Fatigue and fibromyalgia syndrome: clinical and neurophysiologic pattern.

The concept of 'fatigue' is strictly related to parameters of the setting in which fatigue is measured. Therefore, it is mandatory to provide a definition of fatigue and the modalities of its use. This is of pivotal importance with regard to the fibromyalgia (FM) syndrome, where fatigue is the most invalidating symptom and where, paradoxically, no clear and widely accepted definition of fatigue is available in the literature as yet. In the clinical setting, fatigue can be measured by different methods of various complexity. The simplest technique to assess fatigue involves the use of a visual analogue scale (VAS); however, a number of scales with differing levels of complexity are available for use. It is, often, difficult to detach the term 'fatigue' from tiredness and task failure, which correspond to two completely distinguished forms of fatigue: one with central origin (tiredness) and another which is localised within the muscle (peripheral muscle fatigue). The former is related to changes in motor-unit-recruitment strategies, whereas the latter is attributed to changes in membrane properties. To extensively assess fatigue and, partially, to avoid confusion among the types of fatigue described above, a number of laboratory tests have been developed; among these, there are multichannel surface electromyography (EMG) recordings. Using this type of an approach, it is possible the estimation of motor unit location within the muscle, the decomposition of the surface EMG (sEMG) interference signal into constituent trains of motor unit action potentials (MUAPs) and the analysis of single unit properties.