Acoustic, Aerodynamic, Morphometric, and Perceptual Changes During and After Semi-Occluded Vocal Tract Exercise: An Integrative Review

Abstract

Objective: The body of literature discussing the acoustic, aerodynamic, perceptual, and morphometric changes that occur during and after semi-occluded vocal tract exercise (SOVTE) has dramatically expanded within the past 20 years. The current study integrates the literature on SOVTE from the 1990s onward, reviewing the technique's clinically relevant effects in vocally untrained adults both with and without dysphonia. The study aims to give clinicians actionable information on how SOVTEs alter vocal function in both normal and pathological states. Methods: The author queried the MEDLINE database for combinations of search terms related to semi-occluded vocal tract exercise. To improve article identification, the author also performed iterative citation webbing in which the reference lists of each article selected for full-text screening were cross-referenced against the articles returned in the initial MEDLINE search. Articles identified by iterative citation webbing that did not appear in the initial MEDLINE search were then screened individually. The second author independently verified adherence to the review's inclusion and exclusion criteria in both the initial search and data extraction phases. Results: The initial MEDLINE search returned 869 articles, 111 of which passed the title and abstract screening phase. Iterative citation webbing returned an additional 20 studies, resulting in a total of 131 articles that qualified for full-text screening. 53 articles passed full-text screening and were included in the current review. Discussion: Semi-occluded vocal tract exercise increases the inertive reactance of the glottis, vocal tract, and air column, as well as introducing flow resistance at the level of the lips or velopharyngeal port. Each of these mechanisms yields downstream acoustic, aerodynamic, morphometric, and perceptual changes, including reductions in phonation threshold pressure and perceived phonatory effort, improved spectral characteristics of the acoustic signal, attenuation of vocal fold impact stress as indexed by various metrics such as maximum area declination rate, and alterations to the physical dimensions of the vocal tract. Although few studies examined the duration of these changes post exercise, several RCTs support the assertion that SOVTE's effects can be sustained with regular practice over weeks or months, regardless of current vocal health status.