Craniofacial Muscle Pain: Review of Mechanisms and Clinical Manifestations

Epidemiologic surveys of temporomandibular disorders (TMD) have demonstrated that a considerable proportion of the population—up to 5% or 6%—will experience persistent pain severe enough to seek treatment. Unfortunately, the current diagnostic classification of craniofacial muscle pain is based on descriptions of signs and symptoms rather than on knowledge of pain mechanisms. Furthermore, the pathophysiology and etiology of craniofacial muscle pain are not known in sufficient detail to allow causal treatment. Many hypotheses have been proposed to explain cause-effect relationships; however, it is still uncertain what may be the cause of muscle pain and what is the effect of muscle pain. This article reviews the literature in which craniofacial muscle pain has been induced by experimental techniques in animals and human volunteers and in which the effects on somatosensory and motor function have been assessed under standardized conditions. This information is compared to the clinical correlates, which can be derived from the numerous cross-sectional studies in patients with craniofacial muscle pain.

The experimental literature clearly indicates that muscle pain has significant effects on both somatosensory and craniofacial motor function. Typical somatosensory manifestations of experimental muscle pain are referred pain and increased sensitivity of homotopic areas. The craniofacial motor function is inhibited mainly during experimental muscle pain, but phase-dependent excitation is also found during mastication to reduce the amplitude and velocity of jaw movements. The underlying neurobiologic mechanisms probably involve varying combinations of sensitization of peripheral afferents, hyperexcitability of central neurons, and imbalance in descending pain modulatory systems. Reflex circuits in the brain stem seem important for the adjustment of sensorimotor function in the presence of craniofacial pain. Changes in somatosensory and motor function may therefore be viewed as consequences of pain and not factors leading to pain. Implications for the diagnosis and management of persistent muscle pain are discussed from this perspective.

temporomandibular disorders; somatosensory cortex; sensorimotor integration; pain measurement; referred pain mechanisms; mastication; nervous system physiology

1. Zarb GA, Carlsson GE, Rugh JD. Clinical management. In: Zarb GA, Carlsson GE, Sessle BJ, Mohl ND (eds). Temporomandibular Joint and Masticatory Muscle Disorders. Copenhagen: Munksgaard, 1994:529–548.

2. Okeson JP (ed). Orofacial Pain. Guidelines for Assessment, Diagnosis, and Management. Chicago: Quintessence, 1996.

3. Dworkin SF, LeResche L (eds). Research Diagnostic Criteria for Temporomandibular Disorders: Review, Criteria, Examinations and Specifications, Critique. J Craniomandib Disord Facial Oral Pain 1992;6:301–355.

4. Woolf CJ, Bennett GJ, Doherty M, Dubner R, Kidd B, Koltzenburg M, et al. Towards a mechanism-based classification of pain? Pain 1998;77:227–229.

5. Chapman CR, Casey KL, Dubner R, Foley KM, Gracely RH, Reading AE. Pain measurement: An overview. Pain 1985;22:1-31.

6. Gracely RH, Reid KI. Orofacial pain measurement. In: Fricton JR, Dubner R (eds). Advances in Pain Research and Therapy. Vol 21: Orofacial pain and temporomandibular disorders. New York: Raven Press, 1995:117–143.

7. Arendt-Nielsen L. Induction and assessment of experimental pain from human skin, muscle, and viscera. In: Jensen TS, Turner JA, Wiesenfeld-Hallin Z (eds). Progress in Pain Research and Management. Vol 8: Proceedings of the 8th World Congress on Pain. Seattle: IASP Press, 1997:393–425.

8. Svensson P, Arendt-Nielsen L. Experimental models to induce and assess muscle pain. J Electromyogr Kinesiol 1995;5:131–134.

9. Newham DJ, Edwards RHT, Mills KR. Skeletal muscle pain. In: Wall PD, Melzack R (eds). Textbook of Pain. Edinburgh: Churchill Livingstone, 1994:423–440.

10. Vecchiet L, Giamberardino MA, Marini I. Immediate muscular pain from physical activity. In: Tiengo M, Eccles J, Cuello AC, Ottoson D (eds). Advances in Pain Research and Therapy. Vol 10: Pain and mobility. New York: Raven Press, 1987:193–206.

11. Mense S. Nociception from skeletal muscle in relation to clinical muscle pain. Pain 1993;54:241–289.

12. Meyers DE, McCall WD Jr. Head pain as a result of experimental ischemic exercise of the temporalis muscle. Headache 1983;23:113–116.

13. Göbel H, Cordes P. Circadian variation of pain sensitivity in pericranial musculature. Headache 1990;30:418–422.

14. Scott DS, Lundeen TF. Myofascial pain involving the masticatory muscles: An experimental model. Pain 1980;8:207–215.

15. Bowley JF, Gale EN. Experimental masticatory muscle pain. J Dent Res 1987;66:1765–1769.

16. Clark GT, Adler RC, Lee JJ. Jaw pain and tenderness levels during and after repeated sustained maximum voluntary protrusion. Pain 1991;45:17–22.

17. Glaros AG, Tabacchi KN, Glass EG. Effect of parafunctional clenching on TMD pain. J Orofac Pain 1998;12:145–152.

18. Glaros AG, Baharloo L, Glass EG. Effect of parafunctional clenching and estrogen on temporomandibular disorder pain. J Craniomandib Pract 1998;16:78–83.

19. Plesh O, Curtis DA, Hall LJ, Miller A. Gender differences in jaw pain induced by clenching. J Oral Rehabil 1998;25:258–263.

20. Svensson P, Arendt-Nielsen L. Effects of 5 days of repeated submaximal clenching on masticatory muscle pain and tenderness: An experimental study. J Orofac Pain 1996;10:330–338.

21. Arima T, Svensson P, Arendt-Nielsen L. Capsaicin-induced muscle hyperalgesia in the exercised and non-exercised human masseter muscle. J Orofac Pain 2000;14:213–223.

22. Madeleine P, Voigt M, Arendt-Nielsen L. Subjective, physiological and biomechanical responses to prolonged manual work performed standing on hard and soft surfaces. Eur J Appl Physiol 1998;77:1–9.

23. Jensen K, Bülow P, Hansen H. Experimental tooth-clenching in common migraine. Cephalalgia 1985;5:245–251.

24. Jensen R, Olesen J. Initiating mechanisms of experimentally induced tension-type headache. Cephalalgia 1996;16:175–182.

25. Jensen R. Pathophysiological Mechanisms of Tension-type Headache. A Review of Epidemiological and Experimental Studies [thesis]. Copenhagen: Univ of Copenhagen, 1999.

26. Abraham WM. Factors in delayed muscle soreness. Med Sci Sports 1977;9:11–20.

27. Howell JN, Chleboun G, Conatser R. Muscle stiffness, strength loss, swelling and soreness following exercise-induced injury in humans. J Physiol (Lond) 1993;464:183–196.

28. MacIntyre DL, Reid WD, McKenzie DC. Delayed muscle soreness. The inflammatory response to muscle injury and its clinical implications. Sports Med 1995;20:24–40.

29. Round JM, Jones DA, Cambridge G. Cellular infiltrates in human skeletal muscle: Exercise induced damage as a model for inflammatory disease? J Neurol Sci 1987;82:1–11.

30. Brown SJ, Child RB, Day SH, Donnelly AE. Indices of skeletal muscle damage and connective tissue breakdown following eccentric muscle contractions. Eur J Appl Physiol 1997;75:369–374.

31. Fridén J. Muscle soreness after exercise: Implications of morphological changes. Int J Sports Med 1984;5:57–66.

32. Hutchins MO, Skjonsby HS, Brazeau GA, Parikh UK, Jenkins RM. Weakness in mouse masticatory muscles by repetitive contractions with forced lengthening. J Dent Res 1995;74:642–648.

33. Christensen LV. Facial pain and internal pressure of masseter muscle in experimental bruxism in man. Arch Oral Biol 1971;16:1021–1031.

34. Arima T, Svensson P, Arendt-Nielsen L. Experimental grinding in healthy subjects: A model for postexercise jaw muscle soreness. J Orofac Pain 1999;13:104–114.

35. Vecchiet L, Dragani L, Bigontina P, Obletter G, Giamberardino MA. Experimental referred pain and hyperalgesia from muscles in humans. In: Vecchiet L, Albe-Fessard D, Lindblom U (eds). Pain Research and Clinical Management. Vol 7: New trends in referred pain and hyperalgesia.Amsterdam: Elsevier, 1993:239–249.

36. Svensson P, Beydoun A, Morrow TJ, Casey KL. Human intramuscular and cutaneous pain: Psychophysical comparisons. ExpBrain Res 1997;114:390–392.

37. Laursen RJ, Graven-Nielsen T, Jensen TS, Arendt-Nielsen

L. The effect of compression and regional anaesthetic block on referred pain intensity in humans. Pain 1999;80:257–263.

38. Torebjörk HE, Ochoa JL, Schady W. Referred pain from intraneural stimulation of muscle fascicles in the median nerve. Pain1984;18:145–156.

39. Simone DA, Marchettini P, Caputi G, Ochoa JL. Identification of muscle afferents subserving sensation of deep pain in humans. J Neurophysiol 1994;72:883–889.

40. Marchettini P, Simone DA, Caputi G, Ochoa JL. Pain from excitation of identified muscle nociceptors in humans. Brain Res 1996;740:109–116.

