Indian Journal of Pain

: 2017  |  Volume : 31  |  Issue : 1  |  Page : 41--49

Strength-duration curve: A measure for assessing pain in trapezius spasm

Shivani Chowdhury Salian, Gauri Namdeo Tulsankar 
 Department of Electrotherapy and Electrodiagnosis, School of Physiotherapy, Medical College Building, D.Y. Patil University, Navi Mumbai, Maharashtra, India

Correspondence Address:
Shivani Chowdhury Salian
Department of Electrotherapy and Electrodiagnosis, School of Physiotherapy, 6th Floor, Medical College Building, D.Y. Patil University, Nerul, Sector 7, Navi Mumbai, Maharashtra


Objective: To ascertain the normative values of Strength-duration curve parameters for motor and sensory nerves of trapezius muscle in asymptomatic subjects and compare them with subjects suffering from chronic bilateral trapezius spasm. Methods: A multi-centric cross-sectional study was conducted to compare the Strength-duration curve parameters derived from normal healthy group of subjects (n = 100) and with subjects presenting with bilateral trapezius spasm (n = 100). The subjects were recruited after explaining the procedure in the language best understood by them and a written consent was procured before starting the research. Results: Level of significance was set at P value < 0.05. Rheobase values were found to be lower, whereas, chronaxie, rise time and pulse ratio were increased for both sensory and motor nerve in subjects with trapezius spasm as compared to normal healthy subjects. Discussion: Strength-duration curve parameters can be an important addition to the objective measurement tools of pain assessment and prognosis in musculoskeletal painful conditions. Conclusion: SDC can be effectively used an objective measure of assessment of pain in patients suffering from trapezius spasm. Clinical Implications: On the same lines further studies can be conducted and applied in various neuromuscular conditions.

How to cite this article:
Salian SC, Tulsankar GN. Strength-duration curve: A measure for assessing pain in trapezius spasm.Indian J Pain 2017;31:41-49

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Salian SC, Tulsankar GN. Strength-duration curve: A measure for assessing pain in trapezius spasm. Indian J Pain [serial online] 2017 [cited 2020 Jan 21 ];31:41-49
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The trapezius muscle, which adjusts the movement and location of the scapula, plays an important role in both the stability and movement of the shoulder joints.[1] The motor function is supplied by the spinal accessory nerve (XI), whereas the cutaneous supply is through the ventral rami of C3 and C4 cervical nerves also known as supraclavicular nerve.[2] Trapezius myalgia, which is a chronic pain of the upper trapezius muscle, is a complex and multifactorial condition.[3],[4] Individuals complaining of trapezius myalgia present with sensations of localized muscle pain, tenderness at palpation, stiffness, and constant muscle fatigue [5],[6] in the upper trapezius.

Patients suffering from acute or chronic pain provide valuable opportunities to study the mechanisms of pain and analgesia. The measurement of pain is therefore essential to determine the intensity, perceptual qualities, and time course of the pain so that the differences among pain syndromes can be ascertained and investigated. Furthermore, measurement of these variables provides valuable clues that help in the differential diagnosis of the underlying causes of pain.[7] Thus, if the study of pain in people is to have a scientific foundation, it is essential to measure it. Various subjective methods to assess pain include verbal and numerical self-rating scales, visual analog scales, behavioral observation scales, and physiological responses,[7] while there are even standard objective methods of measuring pain which includes pain pressure threshold with the help of a pressure algometer.

Various authors have carried out studies in the past who have given clinical evidence of use of electrodiagnostic measures such as strength-duration (S-D) properties which show variations in various pathological conditions.[8],[9],[10],[11],[12] The alterations in extracellular electrolyte concentrations in musculoskeletal pain conditions lead to membrane depolarization, leading to an increase in axonal excitability.[13] There are studies even in which the clinicians have used cutaneous electrical stimulation for assessing peripheral nerve function.[14] Studies have also found differences existing between electrophysiological response of sensory and motor fibers. However, not much research has been done to consider similar electrophysiological changes at a painful musculoskeletal site. This formed the basis of our hypothesis of using S-D curve (SDC) parameters as an objective measure of assessing pain.

