Journal of Oral & Facial Pain and Headache 31 Can Experimentally Evoked Pain in the Jaw Muscles or Temporomandibular Joint Affect Anterior Bite Force in Humans? Aims: To test the hypothesis that experimental pain in the masseter muscle or temporomandibular joint (TMJ) will decrease the anterior maximum voluntary bite force (MVBF) and jaw muscle activity in relation to the perceived effort. Methods: Sixteen volunteers participated in two experimental sessions. Participants were injected with 0.2 mL of monosodium glutamate (1.0 M) into either the masseter muscle or TMJ. The MVBF and corresponding electromyographic (EMG) activity of the masseter, anterior temporalis, and digastric muscles were recorded 10 times at an interval of 2 minutes before and after injection. Pain was measured using a visual analog scale and McGill Pain Questionnaire. In addition, participants were asked how they perceived the interference of pain on their biting performance. The data analysis included a two-way analysis of variance model and t test. Results: There was no significant difference in peak pain intensity (P = .066) and duration of pain (P = .608) between painful muscle and TMJ injections, but TMJ injection produced a significantly larger area under the curve (P = .005) and a significantly higher pain rating index (P = .030). Pain in the muscle (P = .421) and TMJ (P = .057) did not significantly change the MVBF from baseline levels. The EMG activity also did not differ significantly from baseline levels during muscle pain. However, there was a significant increase (P = .028) in the EMG activity of the anterior temporalis and a significant decrease (P = .010) in the EMG activity of the anterior digastric muscle compared to baseline during TMJ pain. Subject-based reports also revealed that in the majority of cases (62.5%), pain did not interfere with the MVBF task. Conclusion: Experimental pain from either masseter muscle or TMJ did not affect the MVBF, in accordance with the subject-based reports. Jaw muscle activity, except for EMG activity of the anterior temporalis and anterior digastric muscles during TMJ pain, also remained unaffected by pain. The findings suggest that it is not pain in itself but rather how pain is perceived that may lead to adaptation of motor function, supporting an integrated pain adaptation model. J Oral Facial Pain Headache 2015;29:31–40. doi: 10.11607/ofph.1268 Key words: electromyography, integrated pain adaptation model, monosodium glutamate T he term temporomandibular disorders (TMD) encompasses a wide variety of clinical conditions and pathologic states that involve the masticatory muscles and/or the temporomandibular joints (TMJs) along with the associated structures. 1–3 Pain in the masticatory muscles and ⁄or TMJs, as well as disordered jaw function such as limitation in mandibular movement and joint noises, are often considered to be the cardinal signs of TMD. 1,2 Clinically, the majority of TMD patients can be classifed into those with problems associated with the masticatory musculature or with problems within the TMJ itself. Distinguishing these two categories is often considered a clinical challenge. 4,5 It has been reported that pain in the musculoskeletal system (masti- catory muscles and TMJ) causes signifcant changes in the masticato- ry movement pattern, maximum occlusal force, and electromyographic (EMG) activity as compared to individuals without such pain. 6,7 EMG data from animal studies have also indicated that noxious stimulation of the TMJ might have a different motor effect compared to noxious Abhishek Kumar, BDS PhD Scholar Eduardo Castrillon, DDS, MSc, PhD Associate Professor Peter Svensson, DDS, PhD, Dr Odont Professor Section of Clinical Oral Physiology Department of Dentistry Aarhus University Aarhus, Denmark and Scandinavian Center for Orofacial Neurosciences Correspondence to: Dr Abhishek Kumar Section of Clinical Oral Physiology Department of Dentistry Aarhus University Aarhus 8000, Denmark Fax: +45 8619 5665 Email: a.kumar@odontologi.au.dk ©2015 by Quintessence Publishing Co Inc. © 2015 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.