41. Simone DA, Marchettini P, Ochoa JL. Primary afferent nerve fibers that contribute to muscle pain sensation in humans. Pain Forum 1997;6:207–212.

42. Fujisawa M, Shoji S, Ishibashi K, Clark GT. Pressure-pain threshold with and without iontophoretic anesthesia of the masseter muscle in asymptomatic males. J Orofac Pain 1999;13:97–103.

43. Reid KI, Carlson C, Rayens MK, Gracely RH. The influence of cutaneous tissue afferents on masticatory pain-pressure thresholds. J Orofac Pain 1996;10:324–329.

44. Graven-Nielsen T, Babenko V, Svensson P, Arendt-Nielsen L. Experimentally-induced muscle pain induces hypoalgesia in heterotopic deep tissues, but not in homotopic deep tissues. Brain Res 1998;787:203–210.

45. Kosek E, Ekholm J, Hansson P. Pressure pain thresholds in different tissues in one body region. The influence of skin sensitivity in pressure algometry. Scand J Rehabil Med 1999;31:89–93.

46. Simons DJ, Day E, Goodell H, Wolff HG. Experimental studies on headache: Muscles of the scalp and neck as sources of pain. Assoc Res Nerv Ment Dis 1943;23: 228–244.

47. Göbel H, Weigle L, Kropp P, Soyka D. Pain sensitivity and pain reactivity of pericranial muscles in migraine and tension-type headache. Cephalalgia 1992;12:142–151.

48. Mense S. Nervous outflow from skeletal muscle following chemical noxious stimulation. J Physiol (Lond) 1977;267:75–88.

49. Kantor TG, Jarvik ME, Wolff BB. Bradykinin as a mediator of human pain. Proc Soc Exp Biol Med 1967;126:505–507.

50. Jensen K, Tuxen C, Pedersen-Bjergaard U, Jansen I, Edvinsson L, Olesen J. Pain and tenderness in human temporal muscle induced by bradykinin and 5-hydroxytryptamine. Peptides 1990;11:1127–1132.

51. Babenko V, Graven-Nielsen T, Svensson P, Drewes A, Jensen TS, Arendt-Nielsen L. Experimental human muscle pain induced by intramuscular injections of bradykinin, serotonin, and substance P.Eur J Pain 1999;3:93–102.

52. Ernberg M, Lundeberg T, Kopp S. Pain and allodynia/hyperalgesia induced by intramuscular injection of serotonin in patients with fibromyalgia and healthy individuals. Pain 2000;85:31–39.

53. Babenko V, Graven-Nielsen T, Svensson P, Drewes AM, Jensen TS, Arendt-Nielsen L. Experimental human muscle pain and muscular hyperalgesia induced by combinations of serotonin and bradykinin. Pain 1999;82:1–8.

54. Mense S, Hoheisel U, Kaske A, Reinert A. Muscle pain: Basic mechanisms and clinical correlates In: Jensen TS, Turner JA, Wiesenfeld-Hallin Z (eds). Progress in Pain Research and Management. Vol 8: Proceedings of the 8th World Congress on Pain. Seattle: IASP Press, 1997:479–496.

55. Cairns BE, Sessle BJ, Hu JW. Evidence that excitatory amino acid receptors within the temporomandibular joint region are involved in the reflex activation of the jaw muscles. J Neurosci 1998;18:8056–8064.

56. Cairns BE, Sessle BJ, Hu JW. Activation of peripheral GABA-A receptors inhibits temporomandibular joint-evoked jaw muscle activity. J Neurophysiol 1999;81:1966–1969.

57. Jensen K, Tuxen C, Pedersen-Bjergaard U, Jansen I. Pain, tenderness wheal and flare induced by substance-P, bradykinin, and 5-hydroxytryptamine in humans. Cephalalgia 1991;11:175–182.

58. Pedersen-Bjergaard U, Nielsen LB, Jensen K, Edvinsson L, Jansen I, Olesen J. Calcitonin gene-related peptide, neurokinin A, and substance P: Effects on nociception and neurogenic inflammation in human skin and temporal muscle. Peptides 1991;12:333–337.

59. Kilo S, Schmelz M, Koltzenburg M, Handwerker HO. Different patterns of hyperalgesia induced by experimental inflammation in human skin. Brain 1994;117:385–396.

60. Witting N, Svensson P, Gottrup H, Arendt-Nielsen L, Jensen TS. Intramuscular and intradermal injection of capsaicin: A comparison of local and referred pain. In: Devor M, Rowbotham MC, Wiesenfeld-Hallin Z (eds). Progress in Pain Research and Management. Vol 16: Proceedings of the 9th World Congress on Pain, Vienna, Austria. Seattle: IASP Press, 2000.

61. Petty BG, Cornblath DR, Adornato BT, Chaudhry V, Flexner C, Wachsman M, et al. The effects of systematically administered recombinant human nerve growth factor in healthy human subjects. Ann Neurol 1994;36:244–246.

62. Stohler CS. Masticatory myalgias. Emphasis on the nerve growth factor–estrogen link. Pain Forum 1997;6:176–180.

63. LeResche L, Saunders K, Von Korff MR, Barlow W, Dworkin SF. Use of exogenous hormones and risk of temporomandibular disorder pain. Pain 1997;69:153–160.

64. Dubner R, Sessle BJ, Storey AT. The Neural Basis of Oral and Facial Function. New York: Plenum Press, 1978.

65. Sessle BJ. The neural basis of temporomandibular joint and masticatory muscle pain. J Orofac Pain 1999;13:238–245.

66. Sessle BJ. Masticatory muscle disorders: Basic science perspectives. In: Sessle BJ, Bryant PS, Dionne RA (eds). Progress in Pain Research and Management. Vol 4: Temporomandibular disorders and related pain conditions. Seattle: IASP Press, 1995:47–61.

67. Sessle BJ. Acute and chronic craniofacial pain: Brain stem mechanisms of nociceptive transmission and neuroplasticity, andtheir clinical correlates. Crit Rev Oral Biol Med 2000;11:57–91.

68. Capra NF, Dessem D. Central connections of trigeminal primary afferent neurons: Topographical and functional considerations. Crit Rev Oral Biol Med 1992;4:1–52.

69. Hannam AG, Sessle BJ. Temporomandibular neurosensory and neuromuscular physiology. In: Zarb GA, Carlsson GE, Sessle BJ, Mohl ND (eds). Temporomandibular Joint and Masticatory Muscle Disorders. Copenhagen: Munksgaard, 1994:67–100.

70. Lund JP, Sessle BJ. Neurophysiological mechanisms. In: Zarb GA, Carlsson GE, Sessle BJ, Mohl ND (eds). Temporomandibular Joint and Masticatory Muscle Disorders. Copenhagen: Munksgaard, 1994:188–207.

71. Broton JG, Sessle BJ. Reflex excitation of masticatory muscles induced by algesic chemicals applied to the temporomandibular joint of the cat. Arch Oral Biol 1988;33:741–747.

72. Hu JW, Sessle BJ, Raboisson P, Dallel R, Woda A. Stimulation of craniofacial muscle afferents induces prolonged facilitatoryeffects in trigeminal nociceptive brainstem neurons. Pain 1992;48:53–60.

73. Hu JW, Yu X-M, Vernon H, Sessle BJ. Excitatory effects on neck and jaw muscle activity of inflammatory irritant applied to cervical paraspinal tissues. Pain 1993; 55:243–250.

74. Yu X-M, Sessle BJ, Hu JW. Differential effects of cutaneous and deep application of inflammatory irritant on mechanoreceptive field properties of trigeminal brain stem nociceptive neurons. J Neurophysiol 1993;70:1704–1707.

75. Yu X-M, Sessle BJ, Vernon H, Hu JW. Effects of inflammatory irritant application to the rat temporomandibular joint on jaw and neck muscle activity. Pain 1995;60:143–149.

76. Yu X-M, Sessle BJ, Haas DA, Izzo A, Vernon H, Hu JW. Involvement of NMDA receptor mechanisms in jaw electromyographic activity and plasma extravasation induced by inflammatory irritant application to temporomandibular joint regions of rats. Pain 1996;68:169–178.

77. Bereiter DA, Benetti AP. Excitatory amino release within spinal trigeminal nucleus after mustard oil injection into the temporomandibular joint region of the rat. Pain 1996;67:451–459.

78. Bakke M, Hu JW, Sessle BJ. Involvement of NK-1 and NK-2 tachykinin receptor mechanisms in jaw muscle activity reflexly evoked by inflammatory irritant application to the rat temporomandibular joint. Pain 1998;75:219–227.

79. Bakke M, Hu JW, Sessle BJ. Morphine application to peripheral tissues modulates nociceptive jaw reflex. Neuroreport 1998;9:3315–3319.

80. Imbe H, Ren K. Orofacial deep and cutaneous tissue inflammation differentially upregulates preprodynorphin mRNA in thetrigeminal and paratrigeminal nucleus of the rat. Brain Res Mol Brain Res 1999;67:87–97.

81. Kellgren JH. Observations on referred pain arising from muscle. Clin Sci 1938;3:175–190.

82. Kellgren JH. On the distribution of pain arising from deep somatic structures with charts of segmental pain areas. Clin Sci 1939;4:36–46.