To begin with, since there was an unavailability of normative range of data for motor and sensory SDC parameter values in the asymptomatic Group 1 of individuals, we had to establish and find a normative range of data in asymptomatic individuals and form a base value which could be compared with the individuals presenting with chronic bilateral trapezius spasm considering the age- and gender-matched criteria, whereby we could ensure our hypothesis of using S-D properties and evaluating their variations.


A multicenter, cross-sectional, prospective study including two age- and gender-matched groups of individuals between the age group 18 and 25 years; Group 1 (n = 100) comprising asymptomatic individuals (Control group) and Group 2 (n = 100) comprising individuals presenting with chronic bilateral trapezius spasm (pain for more than 6 months); was conducted in a period of 1 year. Individuals with unilateral trapezius spasm and open wound over trapezius muscle region, individuals with cervical region pain, individuals with pain radiating to upper limb, individuals with impaired sensation and any dermatological condition were excluded from the study [Figure 1].{Figure 1}

The individuals were made to understand the purpose of this research and their role in the study in the language best understood by them with the help of an information sheet. Informed consent was obtained from all the individuals before enrolling them for the study. The study protocol was passed through the Ethics Committee of D.Y. Patil University, Nerul, Navi Mumbai. Sample size calculation was done based on the pilot study which was approximately 194, and thus, a total of 200 individuals were taken in this study.

Both the groups were tested for pain threshold and pain tolerance over trapezius muscle using a pressure algometer. Pressure pain threshold (PPT) was taken to be the most tender point in over the trapezius and the pressure algometer applied at the sensitive spot, increasing the pressure slowly and continuously (at a rate of 1 kg/s by counting 1/1000, 2/1000) asking the patient to respond by saying “yes” when he/she started to feel discomfort. The pressure at which the individual responds was considered as the pain pressure threshold for that individual (documented in kg). Pressure pain tolerance was the pressure over the skin that was gradually increased further by 1 kg/s until the individual responded by saying “stop.” This point was considered as the pressure tolerance of that individual (documented in kg) [Figure 2] for algometer placement].{Figure 2}

The assessment of SDC parameters for both the group of individuals was carried with the help of a Diagnostic Muscle Stimulator (3, microcontroller based) using interrupted galvanic current. It comprises a unipolar direct current output at different rates and pulse widths. Pulse widths ranging from 0.01 to 300 ms were used. Pulse rate was considered at 1 pulse/s with variable intensity between 0 and 120 V. Before the current was applied, the skin resistance was reduced by washing and soaking in warm water and any abrasions were protected. The reference electrode was placed on the midline of the body (C7) and the active pen electrode over the belly of the trapezius [Figure 3]. For motor threshold, spinal accessory nerve (motor supply for trapezius muscle) was stimulated and current was applied using the longest stimulus first and increased until the minimum observable contraction was obtained. This was visually assessed or by palpation of the tendon. For sensory threshold, supraclavicular nerve (C3, C4) (cutaneous supply over the region of the upper trapezius muscle) was stimulated and current was applied using the longest stimulus first and increased until the minimum sensation was perceived by the individual. In this case, the individual's feedback was important. The magnitude of the current (or voltage) was noted and the impulse was shortened. The procedure was repeated for each length of stimulus in turn, the magnitude of current being increased as required. SDC was plotted from the results of the test. For Group 2 individuals presenting trapezius spasm, visual analog score (VAS) was used to assess pain as subjective measure of pain.{Figure 3}


The data obtained from both the group of individuals were analyzed using Statistical Package for the Social Sciences (SPSS version 16.0, SPSS is software which is used to do statistical analysis. It was acquired by IBM in 2009. IBM SPSS Statistics). Normal ranges of pressure algometric parameters and SDC parameters for both spinal accessory nerve (motor supply for trapezius muscle) and supraclavicular nerve (C3, C4) (cutaneous supply over the region of the upper trapezius muscle), in asymptomatic subjects is represented in [Table 1]. The similar parameters in Group 2 patients presenting with trapezius spasm are represented in [Table 2].{Table 1}{Table 2}

According to the mean values, both pain threshold and pain tolerance were reduced in the individuals with trapezius spasm as compared to normal subjects. Rheobase was reduced in trapezius spasm Group 2 as compared to asymptomatic individuals while chronaxie and rise time were found to be increased in trapezius spasm group for both motor and sensory nerves. Pulse ratio being a nonparametric data, mean ranks was considered in this case. According to the mean ranks, pulse ratio was found to be increased in trapezius spasm group as compared to the asymptomatic group for both the nerves.

