor end plate region of the dominant brachial biceps muscle (BBM) in a double-blind manner. The nerve CMAP was obtained by stimulating the musculocutaneous nerve and recording from the BBM using surface-electrodes. Muscle CMAPs were obtained by direct muscle stimulation with subdermal electrodes placed subcutaneously in the distal third of the muscle. A stimuli-response curve of the amplitudes from muscle CMAP was obtained by stimulating from 10 to 90 mA.
Results
There was a decrease of the nerve CMAP amplitudes after infusion of isotonic saline (from 13.78mV to 12.16 mV), p-value 0.0007 and of hypertonic saline (from 13.35 mV to 10.85 mV), p-value 0.0000. The percent decrease from before to after infusion was larger in the hypertonic saline group (19.37%) compared to the isotonic saline group (12.18%), p-value 0.025.
There was a decrease of the amplitudes of the muscle CMAP after infusion of both isotonic (at 90 mA from 13.84mV to 10.32 mV, p value 0.001) and of hypertonic saline (at 90 mA from 14.01 mV to 8.19 mV, p value 0.000). The percent decrease was larger in the hypertonic saline group compared to the isotonic saline group for all the stimulations intensities. At 90 mA we saw a 42% decrease in the hypertonic saline group and 24.5% in the isotonic saline group, p value 0.005.
There were no changes in conduction velocity.
Conclusion
We found a larger amplitude decrease of the muscle and nerve potentials following hypertonic saline infusion compared with that of isotonic saline. We suggest that this deferential outcome of hypertonic saline on muscle CMAP may be linked to the nociceptive effect on muscle fibre membrane excitability.
Implications
The study supplies with some evidence of the peripheral effect of muscle pain. However, further trials with other nociceptive substances such as capsaicin should be performed.
1 Introduction
The underlying neurophysiologic mechanisms of muscle pain remain still a challenge. Both peripheral and central mechanisms have been studied [1,2]. An extensive amount of studies have investigated the peripheral mechanisms of muscle pain evaluating the excitability and conduction velocity of single motor units after induction of experimental muscle pain [3,4]. Methods measuring the M wave amplitude using surface EMG have been applied [5,6,7] and there has not been reported any effect of hypertonic saline on the M wave amplitude. The general shortcoming of these studies has been the fact that since the nociceptive substances were injected deep into the muscle, the muscle fibres, which mostly contributed to the M-wave were probably not reached by the saline. Some later studies have used intramuscular EMG (wire EMG) and the single motor unit technique, usually based on the estimation of delay between two or more signals recorded along the direction of propagation of the action potentials during low level of contraction. Those studies have reported no change in the discharge rate, spike-triggered average twitch torque, action potential propagation velocity, action potential amplitude and duration, and the recruitment threshold of the motor units during low level of contraction [3,4,8] following the injection of nociceptive substances.
Direct muscle stimulation (DMS) introduced by Rich et al. [9] has been used to differentiate myopathy from neuropathy in critical illness where muscle fibre conduction velocities [10] and amplitudes of muscle compound muscle action potential (CMAP) [11] decreased. It has been suggested that amplitudes of the potentials evoked by DMS and [12] could be used as a measure of muscle fibre membrane excitability. In a previous study [13], where pain was evoked by a bolus of hypertonic saline, we found that experimental pain reduced the amplitudes of the interference pattern of the EMG. We found no change of the amplitudes of the CMAP evoked by electrical stimuli of the nerve. We could not exclude that the evoked pain led to a decrease in muscle fibre excitability, and thereby had a peripheral effect on the muscle. To our knowledge there are no studies that have examined the effects of the experimental pain on muscle fibre excitability as measured by the amplitudes of the potentials evoked by DMS in a muscle at rest.
Intramuscular injection of hypertonic saline induces muscle pain and the quality of the induced pain mimics clinical muscle pain [14,15]. Hypertonic saline is a non-specific agent that excites both group III and group VI muscle receptors [16,17]. The highNa+ concentration (5.8%) is considered to be the effective stimulus [18], either by its direct action on the nociceptors [19] or possibly by inducing release of glutamate [20] followed by an excitation of muscle nociceptors.