83. Lewis T, Kellgren JH. Observations relating to referred pain, visceromotor reflexes and other associated phenomena. Clin Sci 1939;4:47–71.

84. Stohler CS, Zhang X, Ashton-Miller JA. An experimental model of jaw muscle pain in man. In: Davidovitch D (ed). The Biological Mechanisms of Tooth Movement and Craniofacial Adaptation. Columbus: Ohio State University College of Dentistry, 1992:503–511.

85. Lewis T. Suggestions relating to the study of somatic pain. Br Med J 1938;12:321–325.

86. Inman VT, Saunders JB. Referred pain from skeletal structures. J Nerv Ment Dis 1944;99:660–667.

87. Harman JB. The localization of deep pain. Br Med J 1948;31:188–192.

88. Feinstein B, Langton JNK, Jameson RM, Schiller F. Experiments on pain referred from deep somatic tissues. J Bone Joint Surg [Am] 1954;36:981–997.

89. Wolff BB, Potter JL, Vermeer WL, McEwen C. Quantitative measures of deep somatic pain: Preliminary study with hypertonic saline. Clin Sci 1961;20:345–350.

90. Jarvik ME, Wolff BB. Differences between deep pain responses to hypertonic and hypotonic saline solutions. J Appl Physiol1962;17:841–843.

91. Wolff BB, Jarvik ME. Variations in cutaneous and deep somatic pain sensitivity. Can J Psychol 1963;17:37–44.

92. Wolff BB, Jarvik ME. Relationship between superficial and deep somatic thresholds of pain with a note on handedness. Am J Psychol 1964;77:589–599.

93. Wolff BB, Jarvik ME. Quantitative measures of deep somatic pain: Further studies with hypertonic saline. Clin Sci 1965;28:43–56.

94. Hockaday JM, Whitty CWM. Patterns of referred pain in the normal subject. Brain 1967;90:481–496.

95. Vecchiet L, Galleti R, Giamberardino MA, Dragani L, Marini F. Modifications of cutaneous, subcutaneous and muscular sensory and pain thresholds after the induction of an experimental algogenic focus in the skeletal muscle. Clin J Pain 1988;4:55–59.

96. Jensen K, Norup M. Experimental pain in human temporal muscle induced by hypertonic saline potassium and acidity. Cephalalgia 1992;12:101–106.

97. Veerasarn P, Stohler CS. The effect of experimental muscle pain on the background electrical brain activity. Pain 1992;49:349–360.

98. Stohler CS, Zhang X, Lund JP. The effect of experimental jaw muscle pain on postural activity. Pain 1996;66: 215–221.

99. Stohler CS, Lund JP. Effects of noxious stimulation of the jaw muscles on the sensory experience of volunteer human subjects. In: Stohler CS, Carlson DS (eds). Craniofacial Growth Series. Vol 29: Biological and psychological aspects of orofacial pain. Ann Arbor: The University of Michigan Center for Human Growth and Development, 1994:55–73.

100. Stohler CS, Lund JP. Psychophysical and orofacial motor responses to muscle pain: Validation and utility of an experimental model. In: Morimoto T, Matsuya T, Takada K (eds). Brain and Oral Function. Amsterdam: Elsevier, 1995:227–237.

101. Arendt-Nielsen L, Graven-Nielsen T, Svarrer H, Svensson

P. The influence of low back pain on muscle activity and coordination during gait: A clinical and experimental study. Pain 1996;64:231–240.

102. Obrez A, Stohler CS. Jaw muscle pain and its effect on gothic arch tracings. J Prosthet Dent 1996;75:393–398.

103. Graven-Nielsen 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 Musculoskel Pain 1997;5:49–69.

104. Graven-Nielsen T, Arendt-Nielsen L, Svensson P, Jensen TS. Stimulus-response functions in areas with experimentally induced referred muscle pain: A psychophysical study. Brain Res 1997;744:121–128.

105. Graven-Nielsen T, Arendt-Nielsen L, Svensson P, Jensen TS. Quantification of local and referred muscle pain in humans after sequential i.m. injections of hypertonic saline. Pain 1997;69:111–117.

106. Graven-Nielsen T, Svensson P, Arendt-Nielsen L. Effects of experimental muscle pain on muscle activity and co-ordination during static and dynamic motor function. Electroencephalogr Clin Neurophysiol 1997;105:156–164.

107. Graven-Nielsen T, McArdle A, Phoenix J, Arendt-Nielsen L, Jensen TS, Jackson MJ, Edwards RHT. In vivo model of muscle pain: Quantification of intramuscular chemical, electrical, and pressure changes associated with saline-induced muscle pain in humans. Pain 1997;69:137–143.

108. Graven-Nielsen T, Fenger-Grøn LS, Svensson P, Steengaard-Pedersen K, Arendt-Nielsen L, Jensen TS. Quantification of deep and superficial sensibility in saline-induced muscle pain: A psychophysical study. Somatosens Mot Res 1998;15:46–53.

109. Gerber C, Galantay RV, Hersche O. The pattern of pain produced by irritation of the acromioclavicular joint and the subacromial space. J Shoulder Elbow Surg 1998;7:352–355.

110. Madeleine P, Lundager B, Voigt M, Arendt-Nielsen L. Sensory manifestations in experimental and work-related chronic neck-shoulder pain. Eur J Pain 1998;2:251–260.

111. Madeleine P, Lundager B, Voigt M, Arendt-Nielsen L. Shoulder muscle co-ordination during chronic and acute experimental neck-shoulder pain. An occupational pain study. Eur J Appl Physiol 1999;79:127–140.

112. Stohler CS, Kowalski CJ. Spatial and temporal summation of sensory and affective dimensions of deep somatic pain. Pain 1999;79:165–173.

113. Svensson P, Arendt-Nielsen L, Nielsen H, Larsen JK. Effect of chronic and experimental jaw muscle pain on pressure-pain thresholds and stimulus-response curves. J Orofac Pain 1995;9:347-356.

114. Svensson P, Arendt-Nielsen L, Houe L. Sensory-motor interactions of human experimental unilateral jaw muscle pain: A quantitative analysis. Pain 1996;64:241–249.

115. Svensson P, Houe L, Arendt-Nielsen L. Bilateral experimental muscle pain changes electromyographic activity of human jaw-closing muscles during mastication. Exp Brain Res 1997;116:182–185.

116. Svensson P, Arendt-Nielsen L, Houe L. Muscle pain modulates mastication: An experimental study in humans. J Orofac Pain 1998;12:7–16.

117. Svensson P, Graven-Nielsen T, Arendt-Nielsen L. Mechanical hyperesthesia of human facial skin induced by tonic painful stimulation of jaw-muscles. Pain 1998;74:93–100.

118. Svensson P, Graven-Nielsen T, Matre D, Arendt-Nielsen

L. Experimental muscle pain does not cause long-lasting increases in resting electromyographic activity. Muscle Nerve 1998;21:1382–1389.

119. Svensson P, De Laat A, Graven-Nielsen T, Arendt-Nielsen

L. Experimental jaw-muscle pain does not change heteronymous H-reflexes in the human temporalis muscle. Exp Brain Res 1998;121:311–318.

120. Svensson P, McMillan AS, Graven-Nielsen T, Wang K, Arendt-Nielsen L. Modulation of an inhibitory reflex in single motor units in human masseter by tonic painful stimulation. Pain 1999;83:441–446.

121. Wang K, Svensson P, Arendt-Nielsen L. Modulation of exteroceptive suppression periods in human jaw-closing muscles by local and remote experimental muscle pain. Pain 1999;82:253–262.

122. Wang K, Arima T, Arendt-Nielsen L, Svensson P. EMG-force relationships are influenced by experimental jaw-muscle pain. J Oral Rehabil 2000;27:394–402.

123. Zhang X, Ashton-Miller JA, Stohler CS. A closed-loop system for maintaining constant experimental muscle pain in man. IEEE Trans Biomed Eng 1993;40:344–352.

124. Graven-Nielsen T. Sensory Manifestations and Sensory-Motor Interactions During Experimental Muscle Pain in Man [thesis]. Aalborg, Denmark: Aalborg University, 1997.

125. Iggo A. Non-myelinated afferent fibres from mammalian skeletal muscle. Proc Physiol Soc 1961;155:52–53.

126. Paintal AS. Functional analysis of group III afferent fibres of mammalian muscles. J Physiol (Lond) 1960;152: 250–270.

127. Foreman RD, Schmidt RF, Willis WD. Convergence of muscle and cutaneous input onto primate spinothalamic tract neurons. Brain Res 1977;124:555–560.

128. Foreman RD, Schmidt RF, Willis WD. Effects of mechanical and chemical stimulation of fine muscle afferents upon primate spinothalamic tract cells. J Physiol (Lond) 1979;286:215–231.

129. Amano N, Hu JW, Sessle BJ. Responses of neurons in feline trigeminal subnucleus caudalis (medullary dorsal horn) to cutaneous, intraoral, and muscle afferent stimuli. J Neurophysiol 1986;55:227–243.

130. Kawakita K, Dostrovsky JO, Tang JS, Chiang CY. Responses of neurons in the rat thalamic nucleus submedius to cutaneous,muscle and visceral nociceptive stimuli. Pain 1993;55:327–338.

