Effect of Sustained Jaw Opening on Fatigue, Pressure Pain Sensations, and Stiffness in the Masticatory Muscles

Abstract

Although the effects of sustained jaw-closing activities on somatosensory sensitivity and fatigue have been investigated, the effects of sustained jaw-opening activities on somatosensory sensitivity in the masticatory muscles remain unclear. Therefore, this study aimed to investigate the effects of sustained jaw opening（e.g. during dental treatment）on fatigue, pressure pain sensations, and stiffness in the masticatory muscles. A total of 35 healthy volunteers performed 30 minutes each of two jaw motor tasks, with a 1-week interval between tasks: unassisted jaw opening and jaw opening assisted by a mouth prop set at the right tooth. The pressure pain threshold（PPT）and muscle stiffness of the left masseter were measured before and after both jaw motor tasks（at 0 and 30 minutes）. Masticatory muscle fatigue was measured every 10 minutes（at 0, 10, 20, and 30 minutes）during each jaw motor task. No significant differences in the PPT were found between before and after the assisted jaw-opening task, but the PPT was significantly lower after compared with before unassisted jaw opening（P < 0.05）. No significant differences in left masseter muscle stiffness were found between any measurement point in either jaw motor task. Masticatory muscle fatigue was significantly higher after 10, 20, and 30 minutes of each jaw motor task compared with that at baseline（0 minutes）. However, no significant differences in masticatory muscle fatigue were found at any measurement point between both jaw motor tasks. The present results suggest that sustained jaw opening during dental treatment may affect pressure pain sensations in the masticatory muscles. In addition, masticatory muscle pain due to sustained jaw opening seems to be affected not only by jaw-opening but also jaw-closing muscle（masseter muscle）.