We hypothesized that following experimental muscle pain, the amplitudes of DMS potentials would decrease due to a decrease in muscle fibre excitability.
2 Materials and methods
We included 50 healthy, non-medicated men in the study. All subjects had no previous history of chronic muscle disease or long-lasting musculoskeletal pain. Subjects were excluded if they reported abuse of drugs, alcohol or if they had a recent history of acute psychiatric disease. Subjects participated in a previously published study [21] concerning differences between strength trained subjects and untrained subjects. Since there was no difference in baseline muscle CMAP amplitudes between trained and untrained subjects, the data from both groups were pooled in the present study.
All subjects gave informed consent to the experimental protocol, which was performed according to the Helsinki Declaration. The Central Denmark Region Committee on Biomedical Research Ethics had no comments to the use of inserted needles and thus approved the project (case number 20070012). Subjects were compensated for their participation in the experiment.
2.1 Neurophysiologic procedures
Subjects lay supine on an examination couch with the dominant arm in the supinated position. All experiments were performed in the same room at a constant room temperature of 22-24°C. The temperature of the skin was kept constant at ≥33 °C and was similar in the hypertonic and isotonic group (Table 1). All neurophysiologic measures were performed by the same examiner (LD). A counterpoint EMG system (Alpine Biomed, Skovlunde, Denmark) was used for all measurement, with a gain of 50 μV per division and sweep speed of 10 ms. per division for recording muscle CMAP. For recording nerve CMAP a gain of 5 mV per division and a sweep speed of 5 ms were used. Sampling rate was 12 kHz. The experiments were performed on the dominant arm of the subjects.
Subject and experiment characteristics.
| Hypertonic saline n 48 | Isotonic saline n 15 | p value | |
|---|---|---|---|
| DMS electrode distance (cm) | 3.04 (0.46) | 2.80 (0.25) | 0.066 |
| Infused saline (ml) | 2.65 (0.32) | 2.57 (0.15) | 0.850 |
| Surface temperature (°C) | 33.73 (0.61) | 23.07 (2.99) | 0.114 |
| BMI (kg/m2) | 23.14 (2.42) | 23.34 (1.96) | 0.617 |
| Age (years) | 23.10 (4.11) | 24.11 (0.82) | 0.263 |
Values represent mean and SD. No significant differences between subjects infused with hypertonic saline leading to pain and isotonic saline as control
2.2 Localization of the motor point region
The motor point was found by using a previously described method [22]. The motor point band was marked by a dotted line and was generally 1-2 cm wide band located approximately in the middle of the biceps muscle. The muscle CMAP stimulation electrodes were placed one cm distal and the respective recording electrodes were placed additionally three cm distal from the stimulation site. Thus, all the measurements were located in the distal part of the muscle (Fig. 1).
Experimental settings. Experiments conducted on dominant brachial biceps muscle: (1) reference electrode; (2) nerve CMAP; (3) muscle CMAP; (4) saline infusion; (5) the motor point.
2.3 Nerve compound muscle action potential recordings
The recording surface electrode (Alpine Biomed. Pre-gelled Surface Electrodes, 9 mm × 6 mm, pick-up area 300 mm2) was placed approximately 2.5 cm distally to the motor point region and the reference electrode placed over the tendon of the brachial biceps muscle (Fig. 1). Both areas were prepared by alcohol swabs and abrasion to lower the skin resistance. The hand-held surface stimulator was placed at the axilla over the musculocutaneous nerve and excited the nerve with a current between 15 mA and 99 mA (0.2 ms, 1.0 Hz). When increasing current of the stimulations failed to increase the amplitude of the potential, the potential was recorded as supramaximal (10% over max current) and stored for analysis. Approximately four stimulations were used to achieve supramaximal recording. Only the supramaximal response was recorded for the analysis. Latencies and amplitudes from peak to peak were set manually and extracted for analysis.