131. Laursen RJ, Graven-Nielsen T, Jensen TS, Arendt-Nielsen

T. The effect of differential and complete nerve block on experimental muscle pain in humans. Muscle Nerve 1999;22:1564–1570.

132. Sjøgaard G, Kiens B, Jørgensen K, Saltin B. Intramuscular pressure, EMG and blood flow during low-level prolonged static contraction in man. Acta Physiol Scand 1986;128:475–484.

133. Møller E, Rasmussen OC, Bonde-Petersen F. Mechanisms of ischemic pain in human muscles of mastication: Intramuscular pressure, EMG, force and blood flow of the temporal and masseter muscles during biting. In: Bonica JJ, Liebeskind JC, Albe-Fessard DG (eds). Advances in Pain Research and Therapy. Vol 3: Proceedings of the Second World Congress on Pain. New York: Raven Press, 1979:271–281.

134. Lupton DE. Psychological aspects of temporomandibular joint dysfunction. J Am Dent Assoc 1969;79:131–136.

135. Steinbrocker O, Isenberg SA, Silver M, Neustadt D, Kuhn P, Schittone M. Observations on pain produced by injection of hypertonic saline into muscles and other supportive tissues. J Clin Inv 1953;32:1045–1051.

136. Graven-Nielsen T, Gibson S, Svensson P, Arendt-Nielsen

L. Sensitivity to pressure stimuli in capsaicin-induced referred pain areas [abstract]. Proceedings from the Third Congress of European Federation of IASP Chapters, 2000:213.

137. Arendt-Nielsen L, Graven-Nielsen T, Svensson P, Jensen TS. Temporal summation in muscles and referred pain areas: An experimental human study. Muscle Nerve 1997;20:1311–1313.

138. Hoheisel U, Mense S, Simons DG, Yu X-M. Appearance of new receptive fields in rat dorsal horn neurons following noxious stimulation of skeletal muscle: A model for referral of muscle pain? Neurosci Lett 1993;153:9–12.

139. Sharav Y, McGrath P, Dubner R. Masseter inhibitory periods and sensations evoked by electrical tooth pulp stimulation in patients with oral-facial pain and dysfunction. Arch Oral Biol 1982;27:305–310.

140. Davidson RM, Gale EN. Cutaneous sensory thresholds from skin overlying masseter and forearm in MPD patients and controls. J Dent Res 1983;62:555–558.

141. Hagberg C, Hellsing G, Hagberg M. Perception of cutaneous electrical stimulation in patients with craniomandibular disorders. J Craniomandib Disord Facial Oral Pain 1990;4:120–125.

142. Hollins M, Sigurdsson A, Fillingim L, Goble AK. Vibrotactile threshold is elevated in temporomandibular disorders. Pain 1996;67:89–96.

143. Hollins M, Sigurdsson A. Vibrotactile amplitude and frequency discrimination in temporomandibular disorders. Pain 1998;75:59–67.

144. Fillingim RB, Fillingim LA, Hollins M, Sigurdsson A, Maixner W. Generalized vibrotactile allodynia in a patient with temporomandibular disorder. Pain 1998;78:75–78.

145. Maixner W, Fillingim R, Sigurdsson A, Kincaid S, Silva S. Sensitivity of patients with painful temporomandibular disorders to experimentally evoked pain: Evidence for altered temporal summation of pain. Pain 1998;76:71–81.

146. Coderre TJ, Katz J, Vaccarino AL, Melzack R. Contribution of central neuroplasticity to pathological pain: Review of clinical and experimental evidence. Pain 1993;52:259–285.

147. Sörensen J, Graven-Nielsen T, Henriksson K-G, Bengtsson M, Arendt-Nielsen L. Hyperexcitability in fibromyalgia. J Rheumatol 1998;25:152–155.

148. Price DD, Harkins SW. Combined use of experimental pain and visual analogue scales in providing standardized measurement of clinical pain. Clin J Pain 1987;3:1–8.

149. Langemark M, Jensen K, Jensen TS, Olesen J. Pressure pain thresholds and thermal nociceptive thresholds in chronic tension-type headache. Pain 1989;38:203–210.

150. Arroyo JF, Cohen ML. Abnormal responses to electrocutaneous stimulation in fibromyalgia. J Rheumatol 1993;20:1925–1931.

151. Vecchiet L, Giamberardino MA, de Bigontina P, Dragani

L. Comparative sensory evaluation of parietal tissues in painful and nonpainful areas in fibromyalgia and myofascial pain syndrome. In: Gebhart GF, Hammond DL, Jensen TS (eds). Progress in Pain Research and Management. Vol 2: Proceedings of the 7th World Congress on Pain. Seattle: IASP Press, 1994:177–185.

152. Bendtsen L, Jensen R, Olesen J. Decreased pain detection and tolerance thresholds in chronic tension-type headache. Arch Neurol 1996;53:373–376.

153. Maixner W, Fillingim R, Booker D, Sigurdsson A. Sensitivity of patients with painful temporomandibular disorders to experimentally evoked pain. Pain 1995;63:341–351.

154. Maixner W, Fillingim RB, Kincaid S, Sigurdsson A, Harris MB. Relationship between pain sensitivity and resting arterial blood pressure in patients with painful temporomandibular disorders. Psychosom Med 1997;59:503–511.

155. McMahon SB, Bennett DLH, Michael GJ, Priestley JV. Neurotrophic factors and pain. In: Jensen TS, Turner JA, Wiesenfeld-Hallin Z (eds). Progress in Pain Research and Management. Vol 8: Proceedings of the 8th World Congress on Pain. Seattle: IASP Press, 1997:353–379.

156. Reinert A, Kaske A, Mense S. Inflammation-induced increase in the density of neuropeptide-immunoreactive nerve endings in rat skeletal muscle. Exp Brain Res 1998;121:174–180.

157. Jensen K. Quantification of tenderness by palpation and use of pressure algometers. In: Fricton JR, Awad E (eds). Advancesin Pain Research and Therapy. Vol 17: Myofascial pain and fibromyalgia. New York: Raven Press, 1990:165–181.

158. Headache Classification Committee of the International Headache Society. Classification and diagnostic criteria for headache disorders, cranial neuralgias and facial pain. Cephalalgia 1988;8:1–96.

159. Jensen K, Andersen HØ, Olesen J, Lindblom U. Pressure-pain threshold in human temporal region: Evaluation of a new pressure algometer. Pain 1986;25:313–323.

160. Reeves JL, Jaeger B, Graff-Radford SB. Reliability of the pressure algometer as a measure of myofascial trigger point sensitivity. Pain 1986;24:313–321.

161. Ohrbach R, Gale EN. Pressure-pain thresholds in normal muscles: Reliability, measurement effects and topographic differences. Pain 1989;37:257–263.

162. List T, Helkimo M, Falk G. Reliability and validity of a pressure threshold meter in recording tenderness in the masseter muscle and the anterior temporalis muscle. J Craniomandib Pract 1989;7:223–229.

163. Petersen KL, Brennum J, Olesen J. Evaluation of pericranial myofascial nociception by pressure algometry: Reproducibility and factors of variation. Cephalalgia 1992;12:33–37.

164. Sand T, Zwart JA, Helde G, Bovim G. The reproducibility of cephalic pain pressure thresholds in control subjects and headache patients. Cephalalgia 1997;17:748–755.

165. Isselée H, De Laat A, Bogaerts K, Lysens R. Short-term reproducibility of pressure-pain thresholds in masticatory muscles measured with a new algometer. J Orofac Pain 1998;12:203–209.

166. Goulet J-P, Clark GT, Flack VF, Liu C. The reproducibility of muscle and joint tenderness detection methods and maximum mandibular movement measurement for the temporomandibular system. J Orofac Pain 1998;12:17–26.

167. List T, Helkimo M, Karlsson R. Influence of pressure rates on the reliability of a pressure threshold meter. J Craniomandib Disord Facial Oral Pain 1991;5:173–178.

168. McMillan AS, Lawson ET. Effect of tooth clenching and jaw opening on pain-pressure thresholds in the human jaw muscles. J Orofac Pain 1994;8:250–257.

169. Lavigne GJ, Thon MT, Rompré P, Duncan GH, Goulet JP. Sensory descriptors from touch to pressure-pain and experimental variables: A psychophysiological study in human on pressure-pain threshold. In: Gebhart GF, Hammond DL, Jensen TS (eds). Progress in Pain Research and Management. Vol 2: Proceedings of the 7th World Congress on Pain. Seattle: IASP Press, 1994:831–842.

170. Ohrbach R, Crow H, Kamer A. Examiner expectancy effects in the measurement of pressure pain thresholds. Pain 1998;74:163–170.

171. Bendtsen L, Jensen R, Jensen NK, Olesen J. Muscle palpation with controlled finger pressure: New equipment for the studyof tender myofascial tissues. Pain 1994;59: 235–239.

172. Bendtsen L, Jensen R, Olesen J. Qualitatively altered nociception in chronic myofascial pain. Pain 1996;65:259–264.

173. Bendtsen L, Nørregaard J, Jensen R, Olesen J. Evidence of qualitatively altered nociception in patients with fibromyalgia. Arthritis Rheum 1997;40:98–102.

