Surface electromyography is an excellent non-invasive procedure to evaluate the physiological processes of musculoskeletal. The inclusion of surface electromyography in the diagnosis of temporomandibular disorder has been discussed among researchers, since patient exhibit a variety of muscle disorders. Temporomandibular Disorder is diagnosed and classified from its main clinical signs and symptoms and trough the Research Diagnostic Criteria for Temporomandibular Disorders. RDC/TMD is the gold standard for diagnosis. Besides the typical muscle disorders, pain is the symptom most reported by patients with dysfunction. This pain changes masticatory muscle activation, causing the muscle hyperactivity. Variable Duty Factor analyzes the percentage of total recording time at which the EMG muscle activation was above a predetermined threshold from the bite force. The analysis of muscle activity provided by this variable could be important to study deeply the changes arising from the dysfunction. So it is important to test the psychometric properties of this variable, such as reliability and responsiveness. The reliability of a variable refers to the degree of producing equal results, when applying repeated measures by the same examiner from the same subject. Responsiveness is the ability of a variable to detect clinically important changes over a specified period of time preset. This study will perform part of the validation of variable Duty Factor testing its reliability and responsiveness to pain in 7 female volunteers. Surface electromyography will be recorded in the rest position from masseter, temporalis and suprahioideos muscles in the presence of moderate pain (over 4cm in Visual Numeric Scale) and the absence or reduction of this pain, after applying a painkiller resource, the Transcutaneous Electrical Nerve Stimulation (45 minutes). Data processing and statistical variable will be based on mean values and standard deviations of percentage of the time muscle activation above the threshold of 10% of the value of maximum bite force during rest. The reliability will be tested with the intra-class correlation coefficient (2,1) with a confidence interval of 95%. The responsiveness of the variable will be analyzed in two electromyographic tests made one before and one after painkiller resource application, using the Effect Size and Mean Standardized Response. The results would contribute to the validation of one more factor in aiding the diagnosis of temporomandibular disorders and also in the investigation of the effects of pain in the physiological change in neuromuscular activation.
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