Experimental pain is a method used to simulate painful symptoms related to Temporomandibular Disorders (TMDs) and its effect on muscle activity can be studied by electromyographic exams. The aim of this study is to assess the effect of pain hypervigilance and experimental pain in the sternocleidomastoid muscle on the pressure pain threshold and electrical activity of the Temporal Anterior, superficial and deep portions of the Masseter and Sternocleidomastoid muscles, before and during prolonged unilateral mastication, as well as the feeling of muscle fatigue and pain expectation. After the approval of the Research Ethics Committee, 40 patients (20 men and 20 women) with normal health or mild system diseases and without TMD symptoms will be selected. They will respond to the Diagnostic Criteria for Temporomandibular Disorders (DC/TMD) and Pain Hypervigilance and Awareness (PVAQ) questionnaires. All patients will be submitted to two evaluations: 1 - at the first moment the questionnaires will be applied, the pressure pain threshold exam (algometry) and determination of the habitual mastication side will be performed; in addition, surface electromyography exams of the Anterior Temporal, Masseter (superficial portion) and Sternocleidomastoid muscles will be made, as well as fine wire electromyography of the deep portion of the Masseter muscle before and during prolonged mastication of chewing gum on habitual mastication side. 2- Two weeks after the first session, a Neuronal Growth Factor (NGF) injection will be performed on the Sternocleidomastoid muscle in the habitual mastication side, the algometry and electromyographic exams will be repeated. During the electromyographic examination the fatigue and the expectation of pain will be evaluated by a visual analogue scale (VAS) in both sessions. The data collected will be submitted to appropriate statistical tests.
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