174. Lindahl O. Experimental muscle pain produced by chemical stimulus. Acta Orthop Scand 1970;40:741–750.

175. Le Bars D, Dickenson AH, Besson J-M. Diffuse noxious inhibitory controls (DNIC). I. Effects on dorsal horn convergent neurones in the rat. Pain 1979;6:283–304.

176. Fitzgerald M. The contralateral input to the dorsal horn of the spinal cord in the decerebrate rat. Brain Res 1982;236:275–287.

177. Arendt-Nielsen L, Graven-Nielsen T, Drewes AM. Referred pain and hyperalgesia related to muscle and visceral pain. IASP Newsletter 1998:3–5.

178. Yu X-M, Mense S. Response properties and descending control of rat dorsal horn neurons with deep receptive fields. Neuroscience 1990;39:823–831.

179. Chiang CY, Sessle BJ, Hu JW. Parabrachial area and nucleus raphe magnus–induced modulation of electrically evoked trigeminal subnucleus caudalis neuronal responses to cutaneous or deep A-fiber and C-fiber inputs in rats. Pain 1995;62:61–68.

180. Gjerstad J, Tjølsen A, Svendsen F, Hole K. Inhibition of evoked C-fibre responses in the dorsal horn after contralateral intramuscular injection of capsaicin involves activation of descending pathways. Pain 1999;80:413–418.

181. Wei F, Dubner R, Ren K. Nucleus reticularis gigantocellularis and nucleus raphe magnus in the brain stem exert opposite effects on behavioral hyperalgesia and spinal Fos proteinexpression after peripheral inflammation. Pain 1999;80:127–141.

182. Schiffman E, Fricton JR, Haley D, Tylka D. A pressure algometer for myofascial pain syndrome: Reliability and validity testing. In: Dubner R, Gebhart GF, Bond MR (eds). Pain Research and Clinical Management. Vol 3: Proceedings of the 5th World Congress on Pain. Amsterdam: Elsevier, 1988:407–413.

183. Ohrbach R, Gale EN. Pressure pain thresholds, clinical assessment, and differential diagnosis: Reliability and validity in patients with myogenic pain. Pain 1989;39:157–169.

184. Reid KI, Gracely RH, Dubner RA. The influence of time, facial side, and location on pain-pressure thresholds in chronic myogenous temporomandibular disorder. J Orofac Pain 1994;8:258–265.

185. McMillan AS, Blasberg B. Pain-pressure threshold in painful jaw muscles following trigger point injection. J Orofac Pain 1994;8:384–390.

186. Hedenberg-Magnusson B, Ernberg M, Kopp S. Symptoms and signs of temporomandibular disorders in patients with fibromyalgia andlocal myalgia of the temporomandibular system. Acta Odontol Scand 1997;55:344–349.

187. Carlson CR, Reid KI, Curran SL, Studts J, Okeson JP, Falace D, et al. Psychological and physiological parameters of masticatory muscle pain. Pain 1998;76:297–307.

188. Schoenen J, Bottin D, Hardy F, Gerard P. Cephalic and extracephalic pressure pain thresholds in chronic tension-type headache. Pain 1991;47:145–149.

189. Kim HS, Chung SC, Kim YK, Lee SW. Pain-pressure threshold in the head and neck region of episodic tension-type headache patients. J Orofac Pain 1995;9:357–364.

190. Bovim G. Cervicogenic headache, migraine and tension headache. Pressure pain threshold measurements. Pain 1992;51:169–173.

191. Jensen R, Rasmussen BK, Pedersen B, Olesen J. Muscle tenderness and pressure pain thresholds in headache. A populationstudy. Pain 1993;52:193–199.

192. Jensen R, Bendtsen L, Olesen J. Muscular factors are of importance in tension-type headache. Headache 1998;38:10–17.

193. Berberich P, Hoheisel U, Mense S. Effects of carrageenan-induced myositis on the discharge properties of group III and IV muscle receptors in the cat. J Neurophysiol 1988;59:1395–1409.

194. Mense S. Sensitization of group IV muscle receptors to bradykinin by 5-hydroxytryptamine and prostaglandin E2. Brain Res 1981;225:95–105.

195. Mense S, Meyer H. Bradykinin-induced modulation of the response behaviour of different types of feline group III and IV muscle receptors. J Physiol (Lond) 1988;398:49–63.

196. Ernberg M, Hedenberg-Magnusson B, Alstergren P, Kopp

S. Effect of local glucocorticoid injection on masseter muscle level of serotonin in patients with chronic myalgia. Acta Odontol Scand 1998;56:129–134.

197. Taiwo YO, Levine JD. Serotonin is a directly-acting hyperalgesic agent in the rat. Neuroscience 1992;48: 485–490.

198. Dubner R. Hyperalgesia in response to injury to cutaneous and deep tissues. In: Fricton JR, Dubner R (eds). Advances in Pain Research and Therapy. Vol 21: Orofacial pain and temporomandibular disorders. New York: Raven Press, 1995:61–71.

199. Millan MJ. The induction of pain: An integrative review. Prog Neurobiol 1999;57:1–164.

200. Sörensen J, Bengtsson A, Bäckman E, Henriksson KG, Bengtsson M. Pain analysis in patients with fibromyalgia: Effects of intravenous morphine, lidocaine and ketamine in patients with fibromyalgia. Scand J Rheumatol 1995;24:360–365.

201. Sörensen J, Bengtsson A, Ahlner J, Henriksson KG, Ekselius L, Bengtsson M. Fibromyalgia: Are there different mechanisms in the processing of pain? A double blind crossover comparison of analgesic drugs. J Rheumatol 1997;24:1615–1621.

202. Arendt-Nielsen L, Andersen OK, Jensen TS. Brief, prolonged and repeated stimuli applied to hyperalgesic skin areas: A psychophysical study. Brain Res 1996;712:165–167.

203. Graven-Nielsen T, Kendall SA, Henriksson KG, Bengtsson M, Sörensen J, Johnson A, et al. Ketamine reduces muscle pain, temporal summation, and referred pain in fibromyalgia patients. Pain 2000;85:483–491.

204. Malow RM, Grimm L, Olson RE. Differences in pain perception between myofascial pain dysfunction patients and normal subjects: A signal detection analysis. J Psychosom Res 1980;24:303–309.

205. Lautenbacher S, Rollman GB, McCain GA. Multi-method assessment of experimental and clinical pain in patients with fibromyalgia. Pain 1994;59:45–53.

206. Lautenbacher S, Rollman GB. Possible deficiencies of pain modulation in fibromyalgia. Clin J Pain 1997;13:189–196.

207. Kosek E, Hansson P. Modulatory influence on somatosensory perception from vibration and heterotopic noxious conditioning stimulation (HNCS) in fibromyalgia patients and healthy subjects. Pain 1997;70:41–51.

208. Henriksson KG. Muscle activity and chronic muscle pain. J Musculoskel Pain 1999;7:101–109.

209. Türp JC, Kowalski CJ, Stohler CS. Temporomandibular disorders: Pain outside the head and face is rarely acknowledged in the chief complaint. J Prosthet Dent 1997;78:592–595.

210. Okeson JP. Bell’s Orofacial Pains, ed 5. Chicago: Quintessence, 1995.

211. Margolis RB, Chibnall JT, Tait RC. Test-retest reliability of the pain drawing instrument. Pain 1988;33:49–51.

212. Türp JC, Kowalski CJ, O’Leary NO, Stohler CS. Pain maps from facial pain patients indicate a broad pain geography. J Dent Res 1998;77:1465–1472.

213. Gandevia SC, Phegan CML. Perceptual distortions of the human body image produced by local anaesthesia, pain and cutaneous stimulation. J Physiol (Lond) 1999;514:609–616.

214. Schmidt-Hansen PT, Svensson P, Jensen TS, Graven-Nielsen T, Arendt-Nielsen L, Bach FW. Referred pain in pericranial muscles following hypertonic saline injection. In: Devor M, Rowbotham MC, Wiesenfeld-Hallin Z (eds). Progress in Pain Research and Management. Vol 16: Proceedings of the 9th World Congress on Pain, Vienna, Austria. Seattle: IASP Press, 2000.

215. Falace DA, Reid K, Rayens MK. The influence of deep (odontogenic) pain intensity, quality, and duration on the incidence and characteristics of referred orofacial pain. J Orofac Pain 1996;10:232–239.

216. Hutchins HC, Reynolds OE. Experimental investigation of the referred pain of aerodontalgia. J Dent Res 1947;26:3–8.

217. Reynolds OE, Hutchins HC. Reduction of central hyperirritability following block anesthesia of peripheral nerve. Am J Physiol 1948;152:658–662.

218. Hoheisel U, Mense S. Long-term changes in discharge behaviour of cat dorsal horn neurones following noxious stimulation of deep tissues. Pain 1989;36:239–247.

219. Whitty CWM, Willison RG. Some aspects of referred pain. Lancet 1958;2:226–231.

220. Laursen RJ, Graven-Nielsen T, Jensen TS, Arendt-Nielsen

L. Referred pain is dependent on sensory input from the periphery: A psychophysical study. Eur J Pain 1997;1:261–269.

221. Mense S. Mechanisms of pain in hind limb muscles: Experimental findings and open questions. In: Sessle BJ, Bryant PS, Dionne RA (eds). Progress in Pain Research and Management. Vol 4: Temporomandibular disorders and related pain conditions. Seattle: IASP Press, 1995:63–69.

