Fatigue can be present as a trait characteristic or a state variable. Thereby, trait fatigue describes the fatigue experienced by an individual over a longer period of time (e.g., weeks, months), which is relatively stable and a symptom of various diseases (e.g., multiple sclerosis, chronic obstructive pulmonary disease). In contrast, activity-induced state fatigue is characterized by an acute and temporary reduction in motor or cognitive performance (i.e., motor or cognitive performance fatigue, respectively) as well as the subjective experience of exhaustion or weariness that occurs in the context of a specific motor or cognitive task (i.e., perceived motor or cognitive fatigue, respectively). Motor performance fatigue (e.g., decrease in maximal voluntary contraction force) depends on the muscle activation characteristics and contractile function of muscles, whereas cognitive performance fatigue (e.g., increase in reaction time) depends on the integrity of the central nervous system. The motor and cognitive task-induced increase in the perception of fatigue is influenced by the psychophysiological state of an individual. Motor and cognitive performance fatigue, as well as perceived motor and cognitive fatigue, are interdependent, depend on several modulating factors (e.g., characteristics of the task and subject), and are susceptible to perturbations in body homeostasis.