[Electronic measurements of the masticatory muscles. 1: Methods of recording]

The application of surface electromyography as an aid in diagnosing craniomandibular dysfunction requires that pathological or treatment-induced changes of the EMG be distinguished with sufficient probability from random variations due to methodological or biological reasons. It was the aim of this study to determine the range of variability of elevator muscle EMG, to find experimental methods for improving reproducibility, and to establish evaluation procedures that allow the recognition of treatment-induced effects in the EMG even in the presence of great random variations. For this we measured the voltage/tension relation of the elevator muscles of 42 persons in the range of 20 to 500 N on up to 20 different days under constant experimental conditions. The majority of measurements revealed a non-linear correlation between EMG activity and resulting force, hence second order polynomials proved to be more appropriate than straight lines for representing the activity/tension correlation. Muscle activity was found to vary randomly from day to day within each individual. These fluctuations appeared as varying axis intersections, initial slopes and curvatures which are the determinants of the voltage/tension curves. In ten selected subjects random fluctuations were significantly higher when electrodes were repositioned by palpation of the muscle belly than with the aid of tattoos.