222. Sessle BJ, Hu JW, Amano N, Zhong G. Convergence of cutaneous, tooth pulp, visceral, neck and muscle afferents onto nociceptive and non-nociceptive neurones in trigeminal subnucleus caudalis (medullary dorsal horn) and its implication for referred pain. Pain 1986;27:219–235.

223. Kojima Y. Convergence patterns of afferent information from the temporomandibular joint and masseter muscle in the trigeminal subnucleus caudalis. Brain Res Bull 1990;24:609–616.

224. Schaible H-G, Schmidt RF, Willis WD. Convergent inputs from articular, cutaneous and muscle receptors onto ascending tract cells in the cat spinal cord. Exp Brain Res 1987;66:479–488.

225. Hoheisel U, Mense S. Response behaviour of cat dorsal horn neurones receiving input from skeletal muscle and other deep somatic tissues. J Physiol (Lond) 1990;426:265–280.

226. Lamour Y, Willer JC, Guilbaud G. Rat somatosensory (SmI) cortex: I. Characteristics of neuronal responses to noxious stimulation and comparison with responses to non-noxious stimulation. Exp Brain Res 1983;49:35–45.

227. Lamour Y, Guilbaud G, Willer JC. Rat somatosensory (SmI) cortex: II. Laminar and columnar organization of noxious and non-noxious inputs. Exp Brain Res 1983;49:46–54.

228. Apkarian AV, Brüggermann J, Shi T, Airapetian LR. A thalamic model for true and referred visceral pain. In: Gebhart GF (ed).Progress in Pain Research and Management. Vol 5: Visceral pain. Seattle: IASP Press, 1995:217–259.

229. Gray RJM, Rothwell PS, Wastell DG. An investigation of pain distribution in patients with temporomandibular joint pain dysfunction syndrome. J Dent 1986;14:114–120.

230. Hagberg C, Hagberg M, Kopp S. Musculoskeletal symptoms and psychosocial factors among patients with craniomandibular disorders. Acta Odontol Scand 1994;52:170–177.

231. Dao TTT, Reynolds WJ, Tenenbaum HC. Comorbidity between myofascial pain of the masticatory muscles and fibromyalgia. J Orofac Pain 1997;11:232–241.

232. Travell JG, Simons DG. Myofascial Pain and Dysfunction. The Trigger Point Manual. Baltimore: Williams & Wilkins, 1983.

233. Fricton JR, Kroening R, Haley D, Siegert R. Myofascial pain syndrome of the head and neck: A review of clinical characteristics of 164 patients. Oral Surg Oral Med Oral Pathol 1985;60:615–623.

234. Lous I. The importance of referred pain in myogenic headache. Headache 1976;16:119–122.

235. Campbell CD, Loft GH, Davis H, Hart DL. TMJ symptoms and referred pain patterns. J Prosthet Dent 1982;47:430–433.

236. Jaeger B. Myofascial referred pain patterns: The role of trigger points. J Calif Dent Assoc 1985;13:27–32.

237. Johansen MK, Graven-Nielsen T, Olesen AS, Arendt-Nielsen L. Generalised muscular hyperalgesia in chronic whiplash syndrome. Pain 1999;83:229–234.

238. Carlson CR, Okeson JP, Falace DA, Nitz AJ, Lindroth JE. Reduction of pain and EMG activity in the masseter region by trapezius trigger point injection. Pain 1993;55:397–400.

239. De Boever JA, Carlsson GE. Etiology and differential diagnosis. In: Zarb GA, Carlsson GE, Sessle BJ, Mohl ND (eds). Temporomandibular Joint and Masticatory Muscle Disorders. Copenhagen: Munksgaard, 1994:171–187.

240. Travell J. Temporomandibular joint dysfunction. J Prosthet Dent 1960;10:745–763.

241. Laskin DM. Etiology of the pain-dysfunction syndrome. J Am Dent Assoc 1969;79:141–153.

242. Møller E. Muscle hyperactivity leads to pain and dysfunction. Position paper. In: Klineberg I, Sessle BJ (eds). Orofacial Pain and Neuromuscular Dysfunction. Oxford: Pergamon Press, 1985:69–92.

243. Parker MW. A dynamic model of etiology in temporomandibular disorders. J Am Dent Assoc 1990;120: 283–290.

244. De Laat A, Svensson P, Macaluso GM. Are jaw and facial reflexes modulated during clinical or experimental orofacial pain? J Orofac Pain 1998;12:260–271.

245. Buchthal F, Rosenfalck P. Spontaneous electrical activity of human muscle. Electroencephalogr Clin Neurophysiol 1966;20:321–336.

246. Møller E. The chewing apparatus. An electromyographic study of the action of the muscles of mastication and its correlation to facial morphology. Acta Physiol Scand Suppl 1966;280:1–229.

247. Lund JP, Donga R, Widmer CG, Stohler CS. The pain-adaptation model: A discussion of the relationship between chronic musculoskeletal pain and motor activity. Can J PhysiolPharmacol 1991;69:683–694.

248. Lund JP, Widmer CG, Schwartz G. What is the link between myofascial pain and dysfunction? In: van Steenberghe D, De Laat A (eds). EMG of Jaw Reflexes in Man. Leuven, Belgium: Leuven University Press, 1989:427–444.

249. Lund P, Nishiyama T, Møller E. Postural activity in the muscles of mastication with the subject upright, inclined, and supine.Scand J Dent Res 1970;78:417–424.

250. Yemm R. Temporomandibular dysfunction and masseter muscle response to experimental stress. Br Dent J 1969;127:508–510.

251. Rao SM, Glaros AG. Electromyographic correlates of experimentally induced stress in diurnal bruxists and normals. J Dent Res 1979;58:1872–1878.

252. Mercuri LG, Olson RE, Laskin DM. The specificity of response to experimental stress in patients with myofascial pain dysfunction syndrome. J Dent Res 1979;58: 1866–1871.

253. Katz JO, Rugh JD, Hatch JP, Langlais RP, Terezhalmy GT, Borcherding SH. Effect of experimental stress on masseter and temporalis muscle activity in human subjects with temporomandibular disorders. Arch Oral Biol 1989;34:393–398.

254. Jensen R, Fuglsang-Frederiksen A, Olesen J. Quantitative surface EMG of pericranial muscles. Reproducibility and variability. Electroencephalogr Clin Neurophysiol 1993;89:1–9.

255. Paesani DA, Tallents RH, Murphy WC, Hatala MP, Proskin HM. Evaluation of the reproducibility of rest activity of the anterior temporal and masseter muscles in asymptomatic and symptomatic temporomandibular subjects. J Orofac Pain 1994;8:402–406.

256. Mooney V, Robertson J. The facet syndrome. Clin Orthopaed Rel Res 1976;115:149–156.

257. Cobb CR, De Vries HA, Urban RT, Luekens CA, Bagg RJ. Electrical activity in muscle pain. Am J Phys Med 1975;54:80–87.

258. Bogduk N, Munro RR. Experimental low back pain, referred pain and muscle spasm [abstract]. J Anat 1979;128:661.

259. Ashton-Miller JA, McGlashen KM, Herzenberg JE, Stohler CS. Cervical muscle myoelectrical response to acute experimental sternocleidomastoid pain. Spine 1990;15:1006–1012.

260. Tsai C-M, Chiang CY, Yu X-M, Sessle BJ. Involvement of trigeminal subnucleus caudalis (medullary dorsal horn) in craniofacial nociceptive reflex activity. Pain 1999;81:115–128.

261. Wall PD, Woolf CJ. Muscle but not cutaneous C-afferent input produces prolonged increases in the excitability of the flexion reflex in the rat. J Physiol (Lond) 1984;356:443–458.

262. Majewski RF, Gale EN. Electromyographic activity of anterior temporal area pain patients and non-pain subjects. J Dent Res 1984;63:1228–1231.

263. Sherman RA. Relationships between jaw pain and jaw muscle contraction level: Underlying factors and treatment effectiveness. J Prosthet Dent 1985;54:114–118.

264. Flor H, Birbaumer N, Schulte W, Roos R. Stress-related electromyographic responses in patients with chronic temporomandibular pain. Pain 1991;46:145–152.

265. Carlson CR, Okeson JP, Falace DA, Nitz AJ, Curran SL, Anderson D. Comparison of psychological and physiological functioning between patients with masticatory muscle pain and matched controls. J Orofac Pain 1993;7:15–22.

266. Lous I, Sheikoleslam A, Møller E. Postural activity in subjects with functional disorders of the chewing apparatus. Scand J Dent Res 1970;78:404–410.

267. Rugh JD, Montgomery GT. Physiological reactions of patients with TM disorders vs symptom-free controls on a physical stress task. J Craniomandib Disord Facial Oral Pain 1987;1:243–250.

268. Burdette BH, Gale EN. The effects of treatment on masticatory muscle activity and mandibular posture in myofascial pain-dysfunction patients. J Dent Res 1988;67:1126–1130.

269. Dolan EA, Keefe FJ. Muscle activity in myofascial pain-dysfunction syndrome patients: A structured clinical evaluation. J Craniomandib Disord Facial Oral Pain 1988;2:101–105.

270. Kapel L, Glaros AG, McGlynn FD. Psychophysiological responses to stress in patients with myofascial pain-dysfunction syndrome. J Behav Med 1989;12:397–406.