2.4 Action potentials by direct muscle stimulation
For stimulation, a stainless steel sub dermal electrode (Alpine Biomed. Disposable Scalp Needle Electrodes, 10 mm (30G)) was placed sub dermal in the distal third of the muscle, one cm distal to the motor point region. Another sub dermal needle was placed 2 cm laterally as an anode. The recording needle (Alpine Biomed. Disposable Scalp Needle Electrodes 10 mm (30G)) was placed three cm distal from the stimulation site (Table 1). All areas were prepared by alcohol swabs. The reference electrode was the same as in the nerve CMAP settings (Fig. 1).
During a pilot study, we experienced that it was not possible to obtain supramaximal stimulation with the DMS technique in contrast to the nerve CMAP. For that reason we performed a stimulus-response curve by stimulating single pulse stimulus duration 0.5 ms at 10,30,50, 70, and 90 mA followed by the same stimuli with a duration of 1 ms. Latencies and amplitudes from peak to peak were set manually for each recording.
The muscle fibre conduction velocity (CV) was calculated by dividing the distance between the stimulating needles and recording needle with the latency. Moreover, we calculated the ratio between supramaximal nerve CMAP amplitude and amplitude of muscle CMAP recorded following 90 mA stimulation.
2.5 Evoked pain
Forty subjects received hypertonic saline 5.8% and ten subjects received isotonic saline 0.9%. The subjects were matched for age, and there was no difference in body mass index (BMI) between subjects (Table 1 ). The injections were given in a double-blind fashion and as an infusion. Based on pilot studies, a 10-min resting interval was set to reach maximum pain. Afterwards the infusion was kept constant for a 15-min period in order to conduct nerve and muscle CMAP during infusion of saline. The infusion period was therefore 25 min. Then, a 15-min interval was set as time for the pain to decline to baseline after the post injection data was collected.
The saline was infused (pump: Alaris asena™ GH) by the rate of 6 ml per hour. A bolus of 0.2 ml (40 ml per hour) initiated the infusion, to reduce the time to reach the maximal pain intensity. The volume of saline infused was similar in the hypertonic and isotonic group (Table 1 ). The theoretical saline volume could thus be 2.7 ml (0.2 ml + (0.1 ml/min × 25 min)).
We used a cannulated monopolar Teflon-coated hypodermic EMG needle (26Gx 50 mm) with a recording surface of 0.65 mm2 (Medtronic Functional Diagnostics) for intramuscular infusion of saline. The needle was placed lateral and 10-15 mm proximal to the active recording needle and approximately one cm into the muscle so that the saline could diffuse into a region that included the area, where the active electrodes for DMS registration were placed (Fig. 1). The distance between the recording and stimulating electrodes in pain group was slightly larger compared to that in the control group, but this difference was not significant (Table 1).
Output from nerve and muscle CMAP before and after induced pain by hypertonic saline. Raw data from one patient. (A) Upper trace depicts nerve CMAP before induced pain by hypertonic saline following stimulation of 35 mA. Lower trace depicts muscle CMAP following stimulation of 10 mA. (B) Upper trace depicts nerve CMAP after induced pain by hypertonic saline following stimulation of 85 mA. Lower trace depicts muscle CMAP following stimulation of 10mA.
2.6 Pain recording
Pain, due to the infusion of saline was recorded. The subjects recorded the pain intensity continuously on an electronic visual analogue scale (VAS) ranging from 0 (no pain) to 10 (unbearable pain) connected to a computer with a sampling rate of one measurement per second throughout the infusion and the 15 min resting period after the infusion had ended. Mean VAS score and area under the curve (AUC) were calculated.
2.7 Paradigm
The measurements of nerve and muscle CMAP were performed three times: (1) before infusion of saline, (2) during the 15min-period where the infusion was kept constant and (3) 15 min after the infusion was stopped. Latencies and amplitudes were extracted from each stimulation (Fig. 2). We used for further analysis only the data from before and after the infusion, as it was not possible to extract adequate recordings from the during-infusion period.
2.8 Statistics
Values are presented as mean and standard derivation unless otherwise stated. Data were analyzed using non-parametric tests since data were not normally distributed. Mann-Whitney test was performed on unpaired data and on paired data a Wilcoxon test was performed. The comparison of changes from before to after saline induction between groups is done on normalized data by calculation of percentage ((before saline - after saline)/before saline). p values < 0.05 were considered significant unless otherwise stated. (The number of subjects (n) differs due to technical problems in some subjects.)
