Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Objectives: To evaluate electrical activity [EMG] and muscular thickness by ultrasonography of sternocleidomastoid [SCM] muscles in young adults with temporomandibular disorders [TMD], and to determine the effects of head posture on these measures. Methods: The study subjects were recruited into a TMD group and a healthy normal control group. The TMD diagnoses were made using the Research Diagnostic Criteria for TMD. The control group subjects did not meet those TMD criteria. The head tilt was clinically evaluated in all subjects. Data were collected three times at rest, maximal clenching, flexion, and extension of the head, bilaterally. Shapiro-Wilks, Mann Whitney, Kruskal Wallis, unpaired t test, ANOVA and Tukey test, and correlation and logistic regression were applied [alpha=0.05]. Results: The TMD group consisted of 19 individuals [age 25.4 +/- 3.8 years]. The cControl groups consisted of 28 individuals [age 25.9 +/- 4.7 years]. The controls showed greater values in activity and thickness for the right SCM than TMD subjects during flexion and relaxation, respectively. The TMD group presented lower thickness for both muscles when the head was tilted to the right, except during extension. During clenching, EMG activity was significantly higher for the TMD group when the head was to the right. The right SCM showed lower activity during flexion in the TMD group. Ultrasonography and EMG were positively correlated for the right SCM during flexion when the head was to the right. For the other positions, the correlations were negative. The TMD group was more likely to present a lower SCM activity during flexion. Conclusions: The SCM thickness and activity are influenced by the presence of TMD during neck and mandibular movements, as well as the side of head tilt.2218998Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)FAPESP [Fundacao de Amparo a Pesquisa do Estado de Sao Paulo, Brazil]