Three types of temporomandibular joint dysfunction (TMD) pain are recognized: arthrogenous, myogenous, and mixed. Their treatment is different, emphasizing the need for the proper differential diagnosis. This diagnosis is currently made on clinical grounds. In an attempt to obtain more objective measurements, we have been performing surface electromyography (sEMG) on masseter muscles, bilaterally, while TMD patients clench their teeth, and have observed different activity patterns. While different patterns may be related to the different types of TMD, our equipment does not allow us to standardize the bite force nor to know what the accompanying bite force may be. This deficiency prohibits us from making any definitive comment about the differences in EMG. Equipment is now available that will permit us to record simultaneously from the masseter and temporalis muscles, bilaterally, as well as the bite force from both sides of the mouth. In addition, fatigue of the muscles can be monitored via the spectral frequency of the EMG signal. With these added measurements, we plan to compare EMG patterns, force production, and fatigability in patients clinically diagnosed with the different types of TMD to see if there is objective physiological evidence that can help in the differential diagnosis. These same procedures can also be used to monitor these patients as they progress through treatment to determine if these objective measures change with the clinical signs of improvement. Since we anticipate three groups of patients, separate one-way ANOVAs will be used to compare the groups 1) on their bite forces (Newtons), 2) RMS values of the accompanying EMG activity (uV), and 3) on their slopes of the median frequency of the different muscles over time. Correlation coefficients will be determined for aspects such as uV versus Newtons, and slopes of median frequency versus Newtons.