Objectives : To investigate to what extent different types of jaw-muscle contractions cause haemodynamic reactions in human masseter muscle. Materials and Methods : Eleven healthy volunteers (seven males : 25.0±2.9 years and four females: 23.3±4.3 years) performed three standardized oral-motor tasks : maximal voluntary contractions (MVC ; duration 5 sec, 3 times repetition), tooth grinding (repetitive left and right side grinding from intercuspal position to canine-to-canine position at 0.5 Hz keeping 50% MVC for a total of 10 times), and 1-min left-side gum chewing at 1 Hz. Haemodynamic characteristics were measured in the left masseter muscle with the use of a laser blood oxygenation monitor (BOML1TRW, OMEGAWAVE INC., Tokyo, Japan). Electromyographic (EMG) activity from right and left masseter muscle was simultaneously monitored (500 Hz sample frequency) during the tasks. 1-ANOVA followed by Dunnett’s test was used. Results : Oxygenated haemoglobin (OXYHb : 13.5±0.2 104 units/mm3) and deoxygenated haemoglobin (deOXYHb : 7.6 ± 0.3 104 units/mm3) did not change significantly during the MVC task (13.9±0.2 and 7.8±0.3 104 units/mm3, respectively, P>0.065), however, the total haemoglobin (TOTALHb : 22.1±0.3 104 units/mm3) showed a significant increase (22.7±0.3 104 units/mm3, P=0.003) during the MVC. Tissue blood oxygen saturation was not changed during the MVC (P=0.164). During the tooth grinding task, OXYHb, deOXYHb, TOTALHb, and tissue blood oxygen saturation (StO2) remained constant (P>0.127). Finally, the chewing task was associated with significant decreases in StO2 (67.9±0.7%, P=0.006) related to a decrease in OXYHb (14.0±0.2 104 units/mm3, P=0.040) compared to baseline (68.8±0.7% and 14.2 ±0.3 104 units/mm3, respectively). Conclusion : These results showed that high-intensity experimental tooth clenching caused constriction-like reactions in the masseter muscle whereas tooth grinding did not cause detectable changes in haemodynamic characteristics of masseter muscle. Finally, the findings indicated that rhythmic dynamic contractions might lead to oxygen deficit in the masseter muscle. The present data may have implications for understanding the potential pathophysiological consequences of different types of oral-motor tasks, e.g., bruxism and prolonged mastication