3 Results
3.1 Evoked pain
Before infusion there were no differences in pain scores between the subjects infused with hypertonic 5.8% (pain group) and isotonic saline 0.9% (control group). Baseline pain was 0.38 (0.52) for the pain group and 0.52 (0.40) for the control group, p-value: 0.066. Subjects infused with hypertonic saline experienced a higher AUC and peak VAS score than the control group. Peak pain after infusion was 3.36 (1.64) for the pain group and 1.45 (1.10) for the control group, p-value: 0.001. Area-under-curve pain after infusion was 3558.43 (2747.92) for the pain group and 1462 (1340.93) for the control group, p-value: 0.005.
3.2 Nerve compound muscle action potential
3.2.1 Nerve CMAP amplitude
Before infusion of saline there were no differences in nerve CMAP amplitudes between the pain group 13.35 mV (4.66) and the control group 13.78 mV (4.13), p-value 0.712.
After infusion of saline the nerve CMAP amplitude decreased to 10.85 mV (4.24) in the pain group, p value (before – after infusion) 0.0000, and in the control group the amplitude decreased to 12.16 mV (4.24), p value (before – after infusion) 0.0007.
The percent decrease was larger in the pain group 19.37% (12.71) compared to the control group 12.18% (10.81), p-value 0.025.
3.2.2 Nerve CMAP latency
Before infusion saline there were no differences in the nerve CMAP latencies between groups.
In the pain group latency was 5.19 m/s (2.58) and in the control group 4.81 m/s (2.31), p-value 0.65. After infusion of saline, the latency was 5.28 m/s (2.45) in the pain group and 5.01 m/s (2.09) in the control group, p-value 0.99.
3.3 Muscle CMAP by direct muscle stimulation
3.3.1 Muscle CMAP amplitude
Before infusion of saline there was no difference in amplitudes between the pain group and the control group. When stimulated with 90 mA the amplitudes were 14.01 mV(4.31) in the pain group and 13.84 mV (6.04) in the control group, p-value 0.846. After infusion there was a decrease of the amplitudes of the muscle CMAP in both groups (Table 2).
Muscle CMAP before and after saline infusion.
| Conduction velocity (m/s) | Amplitude (mV) | |||||
|---|---|---|---|---|---|---|
| Before saline Mean (SD) | After saline Mean (SD) | p value | Before saline Mean | After saline Mean | p value | |
| Hypertonic saline n 48 | ||||||
| 10 mA[*] | 5.87 (2.81) | 5.48 (2.61) | 0.031 | 6.09 (3.92) | 3.61 (2.90) | 0.000 |
| 30 mA[*] | 6.01 (2.67) | 6.05 (2.95) | 0.658 | 8.55 (3.98) | 5.50 (3.57) | 0.000 |
| 50 mA[*] | 6.39 (2.26) | 6.36 (2.16) | 0.906 | 10.30 (4.30) | 6.40 (3.96) | 0.000 |
| 70 mA[*] | 6.64 (1.97) | 6.93 (2.00) | 0.187 | 12.43 (4.49) | 7.41 (3.99) | 0.000 |
| 90 mA[*] | 7.19 (2.30) | 7.20 (1.80) | 0.819 | 14.01 (4.31) | 8.19 (4.18) | 0.000 |
| Isotonic saline n 15 | ||||||
| 10 mA[*] | 5.62 (2.30) | 5.00 (1.69) | 0.798 | 6.65 (4.24) | 5.42 (4.37) | 0.008 |
| 30 mA[*] | 5.62 (1.42) | 5.44 (1.46) | 0.930 | 9.16 (4.21) | 7.68 (4.12) | 0.002 |
| 50 mA[*] | 6.62 (1.91) | 6.44 (2.01) | 0.278 | 10.89 (4.62) | 8.99 (4.70) | 0.003 |
| 70 mA[*] | 6.86 (1.82) | 6.76 (1.66) | 0.687 | 12.39 (4.92) | 9.77 (5.09) | 0.001 |
| 90 mA[*] | 7.46 (1.91) | 7.13 (2.01) | 0.931 | 13.84 (6.04) | 10.32 (5.52) | 0.001 |
Represent mean and SD. Thus indicating that a supramaximal stimulation was not possible, because increasing current lead to increasing amplitude
The percent decrease was larger in the pain group compared to the control group for all the stimulations intensities (Fig. 3). At 90 mA there was a 42% decrease in the pain group and 24.5% in the control group, p value 0.005.
