Jaw muscle silent periods before and after rapid palatal expansion.

1. The electromyographic silent period was observed in jaw-closing muscles during a maximal clench following a tap to the mandibular symphysis in eight subjects before and after rapid palatal expansion. 2. No differences in mean silent period duration were observed among the three jaw-closing muscles examined. 3. A large (r = 0.96) and statistically significant (p less than 0.01) correlation coefficient between two sets of silent period durations at one session indicates that these measurements are repeatable and reliable. 4. The mean silent period duration for the eight patients with malocclusions was 33.5 msec. prior to rapid palatal expansion and 51.2 msec. after the completion of palatal expansion. The difference was statistically significant (p less than 0.01) and represents a 61 percent increase. 5. Patients with symptoms of TMJ dysfunction are not the only group showing a prolonged silent period duration.