To compare the parameters within the same group between the right and left side or between motor and sensory parameters for the parametric data, viz., pain threshold and tolerance values, rheobase, chronaxie, and rise time, paired t-test was used, whereas for the nonparametric data, viz., pulse ratio, Wilcoxon's signed-rank test was used.

To compare the parameters between the trapezius spasm and the asymptomatics for the parametric data, unpaired t-test was used, whereas for the nonparametric data, Mann–Whitney U-test was used. The statistical significance level was chosen as P< 0.05.

Between right and left sides, paired t-test in asymptomatic Group 1 individuals [Table 3]a showed a significant difference in pain threshold, motor rheobase, and sensory rheobase values between the right and left side with a P = 0.00. Similarly, [Table 3]b shows the t values with significance for comparison of parameters in Group 2 patients presenting with trapezius spasm. [Table 1]a represents the normative range of SDC parameters deduced from [Table 1].{Table 3}

Wilcoxon's signed-rank test [Table 4]a and [Table 4]b showed no significant difference in the motor or sensory pulse ratio between the left and the right side in both the groups.{Table 4}

In the asymptomatic Group 1, there was a significant increase in both right and left rheobase in motor nerve than sensory nerve with a P = 0.000 for both, while rise time values were significantly higher in sensory nerve than motor nerve with a significance of 0.007 on the right side and 0.010 on the left. Chronaxie values showed no significant difference. Statistical analysis for asymptomatic group is represented in [Table 5]a. In trapezius spasm group, there was a significant difference in all the parameters between motor and sensory comparison [Table 5]b.{Table 5}

Sensory rheobase values were significantly lower than motor rheobase having P = 0.000 on both sides, whereas sensory chronaxie and sensory rise time values were significantly higher than motor chronaxie and motor rise time, respectively. Sensory chronaxie showed a significant increase on both sides as compared to motor chronaxie with P = 0.004 and 0.005 on the right and left, respectively, while rise time had a P = 0.000 on both sides.

While the asymptomatic group showed no significant difference in both the right- and left-sided pulse ratio between motor and sensory comparison [Table 6]a, there was a significant difference observed in both the right- and left-sided pulse ratio between motor and sensory comparison in the trapezius spasm group [Table 6]b, in which motor pulse ratio was lower as compared to sensory pulse ratio with a P = 0.000 on both sides.{Table 6}

Interclass comparisons

The statistical analysis for comparison of parametric data between the two groups was carried out using unpaired t-test [Table 7].{Table 7}

Pressure algometry parameters

The test showed statistically significant difference between the two groups for both the parameters on the right as well as the left side with P = 0.000. Pain threshold was found to be significantly lower in individuals with trapezius spasm (right 2.3 ± 0.55 kg, left 2.4 ± 0.56 kg) as compared to that of healthy group (right 3.63 ± 1.13 kg, left 3.77 ± 1.11 kg) on both right and left sides. Similarly, pain tolerance was also found to be lower in trapezius spasm group (right 4.14 ± 0.90 kg, left 4.20 ± 0.90 kg) as compared to that of normal (right 7.24 ± 2.70 kg, left 7.40 ± 2.07 kg) bilaterally.

Strength-duration curve parameters

There was a significant difference found in the right as well as left sensory rheobase (P = 0.000). The sensory rheobase was found out to be lower in the trapezius spasm group (right 9.20 ± 3.18 V, left 8.37 ± 3.04 V) as compared to the normal healthy group (right 11.13 ± 2.84 V, left 10.08 ± 2,76 V) on both the sides. The motor rheobase was also found to be significantly lower on the right side (P = 0.001) in the trapezius spasm group (right 13.22 ± 3.96 V) than that of the normal (right 15.00 ± 3.33). There was a significant difference found in the right (P = 0.012) as well as left (P = 0.005) motor chronaxie. The motor chronaxie was found out to be higher in the trapezius spasm group (right 0.018 ± 0.01 ms, left 0.019 ± 0.01 ms) as compared to the normal group (right 0.015 ± 0.006 ms, left 0.015 ± 0.008 ms) on both the sides. The sensory rise time was also found to be significantly higher (P = 0.000) in trapezius spasm group (right 0.57 ± 0.812 ms, left 0.64 ± 0.844 ms) as compared to the normal healthy group (right 0.29 ± 0.529 ms, left 0.30 ± 0.406 ms) on both the sides.