Percentage decrease in muscle CMAP amplitude after saline infusion. Depicts the amplitude decrease in percentage ((before saline – after saline)/before saline) after infusion of saline. A larger decrease is seen in the pain group compared to the control group.
3.3.2 Muscle CMAP CV
Before infusion of saline there was no difference in the CV between groups. When stimulated with 90 mA, CV was 7.19 (2.30) in the pain group and 7.46 (1.91) in the control group, p-value 0.257. After infusion of saline there was no change in CV in both groups and for all stimulations intensities, except when subjects infused with hypertonic saline were stimulated with 10 mA (Table 2).
At this stimulation intensity there was no difference between the percent decrease in CV following hypertonic saline 1.7% (41.2) compared to that following isotonic saline 4.4% (26.3), p-value 0.364.
When the muscle was stimulated from 10 mA to 90 mA both the amplitude and the conduction velocity increased linearly both in the pain group and in the control group (Table 2).
3.4 Nerve/muscle CMAP amplitude ratio
Before infusion of saline there were no differences in nerve/muscle CMAP ratio between the pain group ratio 0.99 (0.29) and the control group ratio 1.14 (0.52), p-value 0.549.
After infusion of saline the nerve/muscle ratio increased to 1.58 (0.84) in the pain group, p value (before – after infusion) 0.0000, and in the control group the ratio increased to 1.42 (0.65) p value (before – after infusion) 0.0038.
There was no difference between the percent increase from before to after infusion between groups. The percent increase in the pain group was 62.86% (68.36) compared to the control group 28.95% (51.17), p-value 0.056.
4 Discussion
In this study we found that injection of saline independent from the concentration reduced significantly the amplitudes of potentials obtained by direct muscle stimulation and by nerve stimulation. The amplitude decrease in both nerve and muscle stimulations were significantly larger in the hypertonic saline group. After infusion of both hypertonic and isotonic saline there was no change in the conduction velocity of the potentials obtained by direct muscle stimulation and no change in latencies of nerve CMAP.
Taken together, these findings may indicate that the effect of hypertonic saline seen on the amplitudes might be on the level of muscle fibres and might have following mechanism: It has been proposed that hypertonic solutions may activate nociceptive primary afferent fibres by shrinking their terminal endings, an effect that results in afferent terminal depolarization by the opening of neuronal stretch-insensitive sodium channels and/or the release of the neuropeptide substance P [23,24] or possibly by inducing release of glutamate [20] Studies have proposed that NMDA-subtype of glutamate receptors might be involved in the regulation of the membrane potential in muscle fibres, most probably through the NO-synthase system [25]. Thus, our findings suggest that the release of nociceptive substances may cause the inability of the muscle fibres to respond to the electric stimulation and thereby causing both nerve CMAP and muscle CMAP amplitude reduction.
These results are in contrast with previous studies using surface and wire EMG and motor unit approach, where there was not found any effect of hypertonic saline on the M wave amplitude or the discharge rate, spike-triggered average twitch torque, action potential amplitude and duration, and the recruitment threshold of the motor units during low level of contraction [3,5,6,7,8]. The explanation for that could be in the methodological differences as the measurements in our study were performed in a resting muscle and not during contraction.
On the other side, our results on muscle fibre CV is in line with several studies that have looked at the effect of pain on muscle fibre conduction velocity measured by means of surface EMG with contradictory results. Falla and Farina [26] found that the average muscle fibre CV of the upper trapezius muscle was higher in people with chronic neck pain and decreased over time during a repetitive upper limb task. Ervilha et al. [6] showed decreased CV of biceps brachia muscle fibres during painful conditions with no change during non-painful condition. While in a later study Falla et al. [27] showed that experimentally induced neck muscle pain resulted in task-dependent changes in cervical agonist/antagonist activity without modifications in muscle fibre CV. Farina et al. [7,28] showed that injection of hypertonic saline did not change muscle fibre conduction velocity or impair neuromuscular transmission.
