EMG amplitude and frequency at different muscular elongations under constant masticatory force or EMG activity

The relation between mechanical muscular activity (5 - 7.5 - 10 - 15 - 20 Kp of masticatory force) and electrical muscular activity (EMG) was studied during prolonged isometric contractions of the human masseter and temporal muscles, at different degrees of muscular elongation (0.5 - 5 - 10 - 20 mm of jaw opening). In Series 1, recordings maintaining muscular force values constant show that EMG activity increases with a shorter muscular elongation (0.5) and decreases with maximal elongation studied (20 mm). The slope of EMG activity curves rises as the force applied increases and muscular elongation decreases. Analyzing the relation between muscular force and EMG activity as a quotient between integrated activity and activity rate, we observed a greater influence of amplitude of the interference EMG pattern in small ranges (up to 7.5 Kp). However, with higher forces, the mechanism of frequency rate is the predominant factor. In Series 2, recordings of masticatory force maintaining EMG constant show that, for any chosen percentage of EMG activity, masticatory force increases with the increase of muscular elongation. Results show that there is an optimal muscular elongation of maximal efficiency.