Pulse ratio

Statistical analysis for pulse ratio was done using Mann–Whitney U-test as it was nonparametric data [Table 8].{Table 8}

The results showed a strong significant difference between the two groups in right sensory and left sensory pulse ratio with a P = 0.000, whereas motor parameters did not show significant difference in trapezius spasm group as compared to the normal group. The parameters which showed significant difference, i.e., sensory pulse ratio, were higher in trapezius spasm group (right mean rank = 115.81, left mean rank = 114.58) as compared to that of the normal (right mean rank = 85.19, left mean rank = 86.42) on both the sides.


Age- and gender-matched individuals were considered for the two groups in this study. Each group had 17 males and 83 females. As a subjective measure of pain in the individuals presenting with chronic bilateral trapezius spasm, VAS scores were assessed in which the individuals were asked to rate the pain which was felt during muscle palpation on VAS separately for the right and left side. The mean values of VAS score in the individuals with trapezius spasm were 4.32 ± 1.22 and 4.23 ± 1.29 on the right and left side, respectively.

Intragroup comparison between right and left side strength-duration curve parameters

In the intragroup comparisons, it was found that on comparing right side parameters with the left, respectively, the right motor rheobase was found to be higher as compared to left motor rheobase in both the groups. Friedli and Meyer in their study on assessing sensory deficit in peripheral neuropathy using SDC also experienced similar findings. They stated that sensory thresholds were lower for the left side as compared to the right side of right-handed people in both upper and lower extremities.[15] This asymmetry in the threshold values may be due to the somatosensory pathways and the dominance of cerebral hemispheres. Somatosensory pathways cross the midline in the medulla. Thus, lower sensory threshold on the right side in right-handed individuals reflects greater involvement of right cerebral hemisphere in perceiving stimuli.[16] The present study consisted of a higher number of right-handed people with a number of 198 individuals, while only two individuals were left-handed dominant. Thus, the above findings strongly support our results of a higher motor and sensory rheobase on the right side as compared to left.

Intragroup comparison of strength-duration curve parameters between spinal accessory (motor) and supraclavicular (sensory) nerve

Intraclass comparison between motor and sensory fibers showed significant differences in rheobase and rise time bilaterally in normal, while the trapezius spasm group included alterations in chronaxie and pulse ratio as well. While rheobase values were higher for motor nerve as compared to sensory nerve, the chronaxie, rise time, and pulse ratio were higher in sensory nerve when compared with that of the motor nerve in both the groups. Researchers had proved that sensory fibers showed three times larger average time constants of a local response as compared to the motor fibers when depolarizing conditioning stimuli was applied.[17] There are also studies which have proved that there was a slower recovery from hyperpolarizing pulses as compared to depolarizing pulses in sensory fibers; however, similar membrane time constant was achieved by both motor and sensory fibers.[18] Mogyoros et al. in their study on assessing S-D properties of human peripheral nerve have concluded that the rheobase was lower for sensory fibers than motor fibers, while the time constant (chronaxie) values of sensory fibers were found to be longer than the motor fibers,[19] which highly supports our findings. These findings suggest a greater resting activation or persistent sodium conductance in the sensory fibers, which adds a slow component to the recovery of threshold from hyperpolarizing pulses and increases the S-D time constant (SDTC).[20]

Difference in pressure pain threshold and pressure pain tolerance between the two groups

The pain threshold, as well as the pain tolerance level, in trapezius spasm was significantly lower as compared to that of the normal asymptomatic group with a significant value of 0.000. The difference observed can be mainly because of the disturbance in the mechanism of central pain inhibition.[21] Nociceptors in the muscle can be sensitized and activated by trauma or mechanical overloading, as well as by endogenous inflammatory mediators which include bradykinin, serotonin, and prostaglandin E2.[22] Sensitization of muscle nociceptors by these endogenous mediators can be one of the reasons why individuals with muscle lesions show tenderness to pressure applied on the muscle.[22] Kosek et al. observed a decrease in PPT values following isometric contraction of quadriceps muscle in patients with fibromyalgia, while it increased in healthy subjects.[23] Thus, the above findings highly support the present results of decreased pain threshold and pain tolerance in trapezius spasm individuals as compared to normal subjects.

