ABSTRACT
Fatigue is a term that has a wide range of meanings. It is a common symptom in many illnesses where fatigue is used to describe feelings of sleepiness, of general lack of energy, and of the need for abnormally high effort to perform tasks, whether physical or mental (e.g., Dinges, 1995; Hossain et al., 2005; Johansson et al., 2009; Smets et al., 1995; Torres-Harding & Jason, 2005; Zwarts et al., 2008). It may be associated with physical weakness or difficulty in performing cognitive tasks. In health, similar feelings of fatigue can occur after physical or mental exertion. Fatigue is also used more specifically in relation to an episode of physical or mental work but even then it has both a perceptual and a performance aspect. That is, during physical or mental exercise people have both sensations of fatigue and a decrement in maximal performance, which can also be described as fatigue (e.g., Boksem & Tops, 2008; Enoka et al., 2011; Gandevia, 2001; Knicker et al., 2011). Finally, fatigue is used to refer to activity- dependent changes of the muscles or nervous system when these lead to a decrease in motor output (e.g., Enoka & Duchateau, 2008; Gandevia, 2001; Taylor & Gandevia, 2008). Ash (1914) described fatigue as ‘a comprehensive term which in its widest application embraces all those immediate and temporary changes, whether of a functional or organic character, which take place within an organism or any of its constituent parts as a direct result of its own exertions, and which tend to interfere with or inhibit the organism’s further activities.’
