Fast Fourier transform analysis of the masseter muscle EMG during reaction to a warning signal.

Frequency changes of the masseter muscle EMG were examined in eight healthy subjects during the reaction time for unilateral jaw biting paradigm. The subjects were instructed to bite on a small piece of wood after the second of two visual signals two seconds apart. In five subjects, the EMG frequencies shifted to lower significantly in both the first (0-1 sec) and the second (1-2 sec) one-second periods after the first signal. In two subjects, the EMG frequencies shifted to lower significantly after the first signal in either the earlier or the later period. In one subject, the EMG frequencies did not change. This slowing of the EMG after the first signal might be related to recruitment of larger motor units. This phenomenon must be related to motor preparation for jaw biting.