Temporomandibular Joint Disorder: An Investigation of Masseter Muscle Activity in Response to Stressful Computer Data Entry

Temporomandibular joint disorder (TMD) is a set of conditions affecting the joint connecting the mandible to the temporal bone and the surrounding structures responsible for controlling jaw movement. Symptoms of TMD are characterized by facial pain and/or muscle tenderness, popping or clicking joint sounds, and a limited range of mandible motion. Previous research suggests that 60-70% of the population may have at least one symptom of TMD while only 5% will seek treatment. There is evidence that psychological stress can factor into the disorder. A number of studies have used electromyography (EMG) to attempt to correlate experiment-induced stress with increased masseter activity; the results from these studies are generally mixed and inconsistent. Similar studies also failed to present a stressful stimulus with suitable external validity. The current study improves upon this research by using EMG to investigate masseter muscle response to stressful computer data entry among symptomatic and control participants. There were 8 symptomatic and 8 control participants recruited. The independent variables in the study were computer condition (neutral vs. stressful) and group (symptomatic vs. controls). The dependent variable was the EMG activity associated with the clenches. The study employed a repeated measures design; all participants completed 20 minutes of non-stressful (neutral) data entry followed by 20 minutes of stress-inducing data entry. The stressful computer program had a variable lag built in and would at times inform the participant that data they entered was incorrect, even though it was not. The EMG activity for participant clenches was analyzed. The clenches were normalized to each participant’s own maximum voluntary clench (MVC) and characterized by count, average duration, average peak, average power, total duration, maximum peak, and total power (defined as the area under the normalized EMG curve). Skin conductivity readings and self-report results confirmed that the participants were significantly more stressed during the stressful data entry. It was found that there was a significant main effect for group for the dependent variables of clench count, total duration, and total power; these mean values were significantly greater for the symptomatic group than for the control group. A significant main effect for computer condition was found for total clench duration; the total duration of the clenches in the stressful data entry was longer than those in the neutral data entry task. Significant interaction effects were found for average power, average peak, and maximum peak; the means of these variables increased for the control participants during the stressful data entry but decreased for the symptomatic group. Possible muscle fatigue and stress adaptation among the symptomatic group was suggested to explain the interaction. Further research in this area needs to be conducted to understand risk factors, such as stress, for TMD.