Vocal fatigue: current knowledge and future directions.

Vocal fatigue is a complex multifaceted clinical phenomenon. Several hypotheses exist concerning its underlying mechanism, and a range of empirical studies have examined its manifestation. This article reviews the literature pertaining to the nature, underlying processes, and salient features of vocal fatigue. First, vocal fatigue is defined, its major symptoms are discussed, and hypotheses concerning its primary physiological and biomechanical mechanisms are considered. Second, studies of experimentally induced vocal fatigue in humans are evaluated. Third, research investigating the clinical and occupational manifestations of vocal fatigue is discussed. Fourth, directions for ongoing research in this area are offered.