Another finding of our study is that the amplitudes of the muscle CMAP obtained with the DMS technique increased linearly with the increasing current and a supramaximal response could not be achieved. In studies where DMS has been used to distinguish between critical illness neuropathy and myopathy [9,12], the muscle CMAP amplitudes are reported to be up to 20 mV with different stimulus strength up to 100 mA. The authors stated that the maximal muscle CMAP (note: not supramaximal) could be achieved with stimulus intensities between 50 and 75 mA. Further studies have used the technique in patient populations [11,29,30] but to our knowledge only one other study has reported normal values of the muscle CMAP in relatively large group of healthy controls [11]. The authors did not perform a stimulus response curve as we did in our study. The stimulus response curve clearly showed that it was not possible to obtain a supramaximal response. Thus, caution is warranted when quantitative measurements of the ratio of supramaximal nerve CMAP and muscle CMAP are used since these techniques use different assumptions.
Several aspects of our study need to be addressed:
Hypertonic saline are known to influence membrane excitability. Several mechanism other than nociceptive are proposed such as the hyperpolarizing shifts in resting membrane potential of muscle fibres [31] or the depression of membrane Ca2+ activation and/or enhancement of K+ activation [32].Thus, infusion with hypertonic saline may not be an adequate substance to test membrane excitability as the effect may not be limited to the nociceptive effect. We propose that future studies should include pain evoked by other nociceptive substances such as capsaicin.
We saw a significant decrease in the amplitudes of potentials obtained by direct muscle stimulation following the injection of isotonic saline as well as after injection of hypertonic saline. We believe that this might be explained by the effect of the injected volume through two mechanisms: (1) due to the increase of the distance between muscle fibres and the recording electrodes and (2) due to the low pass filter effect of the fluid injected. The volume of the fluid injected was similar for both hypertonic and isotonic saline. Therefore we believe that the added effect on the decrease of amplitudes after hypertonic saline was due the effect of the hypertonic saline per se at the level of muscle fibres.
We acknowledge that the repeated assessments in our study could have influenced the results. We believe that the blinded nature of the experiments ensured that the possible changes in stimulations and infusion conditions due to the repeated measurements were evenly distributed between the groups and thus had a little impact on the results.
We found a significant increase of the nerve/muscle amplitude ratio from before to after infusion in both groups and this may be explained by the fact that the nerve CMAP amplitude, recorded with surface electrodes, was a summation of the action potentials from all muscle fibres of the brachial biceps muscle, including both those influenced and not influenced by the saline infusion while the muscle CMAP, recorded with subdermal electrodes, was the summation of action potentials generated only from the muscle fibres influenced by the saline induction. Thus, the ratio increased in both groups as the muscle CMAP amplitude decreased more than the nerve CMAP amplitude.
5 Conclusion
We could demonstrate a larger amplitude decrease of the muscle and nerve potentials following hypertonic saline infusion compared with that of isotonic saline. There was no change in muscle fibre conduction velocity. We suggest that this deferential outcome of hypertonic saline on muscle CMAP may be linked to the nociceptive effect on muscle fibre membrane excitability.
6 Implications
The study supplies with some evidence of the peripheral effect of muscle pain. However, further trials with other nociceptive substances such as capsaicin should be performed.
Highlights
Muscle and nerve compound muscle action potential (CMAP) used to evaluate muscle pain.
Pain evoked by hypertonic saline reduced the muscle and nerve stimulation CMAP amplitudes.
Pain possibly has an effect on the muscle fibre excitability.
Evoked pain may have a peripheral effect.
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Author contributions: All authors discussed and commented on the manuscript.
-
Conflict of interest: None declared.
Acknowledgments
This work was supported by grants from the Danish Agency for Science, Technology and Innovation and the Danish Ministry of Culture.
