Several papers have recently appeared in the literature questioning the usefulness of surface EMG in the management of temporomandibular dysfunction (TMD) patients. One of the concerns is the stability of EMG over time in any one patient. This study was undertaken on normal subjects to determine the level of surface EMG stability over time using a variety of spectral analysis parameters. Four male and seven female subjects, ranging from 24 to 38 years of age, were selected for the study. All subjects had class I occlusions, intact dentition and no history of TMD. Oral and perioral hard and soft tissues were within normal limits. EMG data from the bilateral masseter muscles were obtained during four states of jaw activity: rest, right-sided chewing, left-sided chewing, and 50% maximal clench. Three sets of recordings were obtained in each study session. Three 10-min study sessions were obtained for each subject. Six measures, which included SP-50, central frequency, band width, effective band width, zero crossings and power spectrum, were analysed to assess their reliability and sensitivity over time during the several states of jaw activity. The conclusion of the study was that bilateral masseter EMG band width, effective band width, and power spectrum in normal humans are all reliable enough to be stable over a 6 week period, and sensitive enough to differentiate among jaw rest, right chewing, left chewing and 50% clench; power spectra being the most sensitive.

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The study concerned the nature of the alterations, if any, in muscle activity demonstrable when the mandible shifts from maximum intercuspation into its most retruded physiological relation (i.e. centric relation). An integrator-averager was used to determine micro V average amplitude from masseter and temporal muscles in two maxillo-mandibular positions, centric relation and maximum intercuspation, and three modes, first contact occlusion, chewing and swallowing. Vertical reference marks on the cuspids were used to quantitate horizontal deviation from maximum intercuspation to centric relation. Data were obtained from twelve subjects and analysed for variance. Results demonstrated a significant increase in micro V in all centric relation positions. Statistics yielded an F value of 5.88258 with a probability of 0.005. Results suggest critical limitations in reliance on centric relation as a reference position during clinical therapy.

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