Lower-lip EMG and displacement during bilabial disfluencies in adult stutterers

The purpose of this study was to describe the temporal patterns and spectral characteristics of antagonist lip muscle EMG and lip displacement during bilabial disfluencies in adult stutterers. Selective EMG recordings were obtained in five adult stutterers of mentalis and depressor labii inferior motor units (MENT and DLI) that were activated in a reciprocal manner during fluent speech. During disfluencies (N = 91) these same motor units were activated primarily in a reciprocal manner or DLI was not activated. These findings are not consistent with previous reports that stutterers' disfluencies involve cocontraction and overall increases in the activation levels of antagonist muscles. Spectral analysis of bilabial disfluencies revealed peaks at 5-9 Hz in the MENT EMG spectra of the five subjects and at 5-6 Hz in the displacement spectra of two subjects. A refinement in the classification of stutterers' disfluencies is suggested on the basis of individual differences in lip EMG and displacement spectra, and the mechanisms of stuttering tremor are discussed.