Recruitment patterns of motor units in speech production

Single motor units were recorded with intramuscular electrodes in sites selected to isolate units of the mentalis muscles of two human subjects. Order of recruitment of three groups of motor units was analyzed during repetition of syllables. Within each group motor units showed variable patterns of recruitment over repeated utterances. These recruitment patterns of labial motor units and the patterns observed by Sussman et al. in a jaw opening muscle are used to illustrate issues critical to interpretation of recruitment patterns of motor units active during speech. From extant data, inferences about the size of the motor units active cannot be made; however, the variability of recruitment patterns has significance for hypotheses about the underlying mechanisms of recruitment. Discussion includes the question of the size of motor units as inferred from action potential amplitude, differences in methodology between experiments in speech and those often used to interpret them, and the extent to which it is possible to isolate motor units from a single muscle in electromyography of facial muscles.