271. Gervais RO, Fitzsimmons GW, Thomas NR. Masseter and temporalis electromyographic activity in asymptomatic, subclinical, and temporomandibular joint dysfunction patients. Cranio 1989;7:52–57.

272. Shi CS, Wang HY. Postural and maximum activity in elevators during mandible pre- and post-occlusal splint treatment of temporomandibular joint disturbance syndrome. J Oral Rehabil 1989;16:155–161.

273. Glaros AG, Glass EG, Brockman D. Electromyographic data from TMD patients with myofascial pain and from matched control subjects: Evidence for statistical, not clinical, significance. J Orofac Pain 1997;11:125–129.

274. Liu ZJ, Yamagata K, Kasahara Y, Ito G. Electromyographic examination of jaw muscles in relation to symptoms and occlusion of patients with temporomandibular joint disorders. J Oral Rehabil 1999;26:33–47.

275. Intrieri RC, Jones GE, Alcorn JD. Masseter muscle hyperactivity and myofascial pain dysfunction syndrome: A relationship under stress. J Behav Med 1994;17:479–500.

276. Vaughn R, Pall ML, Haynes SN. Frontalis EMG response to stress in subjects with frequent muscle-contraction headaches. Headache 1977;16:313–317.

277. Van Boxtel A, van der Ven JR. Differential EMG activity in subjects with muscle contraction headaches related to mental effort. Headache 1978;17:233–237.

278. Hursey KG, Holroyd KA, Penzien DB, Holm JE. The influence of pain state on physiological reactivity of tension headache sufferers. Headache 1985;25:79–84.

279. Schoenen J, Gerard P, De Pasqua V, Juprelle M. EMG activity in pericranial muscles during postural variation and mental activity in healthy volunteers and patients with chronic tension type headache. Headache 1991;31: 321–324.

280. Hatch JP, Moore PJ, Cyr-Provost M, Boutros NN, Seleshi E, Borcherding S. The use of electromyography and muscle palpation in the diagnosis of tension-type headache with and without pericranial muscle involvement. Pain 1992;49:175–178.

281. Jensen R, Fuglsang-Frederiksen A, Olesen J. Quantitative surface EMG of pericranial muscles in headache. A population study. Electroencephalogr Clin Neurophysiol 1994;93:335–344.

282. Martin PR, Mathews AM. Tension headaches: Psychophysiological investigation and treatment. J Psychosom Res 1978;22:389–399.

283. Sutton EP, Belar CD. Tension headache patients versus controls: A study of EMG parameters. Headache 1982;22:133–136.

284. Clark GT, Sakai S, Merrill R, Flack VF, McCreary C. Cross-correlation between stress, pain, physical activity, and temporalis muscle EMG in tension-type headache. Cephalalgia 1995;15:511–518.

285. Lund JP, Widmer CG. An evaluation of the use of surface electromyography in the diagnosis, documentation, and treatment of dental patients. J Craniomandib Disord Facial Oral Pain 1989;3:125–137.

286. LeResche L, Dworkin SF, Wilson L, Ehrlich KJ. Effect of temporomandibular disorder pain duration on facial expressions and verbal report of pain. Pain 1992;51:289–295.

287. Sakai F, Ebihara S, Akiyama M, Horikawa M. Pericranial muscle hardness in tension-type headache. Headache 1995;118:523–531.

288. Ashina M, Bendtsen L, Jensen R, Sakai F, Olesen J. Muscle hardness in patients with chronic tension-type headache: Relation to actual headache state. Pain 1999;79:201–205.

289. Travell J, Rinzler S, Herman M. Pain and disability of the shoulder and arm. Treatment by intramuscular infiltration with procaine hydrochloride. JAMA 1942;120:417–422.

290. Simons DG, Mense S. Understanding and measurement of muscle tone as related to clinical muscle pain. Pain 1998;75:1–17.

291. De Vries HA. Quantitative electromyographic investigations of the spasm theory of muscle pain. Am J Phys Med 1966;45:119–134.

292. Johansson H, Sojka P. Pathophysiological mechanisms involved in genesis and spread of muscular tension in occupational muscle pain. Med Hypotheses 1991;135:196–203.

293. Simons DG, Hong C-Z, Simons LS. Prevalence of spontaneous electrical activity at trigger spots and at control sites in rabbit skeletal muscle. J Musculoskel Pain 1995;3:35–48.

294. Fricton JR, Auvinen MD, Dykstra D, Schiffman E. Myofascial pain syndrome: Electromyographic changes associated with local twitch response. Arch Phys Med Rehabil 1985;66:314–317.

295. Lund JP, Stohler CS, Widmer CG. The relationship between pain and muscle activity in fibromyalgia and similar conditions. In: Værøy H, Merskey H (eds). Pain Research and Clinical Management. Vol 6: Progress in fibromyalgia and myofascial pain. Amsterdam: Elsevier, 1993:311–327.

296. Hagberg C. Assessments of bite force: A review. J Craniomandib Disord Facial Oral Pain 1987;1:162–169.

297. Bakke M. Mandibular elevator muscles: Physiology, action, and effect of dental occlusion. Scand J Dent Res 1993;101:314–331.

298. Clark GT, Carter MC. Electromyographic study of human jaw-closing muscle endurance, fatigue and recovery at various isometric force levels. Arch Oral Biol 1985;30:563–569.

299. Christensen LV. Jaw muscle fatigue and pains induced by experimental tooth clenching: A review. J Oral Rehabil 1981;8:27–36.

300. Clark GT, Carter MC, Beemsterboer PL. Analysis of electromyographic signals in human jaw closing muscles at various isometric force levels. Arch Oral Biol 1988;33:833–837.

301. Sheikholeslam A, Møller E, Lous I. Pain, tenderness and strength of human mandibular elevators. Scand J Dent Res 1980;88:60–66.

302. Clark GT, Beemsterboer PL, Jacobson R. The effect of sustained submaximal clenching on maximum bite force in myofascial pain and dysfunction patients. J Oral Rehabil 1984;11:387–391.

303. Bakke M, Michler L, Han K, Møller E. Clinical significance of isometric bite force versus electrical activity in temporal and masseter muscles. Scand J Dent Res 1989;97:539–551.

304. Kroon GW, Naeije M. Electromyographic evidence of local muscle fatigue in a subgroup of patients with myogenous craniomandibular disorders. Archs Oral Biol 1992;37:215–218.

305. Visser A, Kroon GW, Naeije M, Hansson TL. EMG differences between weak and strong myogenous CMD patients and healthy controls. J Oral Rehabil 1995;22:429–434.

306. Van der Glas HW, Lobbezoo F, van der Bilt A, Bosman F. Influence of the thickness of soft tissues overlying human masseter and temporalis muscles on the electromyographic maximal voluntary contraction level. Eur J Oral Sci 1996;104:87–95.

307. Molin C. Vertical isometric muscle forces of the mandible. A comparative study of subjects with and without manifest mandibular pain dysfunction syndrome. Acta Odontol Scand 1972;30:485–499.

308. Helkimo E, Carlsson GE, Yehuda C. Bite force in patients with functional disturbances of the masticatory system. J Oral Rehabil 1975;2:397–406.

309. Shi CS. Proportionality of mean voltage of masseter muscle to maximum bite force applied for diagnosing temporomandibular joint disturbance syndrome. J Prosthet Dent 1989;62:682–684.

310. Buchner R, van der Glas HW, Brouwers JE, Bosman F. Electromyographic parameters related to clenching level and jaw-jerk reflex in patients with a simple type of myogenous craniomandibular disorder. J Oral Rehabil 1992;19:495–511.

311. Gay T, Maton B, Rendell J, Majourau A. Characteristics of muscle fatigue in patients with myofascial pain-dysfunction syndrome. Arch Oral Biol 1994;39:847–852.

312. Koyano K, Kim YJ, Clark GT. Electromyographic signal changes during exercise in human chronic jaw-muscle pain. Arch Oral Biol 1995;40:221–227.

313. Hamada T, Kotani H, Kawazoe Y, Yamada S. Effect of occlusal splints on the EMG activity of masseter and temporal muscles in bruxism with clinical symptoms. J Oral Rehabil 1982;9:119–123.

314. Hagberg C. EMG versus force relationship in painful masseter muscles before and after intramuscular anesthetics and saline injections. Scand J Dent Res 1987;95:259–265.

315. Hagberg C, Hagberg M. Muscular tenderness evaluated by surface EMG frequency analysis of isometric contractions. Scand J Dent Res 1989;97:368–374.

316. Lund JP, Stohler CS. Effects of pain on muscular activity in temporomandibular disorders and related conditions. In: StohlerCS, Carlson DS (eds). Craniofacial Growth Series. Vol 29: Biological and psychological aspects of orofacial pain. Ann Arbor: The University of Michigan Center for Human Growth and Development, 1994:75–91.

317. Sheikholeslam A, Møller E, Lous I. Postural and maximal activity in elevators of mandible before and after treatment of functional disorders. Scand J Dent Res 1982;90:37–46.

318. Bakke M, Møller E. Craniomandibular disorders and masticatory muscle function. Scand J Dent Res 1992;100:32–38.

319. Lyons MF, Cadden SW, Baxendale RH, Yemm R. Twitch interpolation in the assessment of the maximum force generating capacity of the jaw-closing muscles in man. Arch Oral Biol 1996;41:1161–1168.

