Fatigue is a frequently occurring symptom in many patient groups (e.g. multiple sclerosis, traumatic brain injury, Parkinson’s syndrome, and long-COVID). Fatigue negatively affects quality of life and the ability to be employed. Although many factors that contribute to sense of fatigue have been identified, most underlying mechanisms are still unknown. Fatigue has a multifactorial basis and investigating fatigue requires (simultaneous) combinations of different techniques. Fatigue is per definition self-reported and can be measured with questionnaires. In contrast, fatiguability refers to a quantitative decline in performance. This decline can be measured during for instance motor or cognitive tasks and strongly affects a person’s sense of fatigue.
Studying fatigue and fatigability across different patient groups provides a unique approach to further our understanding of fatigue and fatigability. Although the pathophysiological mechanisms fundamental to these illnesses differ, similarities between mechanisms and manifestations of fatigue may exist and will help us to understand the mechanisms underlying fatigue. Among techniques to be used will be nerve and cortical brain stimulation, electromyographical recordings, functional MRI, (sub)maximal exercise tests and various behavioral measurements (force measures, motor learning, reaction times).