Difference in sensory rheobase between the two groups

There was a significant decrease observed in sensory rheobase in trapezius spasm group than in normal, apparently healthy subjects. In a study on PPT and electromyography assessment in fatigued muscles by Persson et al., it was observed that there were bilateral increases in the PPT values suggesting that central pain regulating mechanism was involved and is necessary for sensory function in healthy humans [24],[25] as well as in persons with musculoskeletal pain.[23],[26] Frey, in his research, used a very small exploring electrode and showed that there was a punctuate distribution for the response of sensory mechanism in the skin to electrical stimulation. This distribution is also applicable to the pain and touch spots. He further added that the threshold stimulus shows great variations in strength when the pain spots are stimulated on exploring the skin surface systematically.[27] All these findings explain the changes taking place in the muscle and the reasons for differences in the sensory rheobase of SDC in trapezius spasm.

Difference in motor rheobase and motor chronaxie between the two groups

Motor rheobase in trapezius spasm group was also found to be lower as compared to asymptomatic group. However, there was a significant difference found only in right motor rheobase between the two groups with a P = 0.001. The difference in left motor rheobase was not significant (P = 0.06). Another S-D property called SDTC is an apparent membrane time constant inferred from the relationship between threshold current and stimulus duration.[13] In human peripheral nerve, the S-D relationship is described by Weiss's empirical law as follows:

Q = I × t = Irh (t + SDTC)

Where Q = stimulus charge; I = stimulus current of duration t; and Irh = rheobase current.

In the above formulation, SDTC equates to chronaxie.[13] Thus, SDTC and chronaxie, both being the properties of the nodal membrane, are considered to be closely related to each other. There are studies undertaken in which increased occurrence of ragged-red fibers has been found in trapezius myalgia.[28],[29],[30],[31] Many recent studies on trapezius myalgia have been published which have proved that there is a disturbed situation in chronic painful trapezius muscle.[32],[33],[34] They have reported that there were significantly higher levels of interstitial protons, calcitonin gene-related peptide, bradykinin, substance P, tumor necrosis factor-alpha, interleukin1-beta, serotonin, and norepinephrine in individuals presenting with myofascial trapezius pain when compared with normal healthy subjects.[32] As a result of these biochemical and pathological changes in the muscle, there might be alterations in transmission of impulses during muscle contraction. These changes are seen chiefly due to the noninactivating, voltage-dependent Na+ channels which are active even at resting potential.[35] The pain in muscle spasm is also primarily because of ischemia leading to drop in pH and release of pain-producing substances such as bradykinin, adenosine triphosphate, and H+.[22] Thus, when the trapezius muscle was stimulated, the already existing Na+ ions resulted in earlier contraction as compared to normal. Furthermore, rheobase depends on electrical resistance and state of excitability, whereas chronaxie depends on time relations of excitation process.[36] This supports our present study that the painful trapezius muscle already in spasm showed lower motor rheobase and higher motor chronaxie bilaterally when compared with those of the normal subjects.

Difference in sensory pulse ratio and sensory rise time between the two groups

In the present study, both the rise time and the pulse ratio were found to be higher in trapezius spasm group as compared to normal. However, a significant increase was only seen in the sensory rise time and the sensory pulse ratio on both the sides with right sensory rise time having a significant value of 0.003 and with left sensory rise time, right sensory pulse ratio, and left sensory pulse ratio having a P = 0.000. A very limited literature is available on these parameters. However, Khatri, in his book on Basics of Electrotherapy, has stated that the pulse ratio in the innervated muscle is very small, i.e. around 1, but may vary up to 2.2. He further added that very small or no rise in current is required when impulse is reduced from 100 to 1 ms.[37] Thus, from the above information, it can be said that the pulse ratio and the rise time share a linear relationship with each other such that when the rise time is higher, a larger pulse ratio can be obtained and vice versa. In the denervated muscles, amount of current required to produce a contraction is more, and thus, the ratio is also more than 2.5.[37] Thus, both the parameters give us a picture of the slope of the SDC which helps find the response of the nerve to short-duration impulses. When there is depolarization of membrane, as in ischemia, it produces an increase in axonal excitability along with an increase in slope of current-threshold relationship and SDTC.[13] This statement strongly supports our study, thus establishing the importance of rise time and pulse ratio.

