Changes in voice fundamental frequency following discharge of single motor units in cricothyroid and thyroarytenoid muscles.

This investigation was designed to measure voice Fo changes related to single motor unit (SMU) contractions in the cricothyroid and thyroarytenoid muscles. Four subjects (3 men and 1 woman) were recorded producing a prolonged vowel at modal pitch and loudness levels while simultaneous recordings of electromyograms (EMG) from the muscles were obtained. Voice Fo changes unrelated to SMU firings in the muscles were eliminated using an averaging method previously described by Baer (1979). Results indicate that the time between discharge of the SMU and the peak in Fo change ("Fo Latency") was variable and ranged from 5 to 20 ms for the thyroarytenoid and 6 to 75 ms for the cricothyroid muscle. Distinct oscillations in Fo were always present in recordings from the woman subject and from the men when they phonated at higher-than-modal pitch levels. The findings are discussed in relation to SMU contraction times, biomechanics of the vocal folds, and the presence of jitter in normal voices.