Systematic Review of Literature on Vocal Demand Response: Understanding Physiology, Measurements, and Associated Factors.

Purpose: Considering the conceptual migration from vocal load and vocal loading to vocal demand and vocal demand response, this review of literature aimed to identify physiological explanations, reported measurements, and associated factors (vocal demands) reported in the literature when considering the phonatory response to a vocal demand.

Methods: A systematic review of literature, following the PRISMA Statement, was conducted using Web of Science, PubMed, Scopus, and ScienceDirect. Data were analyzed and presented in two parts. First, a bibliometric analysis, co-occurrence analysis, and content analysis were performed. Three criteria that got article inclusion were defined: (1) written in English, Spanish, and Portuguese; (2) published between 2009 and 2021; and (3) focused on vocal load and loading, vocal demand response, and voice assessment parameters. A total of 54 publications met the criteria and were included in this review. The second part included a conceptual framework based on the content analysis of three aspects of vocal demand response: (1) physiological explanations, (2) reported measurements, and (3) vocal demands.

Results and conclusion: As would be expected since vocal demand response is a relatively new term and not yet commonly used in literature when discussing way that the speakers respond to communicative scenarios, most of the studies reviewed (both historical and recent) still use the term of vocal load and vocal loading. Although there is a broad variety of literature discussing a wide range of vocal demands and voice parameters used to characterize the vocal demand response, results show that there is consistency across the studies. While vocal demand response is unique and intrinsic to the talker, associated factors that contribute to this response include both internal talker and external talker factors. Internal factors include muscle stiffness, viscosity in the phonatory system, vocal fold tissue damage, elevated sound pressure levels during occupational voice demands, extended periods of voice use, suboptimal body posture, difficulties in breathing technique, and sleep disturbances. Associated external factors include the working environment (noise, acoustics, temperature, humidity). In conclusion, although vocal demand response is intrinsic to the speaker, the speaker's response is affected by external vocal demands. However, due to the wide methods to evaluate vocal demand response, it has been difficult to establish its contribution to voice disorders in the general population and, specifically, among occupational voice users. This literature review identified commonly reported parameters and factors that may help clinicians and researchers define vocal demand response.