Thus, the present study which compared the S-D parameters of motor and sensory fibres between the trapezius spasm and normal, apparently healthy subjects determines that there exists changes in the S-D properties in presence of pain and muscle spasm and that these properties can be used as an objective tool for the measurement of pain and prognosis.


From the present study, it is evident that SDC properties can be effectively used as an important objective tool of assessment and prognosis of pain in patients suffering from chronic bilateral trapezius spasm.

Clinical application

SDC can be used as an objective tool for assessment of musculoskeletal pain, is a safe and feasible method of assessment, is easily producible, and can be performed with an easily accessible instrument in the setup.

The normative data on SDC for spinal accessory and supraclavicular nerve found in young adults in the present study can be used in further research purposes.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.


1Cools AM, Declercq GA, Cambier DC, Mahieu NN, Witvrouw EE. Trapezius activity and intramuscular balance during isokinetic exercise in overhead athletes with impingement symptoms. Scand J Med Sci Sports 2007;17:25-33.
2Jean-Pierre B, Croibier A. The cervical plexus and its branches. In: Wilson C, Bonnett C, editors. Manual Therapy for the Peripheral Nerves. Paris: Elsevier Limited; 2007. p. 91.
3Sjörs A, Larsson B, Dahlman J. Physiological responses to low-force work and psychosocial stress in women with chronic trapezius myalgia. Biomed Cent 2009;10:63.
4Ghafouri N, Ghafouri B, Larsson B, Turkina MV, Karlsson L, Fowler CJ, et al. High levels of n-palmitoylethanolamide and n-stearoylethanolamide in microdialysate samples from myalgic trapezius muscle in women. PLoS One 2011;6:e27257.
5Juul-Kristensen B, Søgaard K, Strøyer J, Jensen C. Computer users' risk factors for developing shoulder, elbow and back symptoms. Scand J Work Environ Health 2004;30:390-8.
6Veiersted KB, Westgaard RH, Andersen P. Electromyographic evaluation of muscular work pattern as a predictor of trapezius myalgia. Scand J Work Environ Health 1993;19:284-90.
7Melzack R, Katz J. Pain assessment in adult patients. In: Melzack R, Wall PD, editors. Wall and Melzack's Textbook of Pain. Vol. 6. Canada: Elsevier Limited; 1984. p. 301.
8Genç G, Bek S, Kasikci T, Ulas UH, Demirkaya S, Odabasi Z. Strength-duration time constant in peripheral nerve: No abnormality in multiple sclerosis. Mult Scler Int 2012;2012:390157.
9Melchiorri G, Salsano ML, Boscarino S, Sorge R. Traditional electrodiagnostic testing in lumbar disc herniation. Funct Neurol 2006;21:83-6.
10Horn S, Quasthoff S, Grafe P, Bostock H, Renner R, Schrank B. Abnormal axonal inward rectification in diabetic neuropathy. Muscle Nerve 1996;19:1268-75.
11Krishnan AV, Phoon RK, Pussell BA, Charlesworth JA, Kiernan MC. Sensory nerve excitability and neuropathy in end stage kidney disease. J Neurol Neurosurg Psychiatry 2006;77:548-51.
12Mogyoros I, Kiernan MC, Burke D. Strength-duration properties of sensory and motor axons in carpal tunnel syndrome. Muscle Nerve 1997;20:508-10.
13Kiernan M, Lin CS. Nerve excitability – A clinical translation. In: Aminoff MJ, editor. Aminoff's Electrodiagnosis in Clinical Neurology. California: Elsevier Limited; 2012. p. 348-64.
14Bourguignon G; Study of general physiology Neuromuscular and Sensitive. Paris: Masson; 1923.
15Friedli WG, Meyer M. Strength-duration curve: A measure for assessing sensory deficit in peripheral neuropathy. J Neurol Neurosurg Psychiatry 1984;47:184-9.
