Reliable measurement of incisal bite force for understanding the control of masticatory muscles

•A protocol was designed to assess the incisal bite force and EMG of jaw elevators.•The protocol was reliable for the estimation of incisal bite force steadiness.•Incisal bite force variability scales proportionally to the mean force (5 %MVC to 20 %MVC).•Force and EMG are linearly related for both temporalis and masseter muscles.•EMG median frequency does not change with contraction intensity of jaw elevators. In the present study, we aimed at evaluating the steadiness of incisal bite force during isometric contractions of masticatory muscles. Two separate experiments were carried out in 11 healthy young women. A first experiment was performed to test the reliability of our protocol for measurement of incisal bite force steadiness. The second experiment aimed to evaluate the steadiness of incisal bite force at four submaximal (i.e., percentage of maximum voluntary contraction, MVC) levels (5 %MVC, 10 %MVC, 15 %MVC, and 20 %MVC), along with the bilateral myoelectric activity of two masticatory muscles (temporalis and masseter). The results from the first experiment showed that our protocol is substantially reliable (intraclass correlation coefficient, ICC > 0.80) for estimating force variability and moderate reliable (0.60 < ICC < 0.80) for estimating spectral properties of force signals. In the second experiment, we found that force standard deviation (SD) increased proportionally to the power of mean force, and coefficient of variation (CoV) was higher at low-intensity contractions and maintained at an approximately constant level for high-intensity contractions. The force-EMG relationships were linear for both muscles at the contraction intensities evaluated in the study (5 %MVC to 20 %MVC), and the median frequency did not change with contraction intensity. Therefore, we presented a reliable method to estimate the incisal bite force, along with additional data on force control and myoelectric activity of jaw elevator muscles during isometric steady contractions.