320. Nørregaard J. Muscle Function, Psychometric Scoring and Prognosis in Patients with Widespread Pain and Tenderness (Fibromyalgia) [thesis]. Copenhagen: Univ of Copenhagen, 1998.

321. Bengtsson A, Henriksson K-G, Jorfeldt L, Kågedal B, Lennmarken C, Lindström F. Primary fibromyalgia. A clinical and laboratory study of 55 patients. Scand J Rheumatol 1986;15:340–347.

322. Bäckman E, Bengtsson A, Bengtsson M, Lennmarken C, Henriksson K-G. Skeletal muscle function in primary fibromyalgia. Effect of regional sympathetic blockade with guanethidine. Acta Neurol Scand 1988;77:187–191.

323. Nørregaard J, Bülow PM, Danneskiold-Samsøe B. Muscle strength, voluntary activation, twitch properties, and endurance in patients with fibromyalgia. J Neurol Neurosurg Psychiatry 1994;57:1106–1111.

324. Nørregaard J, Bülow PM, Vestergaard-Poulsen P, Thomsen C, Danneskiold-Samsøe B. Muscle strength, voluntary activation and cross-sectional muscle area in patients with fibromyalgia. Br J Rheumatol 1995;34:925–931.

325. Leino P, Aro S, Hasan J. Trunk muscle function and low back disorders: A ten-year follow-up study. J Chron Dis 1987;40:289–296.

326. Luschei ES, Goldberg LJ. Neural mechanisms of mandibular control: Mastication and voluntary biting. In: Brooks VB (ed). Motor Control. Vol 2: Handbook of physiology. Bethesda, MD: American Physiological Society, 1981:1237–1274.

327. Lund JP. Mastication and its control by the brain stem. Crit Rev Oral Biol Med 1991;2:33–64.

328. Nakamura Y, Katakura N. Generation of masticatory rhythm in the brainstem. Neurosci Res 1995;23:1–19.

329. Stohler CS. A comparative electromyographic and kinesiographic study of deliberate and habitual mastication in man. Arch Oral Biol 1986;31:669–678.

330. Howell PGT, Johnson CWL, Ellis S, Watson IB, Klineberg

I. The recording and analysis of EMG and jaw tracking. I. The recording procedure. J Oral Rehabil 1992;19:595–605.

331. Howell PGT, Ellis S, Johnson CWL, Watson IB, Klineberg

I. The recording and analysis of EMG and jaw tracking.

II. Reproducibility of jaw tracking. J Oral Rehabil 1993;20:33–43.

332. Wilding RJC, Shaikh M. Muscle activity and jaw movements as predictors of chewing performance. J Orofac Pain 1997;11:24–36.

333. Schwartz G, Enomoto S, Valiquette C, Lund JP. Mastication in the rabbit: A description of movement and muscle activity. J Neurophysiol 1989;62:273–287.

334. Neill DJ, Howell PGT. Computerized kinesiography in the study of mastication in dentate subjects. J Prosthet Dent 1986;55:629–638.

335. Throckmorton GS, Teenier TJ, Ellis E. Reproducibility of mandibular motion and muscle activity levels using a commercial computer recording system. J Prosthet Dent 1992;68:348–354.

336. Plesh O, Bishop B, McCall WD Jr. Kinematics of jaw movements during chewing at different frequencies. J Biomech 1993;26:243–250.

337. Cecere F, Ruf S, Pancherz H. Is quantitative electromyography reliable? J Orofac Pain 1996;10:38–47.

338. Balkhi KM, Tallents RH, Goldin B, Catania JA. Error analysis of a magnetic jaw-tracking device. J Craniomandib Disord Facial Oral Pain 1991;5:51–56.

339. Kazazoglu E, Heath MR, Ferman AM, Davis GR. Recording mandibular movement: Technical and clinical limitations of the Sirognathograph. J Orofac Pain 1994;8:165–177.

340. Airoldi RL, Gallo LM, Palla S. Precision of the jaw tracking system JAWS-3D. J Orofac Pain 1994;8:155–164.

341. Naeije M, van der Weijden JJ, Megens CCEJ. OKAS-3D: Optoelectronic jaw movement recording system with six degrees of freedom. Med Biol Eng Comput 1995;33:683–688.

342. Zhang X, Ashton-Miller JA, Stohler CS. Three-dimensional unilateral method for the bilateral measurement of condylar movements. J Biomech 1995;28:1007–1011.

343. Gerstner GE, Parekh VV. Evidence of sex-specific differences in masticatory jaw movement patterns. J Dent Res 1997;76:796–806.

344. Green JR, Moore CA, Ruark JL, Rodda PR, Morvée WT, van Witzenburg MJ. Development of chewing in children from 12 to48 months: Longitudinal study of EMG patterns. J Neurophysiol 1997;77:2704–2716.

345. Ottenhoff FAM, van der Bilt A, van der Glas HW, Bosman F. Control of elevator muscle activity during simulated chewing with varying food resistance in humans. J Neurophysiol 1992;68:933–944.

346. Ottenhoff FAM, van der Bilt A, van der Glas HW, Bosman F. Control of human jaw elevator muscle activity during simulated chewing with varying bolus size. Exp Brain Res 1993;96:501–512.

347. Schwartz G, Lund JP. Modification of rhythmical jaw movements by noxious pressure applied to the periosteum of the zygoma in decerebrate rabbits. Pain 1995;63:153–161.

348. Westberg K-G, Clavelou P, Schwartz G, Lund JP. Effects of chemical stimulation of masseter muscle nociceptors on trigeminal motoneuron and interneuron activities during fictive mastication in the rabbit. Pain 1997;73:295–308.

349. Sessle BJ, Martin RE, Murray GM, Masuda Y, Kemppainen P, Narita N, et al. Cortical mechanisms controlling mastication and swallowing in the awake monkey. In: Morimoto T, Matsuya T, Takada K (eds). Brain and Oral Function. Amsterdam: Elsevier, 1995:181–189.

350. Dao TTT, Lund JP, Lavigne GJ. Pain responses to experimental chewing in myofascial pain patients. J Dent Res 1994;73:1163–1167.

351. Mongini F, Tempia-Valenta G, Conserva E. Habitual mastication in dysfunction: A computer-based analysis. J Prosthet Dent 1989;61:484–494.

352. Feine JS, Hutchins MO, Lund JP. An evaluation of the criteria used to diagnose mandibular dysfunction with the mandibular kinesiograph. J Prosthet Dent 1988;60:374–380.

353. Mohl ND, McCall WD Jr, Lund JP, Plesh O. Devices for the diagnosis and treatment of temporomandibular disorders. Part I:Introduction, scientific evidence, and jaw tracking. J Prosthet Dent 1990;63:198–201.

354. Nielsen IL, Marcel T, Chun D, Miller AJ. Patterns of mandibular movements in subjects with craniomandibular disorders. J Prosthet Dent 1990;63:202–217.

355. Møller E, Sheikholeslam A, Lous I. Response of elevator activity during mastication to treatment of functional disorders. Scand J Dent Res 1984;92:64–83.

356. Nielsen IL, McNeill C, Danzig W, Goldman S, Levy J, Miller AJ. Adaptation of craniofacial muscles in subjects with craniomandibular disorders. Am J Orthod Dentofac Orthop 1990;97:20–34.

357. Kumai T. Difference in chewing patterns between involved and opposite sides in patients with unilateral temporomandibular joint and myofascial pain-dysfunction. Archs Oral Biol 1993;38:467–478.

358. Stohler C, Yamada Y, Ash MM Jr. Antagonistic muscle stiffness and associated reflex behaviour in the pain-dysfunctional state. Helv Odontol Acta 1985;29:13–20.

359. Stohler CS, Ashton-Miller JA, Carlson DS. The effects of pain from the mandibular joint and muscles on masticatory motor behaviour in man. Arch Oral Biol 1988;33:175–182.

360. Dellow PG, Lund JP. Evidence for central timing of rhythmical mastication. J Physiol (Lond) 1971;215:1–13.

361. Westberg K-G, Clavelou P, Sandström G, Lund JP. Evidence that trigeminal brainstem interneurons form subpopulations to produce different forms of mastication in the rabbit. J Neurosci 1998;18:6466–6479.

362. Ashina M, Lassen LH, Bendtsen L, Jensen R, Olesen J. Effect of inhibition of nitric oxide synthase on chronic tension-type headache: A randomized crossover trial. Lancet 1999;353:287–289.

363. Besson JM. The neurobiology of pain. Lancet 1999;353:1610–1615.

364. Matre DA, Sinkjær T, Svensson P, Arendt-Nielsen L. Experimental muscle pain increases the human stretch-reflex. Pain 1998;75:331–339.

365. Raphael KG, Marbach JJ, Gallagher RM, Dohrenwand BP. Myofascial TMD does not run in families. Pain 1999;80:15–22.

366. Flor H, Braun C, Elbert T, Birbaumer N. Extensive reorganization of primary somatosensory cortex in chronic back pain patients. Neurosci Lett 1997;224:5–8.

367. McNamara JA, Seligman DA, Okeson JP. Occlusion, orthodontic treatment, and temporomandibular disorders: A review. J Orofac Pain 1995;9:73–90.