16Friedli WG, Fuhr P, Wiget W. Detection threshold for percutaneous electrical stimuli: Asymmetry with respect to handedness. J Neurol Neurosurg Psychiatry 1987;50:870-6.
17Panizza M, Nilsson J, Roth BJ, Basser PJ, Hallett M. Relevance of stimulus duration for activation of motor and sensory fibers: Implications for the study of H-reflexes and magnetic stimulation. Electroencephalogr Clin Neurophysiol 1992;85:22-9.
18Bostock H, Rothwell JC. Latent addition in motor and sensory fibres of human peripheral nerve. J Physiol 1997;498(Pt 1):277-94.
19Mogyoros I, Lin C, Dowla S, Grosskreutz J, Burke D. Strength-duration properties and their voltage dependence at different sites along the median nerve. Clin Neurophysiol 1999;110:1618-24.
20Kimura J, editor. Somatosensory evoked potential. In: Electrodiagnosis in Diseases of Nerve and Muscle. England: Oxford University Press; 2001. p. 215.
21Buchthal F, Rosenfalck A. Evoked action potentials and conduction velocity in human sensory nerves. Brain Res 1966;3:1-122.
22Mense S. Muscle pain: Mechanisms and clinical significance. Dtsch Arztebl Int 2008;105:214-9.
23Kosek E, Ekholm J, Hansson P. Modulation of pressure pain thresholds during and following isometric contraction in patients with fibromyalgia and in healthy controls. Pain 1996;64:415-23.
24Persson AL, Hansson GA, Kalliomäki A, Moritz U, Sjölund BH. Pressure pain thresholds and electromyographically defined muscular fatigue induced by a muscular endurance test in normal women. Clin J Pain 2000;16:155-63.
25Kosek E, Ekholm J. Modulation of pressure pain thresholds during and following isometric contraction. Pain 1995;61:481-6.
26Persson AL, Hansson GA, Kalliomäki J, Sjölund BH. Increases in local pressure pain thresholds after muscle exertion in women with chronic shoulder pain. Arch Phys Med Rehabil 2003;84:1515-22.
27von Frey M. Contribution to the physiology of pain, Konigl. Sachs. Ges. Wiss., Math. Phys. Class 46; 1894. p. 185-96.
28Larsson SE, Bengtsson A, Bodegård L, Henriksson KG, Larsson J. Muscle changes in work-related chronic myalgia. Acta Orthop Scand 1988;59:552-6.
29Larsson B, Libelius R, Ohlsson K. Trapezius muscle changes unrelated to static work load. Chemical and morphologic controlled studies of 22 women with and without neck pain. Acta Orthop Scand 1992;63:203-6.
30Bengtsson A, Henriksson KG, Larsson J. Muscle biopsy in primary fibromyalgia. Light-microscopical and histochemical findings. Scand J Rheumatol 1986;15:1-6.
31Larsson SE, Bodegård L, Henriksson KG, Oberg PA. Chronic trapezius myalgia. Morphology and blood flow studied in 17 patients. Acta Orthop Scand 1990;61:394-8.
32Shah JP, Phillips TM, Danoff JV, Gerber LH. An in vivo microanalytical technique for measuring the local biochemical milieu of human skeletal muscle. J Appl Physiol 2005;99:1977-84.
33Rosendal L, Larsson B, Kristiansen J, Peolsson M, Søgaard K, Kjaer M, et al. Increase in muscle nociceptive substances and anaerobic metabolism in patients with trapezius myalgia: Microdialysis in rest and during exercise. Pain 2004;112:324-34.
34Rosendal L, Kristiansen J, Gerdle B, Søgaard K, Peolsson M, Kjaer M, et al. Increased levels of interstitial potassium but normal levels of muscle IL-6 and LDH in patients with trapezius myalgia. Pain 2005;119:201-9.
35Mogyoros I, Kiernan MC, Burke D, Bostock H. Strength-duration properties of sensory and motor axons in amyotrophic lateral sclerosis. Brain 1998;121(Pt 5):851-9.
36Buchanan DN, Garven HS. The chronaxie in tetany: The effect on the chronaxie of thyreoparathyreoidectomy, the administration of guanidin and of di-methyl guanidin. J Physiol 1926;62:115-28.
37Khatri S. Basics of Electrotherapy. New Delhi: Jaypee Brothers; 2003.