Abbreviations
- AUC
-
area-under-curve
- CV
-
conduction velocity
- CMAP
-
compound muscle action potential
- DMS
-
direct muscle stimulation
- EMG
-
electromyogram
- MUAP
-
motor unit action potential
- MU
-
motor unit
References
[1] Graven-Nielsen T, Lund H, Arendt-Nielsen L, Danneskiold-Samsoe B, Blid-dal H. Inhibition of maximal voluntary contraction force by experimental muscle pain: a centrally mediated mechanism. Muscle Nerve 2002;26: 708–12.Search in Google Scholar
[2] Graven-Nielsen T, Mense S. The peripheral apparatus of muscle pain: evidence from animal and human studies. Clin J Pain 2001;17:2–10.Search in Google Scholar
[3] Farina D, Arendt-Nielsen L, Roatta S, Graven-Nielsen T. The pain-induced decrease in low-threshold motor unit discharge rate is not associated with the amount of increase in spike-triggered average torque. Clin Neurophysiol 2008;119:43–51.Search in Google Scholar
[4] Falla D, Lindstrøm R, Rechter L, Farina D. Effect of pain on the modulation in discharge rate of sternocleidomastoid motor units with force direction. Clin Neurophysiol 2010;121:744–53.Search in Google Scholar
[5] Hodges PW, Ervilha UF, Graven-Nielsen T. Changes in motor unit firing rate in synergist muscles cannot explain the maintenance of force during constant force painful contractions. J Pain 2008;9:1169–74.Search in Google Scholar
[6] Ervilha UF, Farina D, Arendt-Nielsen L, Graven-Nielsen T. Experimental muscle pain changes motor control strategies in dynamic contractions. Exp Brain Res 2005;164:215–24.Search in Google Scholar
[7] Farina D, Arendt-Nielsen L, Graven-Nielsen T. Experimental muscle pain decreases voluntary EMG activity but does not affect the muscle potential evoked by transcutaneous electrical stimulation. Clin Neurophysiol 2005;116:1558–65.Search in Google Scholar
[8] Farina D, Arendt-Nielsen L, Merletti R, Graven-Nielsen T. Effect of experimental muscle pain on motor unit firing rate and conduction velocity. J Neurophysiol 2004;91:1250–9.Search in Google Scholar
[9] Rich MM, Teener JW, Raps EC, Schotland DL, Bird SJ. Muscle is electrically inexcitable in acute quadriplegic myopathy. Neurology 1996;46:731–6.Search in Google Scholar
[10] Allen DC, Arunachalam R, Mills KR. Critical illness myopathy: further evidence from muscle-fibre excitability studies of an acquired channelopathy. Muscle Nerve 2008;37:14–22.Search in Google Scholar
[11] Trojaborg W, Weimer LH, Hays AP. Electrophysiologic studies in critical illness associated weakness: myopathy or neuropathy—a reappraisal. Clin Neurophysiol 2001;112:1586–93.Search in Google Scholar
[12] Rich MM, Bird SJ, Raps EC, McCluskey LF, Teener FW. Direct muscle stimulation in acute quadriplegic myopathy. Muscle Nerve 1997;20:665–73.Search in Google Scholar
[13] Qerama E, Fuglsang-Frederiksen A, Kasch H, Bach FW, Jensen TS. Effects of evoked pain on the electromyogram and compound muscle action potential of the brachial biceps muscle. Muscle Nerve 2005;31:25–33.Search in Google Scholar
[14] GravenNielsen T, Arendt Nielsen L, Svensson P, Jensen TS. Experimental muscle pain: a quantitative study of local and referred pain in humans following injection of hypertonic saline. J Musculoskeletal Pain 1997;5:49–69.Search in Google Scholar
[15] Graven-Nielsen T. Fundamentals of muscle pain, referred pain, and deep tissue hyperalgesia. Scand J Rheumatol Suppl 2006;122:1–43.Search in Google Scholar
[16] Paintal AS. Functional analysis of group III afferent fibres of mammalian muscles. J Physiol 1960;152:250–70.Search in Google Scholar
[17] Mense S. The pathogenesis of muscle pain. Curr Pain Headache Rep 2003;7:419–25.Search in Google Scholar
