A review of definitions of fatigue - and a step towards a whole definition

Despite its importance to health and safety, there is a long history of disagreement about how to operationalize fatigue when studying exertion in human transport operators. The current article reviews existing definitions, and consequently proposes a new definition as a step forward. A consideration of everyday use of the term finds that people often seem to use fatigue to describe a sensation related to exertion. Formal definitions of fatigue can be divided into a few broad definitions, capturing experiential, physiological and performance aspects of the construct, and many narrow definitions, focusing only on one or two of these aspects. Most existing definitions do not account explicitly for the role of sleep drives and sleepiness. They also fail to account for a wide range of factors associated with transport operator exertion, such as motivation and individual, organizational and environmental factors. Each of these points is assimilated in the derivation of a new “whole definition” of fatigue, in which the experience of human operator fatigue is a central aspect of the fatigue process. Although multidimensional and diffuse, the evolved definition does not detract from the measurement and study of limited aspects of fatigue. Rather, by describing the dynamic complexity of fatigue, it may help make explicit what different studies do or do not measure or account for in terms of the different aspects of fatigue. It is claimed that the proposed definition could be used to help harmonise attempts to study and tackle fatigue in transport health and safety contexts.