Endurance and recovery from a sustained isometric contraction in human jaw-elevating muscles

Ten males sustained maximal voluntary contractions (MVC) of the jaw elevators. Unilateral bite force and electromyographic (EMG) activity were recorded from the right masseter and temporalis. The experiment comprised three endurance trials, and each trial consisted of two sustained 100% MVC clenching tasks. Between the two tasks, the subjects took a randomly assigned rest of either 30, 120 or 300 s. Immediately after each task, they performed an additional brief 100% MVC to check for contractile failure. EMGs were taken from the beginning and end of each sustained 100% MVC and used to determine and compare the EMG centre frequency. The difference in endurance times between the first and second clenchings was greatest for the 30-s rest and progressively decreased as the rest period increased. The beginning EMG centre frequency was significantly greater than the end for each clenching. These findings strongly support the proposal that even though the jaw elevators are resistant to a contractile element failure, a contraction-induced pain is produced, which limits a sustained jaw-closing effort. Recovery from this effort pain is related to the post-contraction blood flow.