[18] Hoheisel U, Unger T, Mense S. Sensitization of rat dorsal horn neurons by NGF-induced subthreshold potentials and low-frequency activation. A study employing intracellular recordings in vivo. Brain Res 2007;1169:34–43.Search in Google Scholar
[19] Mense S. Algesic agents exciting muscle nociceptors. Exp Brain Res 2009;196:89–100.Search in Google Scholar
[20] Ro JY, Capra NF, Lee JS, Masri R, Chun YH. Hypertonic saline-induced muscle nociception and c-fos activation are partially mediated by peripheral NMDA receptors. EurJ Pain 2007;11:398–405.Search in Google Scholar
[21] Duez L, Qerama E, Fuglsang-Frederiksen A, Bangsbo J, Jensen TS. Electrophysio-logical characteristics of motor units and muscle fibres intrained and untrained young male subjects. Muscle Nerve 2010;42:177–83.Search in Google Scholar
[22] Qerama E, Fuglsang-Frederiksen A, Kasch H, Bach FW, Jensen TS. Evoked pain in the motor endplate region of the brachial biceps muscle: an experimental study. Muscle Nerve 2004;29:393–400.Search in Google Scholar
[23] Garland A, Jordan JE, Necheles J, Alger LE, Scully MM, Miller RJ, Ray DW, White SR, Solway J. Hypertonicity, but not hypothermia, elicits substance P release from rat C-fibre neurons in primary culture. J Clin Invest 1995;95:2359–66.Search in Google Scholar
[24] Schumacher MA, Moff I, Sudanagunta SP, Levine JD. Molecular cloning of an N-terminal splice variant of the capsaicin receptor. Loss of N-terminal domain suggests functional divergence among capsaicin receptor subtypes. J Biol Chem 2000;275:2756–62.Search in Google Scholar
[25] Malomouzh AI, Mukhtarov MR, Nikolsky EE, Vyskocil F, Lieberman EM, Urazaev AK. Glutamate regulation of non-quantal release of acetylcholine in the rat neuromuscular junction. J Neurochem 2003;85:206–13.Search in Google Scholar
[26] Falla D, Farina D. Muscle fibre conduction velocity of the upper trapezius muscle during dynamic contraction of the upper limb in patients with chronic neck pain. Pain 2005;116:138–45.Search in Google Scholar
[27] Falla D, Farina D, Dahl MK, Graven-Nielsen T. Muscle pain induces task-dependent changes in cervical agonist/antagonist activity. J Appl Physiol 2007;102:601–9.Search in Google Scholar
[28] Farina D, Arendt-Nielsen L, Graven-Nielsen T. Experimental muscle pain reduces initial motor unit discharge rates during sustained submaximal contractions. J Appl Physiol 2005;98:999–1005.Search in Google Scholar
[29] Bednarik J, Lukas Z, Vondracek P. Critical illness polyneuromyopathy: the electrophysiological components of a complex entity. Intensive Care Med 2003;29:1505–14.Search in Google Scholar
[30] Lefaucheur JP, Nordine T, Rodriguez P, Brochard L. Origin of ICU acquired paresis determined by direct muscle stimulation. J Neurol Neurosurg Psychiatry 2006;77:500–6.Search in Google Scholar
[31] Ferenczi EA, Fraser JA, Chawla S, Skepper JN, Schwiening CJ, Huang CL. Membrane potential stabilization in amphibian skeletal muscle fibres in hypertonic solutions. J Physiol 2004;555:423–38.Search in Google Scholar
[32] Suarez-Kurtz G, Sorenson AL. Inhibition by hypertonic solutions of Ca-dependent electrogenesis in single crab muscle fibres. J Gen Physiol 1977;70:491–505.Search in Google Scholar
[33] Schulte E, Ciubotariu A, Arendt-Nielsen L, Disselhorst-Klug C, Rau G, Graven-Nielsen T. Experimental muscle pain increases trapezius muscle activity during sustained isometric contractions of arm muscles. Clin Neurophysiol 2004;115:1767–78.Search in Google Scholar
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