The Role of Transcranial Magnetic Stimulation in the Study of Fatigue

Fatigue is a frequently reported complaint in a wide variety of conditions, particularly in disorders of the nervous system. Two issues complicate the study of fatigue in humans. The first is that fatigue may be defined in a number of ways, depending on the approach taken. From a functional perspective, fatigue can be described as ‘‘a failure in the capacity of the neuromuscular system to generate force or power.’’ 1 Alternatively, fatigue may be expressed in purely subjective terms (‘‘a sustained sense of exhaustion and decreased capacity for physical and mental work’’ 2). This is not purely a semantic issue because it has important implications for measuring fatigue. For the purpose of clarity, the term fatigue is used here primarily in its functional context. Both of these issues are discussed later in relation to findings using transcranial magnetic stimulation (TMS)