This study aimed to provide normal reference values of surface electromyography (sEMG) and mandibular kinematics in Chinese young adults, compare the sex differences and assess the diagnosis value of these indices.

Temporomandibular disorders (TMD) are one of the leading causes of craniofacial pain, and a high incidence of TMD in young adults has been reported. Previous studies have used sEMG and mandibular kinematic analysis to diagnose TMD. This was an observational study that healthy young adults with individual normal occlusion were strictly selected by diagnosis standard, and TMD patients with disc displacement were recruited. The sEMG signals of the anterior temporalis (TA), masseter (MM), and sternocleidomastoid and digastric were recorded in the mandibular postural positions (MPP) and during maximal voluntary clenching (MVC) with K7 electromyograph. Mandibular kinematics, including maximum opening and opening/closing velocities, were assessed by K7 kinesiograph.

Inclusion Criteria:

• Centred midline and no marked restriction and deviation of mouth opening and closing; ·Overjet and overbite of 1-3 mm, bilateral molar support with molar and cusp relation of Angle's class I;
• Presence of complete permanent dentition, except third molars.

Exclusion Criteria:

• History of local or general trauma;
• Presence of systemic diseases, neurological or psychiatric disorders, muscular diseases, cervical pain, or TMD based on the Research Diagnostic Criteria (RDC);
• Pregnancy;
• Consumption of anti-inflammatory, analgesic, antidepressant, or myorelaxant drugs;
• Presence of parafunctional facets and anamnesis of parafunctional tooth clenching, bruxism, or unilateral chewing;
• Presence of obvious dentition crowding or spacing, malposed, supernumerary or fractured tooth, visible caries, tooth abrasion/hypersensitivity, toothache, periodontal disease, or occlusal discomfort;
• Fixed or removable restorations, tooth filling, or occlusal adjustment that affected the occlusal surfaces;
• Previous or concurrent orthodontic, orthognathic, or TMJ treatment.

Healthy volunteers were recruited from college students of Fujian Medical University, while the temporomandibular disorder participants were recruited through public invitations, via recruitment posters and personal contacts.