title: Effect of experimental pain on EMG-activity in human jaw-closing muscles in different jaw positions.
abstract: We examined the effect of a standardized painful stimulus on the surface EMG-activity of the human jaw-closing muscles at rest and during two levels of jaw opening. Sixteen healthy women participated in two experimental sessions. In randomized order, hypertonic saline (HS: 5.8%) was infused into the left masseter muscle on one occasion, and isotonic saline (IS: 0.9%) on the other. The subjects scored the pain intensity continuously on a 0-10 visual analogue scale (VAS). The subjects were asked to hold the jaw in three different positions (rest, half-maximal, and maximal opening). Before, during, and after infusion, the EMG-activity was recorded from the masseter and temporalis muscles with the jaw in each of the three positions. HS induced significantly higher pain-levels than IS (mean VAS: HS: 5.2+/-1.3, IS: 0.7+/-0.2, P<0.05). At rest, the EMG-activity in most of the jaw muscles increased significantly during both infusions (P<0.05). At half-maximal opening, the EMG-activity in the infused muscle increased significantly with both HS and IS (P<0.05). At maximal opening, the EMG-activity during infusion of HS decreased significantly in the right masseter and temporalis (P<0.05). There was no significant difference in the position of the jaw at rest during infusions. However, the vertical opening distance was significantly decreased during infusion of HS at half-maximal and maximal opening (P<0.05).
  
  
  CONCLUSIONS
  It is concluded that experimental pain affects EMG-activity differentially in jaw-closing muscles in different opening positions of the jaw.
authors.affiliation: Department of Clinical Oral Physiology, School of Dentistry, University of Aarhus, DK-8000 Aarhus C, Denmark. lbhansen@odont.au.dk
authors.forename: Lene
authors.initials: L
authors.lastname: Baad-Hansen
authors.forename: Setsuhiro
authors.initials: S
authors.lastname: Hara
authors.forename: Yoshitsugu
authors.initials: Y
authors.lastname: Marumo
authors.forename: Timothy
authors.initials: T
authors.lastname: Miles
authors.forename: Peter
authors.initials: P
authors.lastname: Svensson
mesh_terms: D000328:Adult; D004576:Electromyography; D005260:Female; D006801:Humans; D007552:Isotonic Solutions / Q000008:administration & dosage; D007568:Jaw / Q000187:drug effects / Q000503:physiopathology*; D008406:Masseter Muscle / Q000187:drug effects / Q000503:physiopathology*; D009043:Motor Activity; D010147:Pain Measurement / Q000295:instrumentation / Q000379:methods*; D012462:Saline Solution, Hypertonic / Q000008:administration & dosage; D055815:Young Adult
chemical_list: D007552:Isotonic Solutions; D012462:Saline Solution, Hypertonic