18.1: What is Fatigue?

Fatigue is a word familiar to many people; it can be used to describe how a person is feeling ("I'm really fatigued from my flight") or describe an apparent alteration in someone's behaviour, for example, a decline in an athlete's performance ("they are fatigued and making lots of mistakes"). Somewhat surprisingly, despite the ubiquitous use of the word in everyday language, scientists have had a hard time describing what fatigue actually is. Fatigue has been described as either a feeling of tiredness, weariness or exhaustion, a measurable decrement in performance of an activity caused by the prior extended performance of the same activity, or an insufficiency of the muscles or central nervous system to maintain physical or mental performance (Hockey, 2013). Fatigue is also often delineated by its proposed cause and consequence: Fatigue has been described as either mental/cognitive or physical/physiological, can be either central or peripheral, or can be acute or chronic. The term mental fatigue is typically used to describe feelings or behaviours which arise from the sustained performance of mentally demanding tasks, whilst physical fatigue is used to describe feelings and behaviours which arise from sustained performance in physically demanding tasks. Central fatigue can mean either mental fatigue or fatigue caused by, or which causes, a change in central nervous system function. Peripheral fatigue usually, but not always, refers to fatigue caused by or which causes changes in muscle function. Acute fatigue describes the temporary fatigue which arises from sustained task performance and which is alleviated by rest. Almost everyone experiences acute fatigue at one point in their lives, perhaps from studying hard for an exam or from a physically demanding hike.

With all these different definitions and delineations, you might have become fatigued! Thankfully for us, and for the purposes of this book chapter, several attempts have recently been made to provide a unified description of fatigue that, as scientists, we can use to describe, study, and treat fatigue (Kluger et al., 2013; I. Penner & Paul, 2017). Many diseases and disorders have their own definitions of chronic fatigue. We will present some of these for cancer, multiple sclerosis and stroke in sections of this chapter. However, as you will see, there are many common traits in each definition, supporting the proposal that a unified taxonomy can be used to describe fatigue (Kluger et al., 2013). Drawing together these disease-specific definitions, we propose the following definition of chronic fatigue, which is applicable across a range of diseases: A persistent feeling or perception of weariness, tiredness or exhaustion that is not alleviated by rest, is not proportional to activity levels and is a disabling negative symptom that interferes with daily activities and impairs quality of life. Importantly, the definition allows us to focus on fatigue as a symptom that is experienced across a range of diseases and disorders. This feeling or perception of fatigue may or may not be accompanied by a measurable decline in physical or mental function and performance, which has been referred to as performance fatigability (Kluger et al., 2013).