Some unresolved issues for the study of fatigue: the way forward

Self-reported aging-related fatigue: a concept description and its relevance to physical therapist practice

Fatigue is one of the most common symptoms experienced by older people, both with and without chronic disease. It is unpleasant and distressing and can affect functioning and quality of life. Fatigue also may inhibit an older person from participating optimally in a physical therapy program aimed at rehabilitation or the promotion of better health. However, confusion surrounds the concept of self-reported aging-related fatigue, not only because of the complexity of the problem itself but also because of lack of clarity of definition and the use of the term "fatigue" to describe a range of different concepts. This article aims to provide clarification of the concept in the context of physical therapist clinical practice. The intention is to increase awareness of the issue among physical therapists, promoting their assessment and consideration of the problem when planning health interventions involving functioning, physical activity, and exercise for older people.

Neurobiological studies of fatigue

Fatigue is a symptom associated with many disorders, is especially common in women and in older adults, and can have a huge negative influence on quality of life. Although most past research on fatigue uses human subjects instead of animal models, the use of appropriate animal models has recently begun to advance our understanding of the neurobiology of fatigue. In this review, results from animal models using immunological, developmental, or physical approaches to study fatigue are described and compared. Common across these animal models is that fatigue arises when a stimulus induces activation of microglia and/or increased cytokines and chemokines in the brain. Neurobiological studies implicate structures in the ascending arousal system, sleep executive control areas, and areas important in reward. In addition, the suprachiasmatic nucleus clearly plays an important role in homeostatic regulation of the neural network mediating fatigue. This nucleus responds to cytokines, shows decreased amplitude firing rate output in models of fatigue, and responds to exercise, one of our few treatments for fatigue. This is a young field but very important as the symptom of fatigue is common across many disorders and we do not have effective treatments.