ABSTRACT
The word fatigue has several meanings; in daily life "fatigue" is often used to express tiredness or weakness, whereas in technical discussion fatigue may include impaired intellectual or motor performance, increased electromyographic (EMG) activity for a given performance, shift of the EMG power spectrum to lower frequencies, or inability of muscle to generate force. A recent NHLBI Workshop defined fatigue as the loss of capacity to develop force and/or velocity in response to a load that is reversible by rest (NHLBI, 1990). As a first approximation, fatigue ofthe respiratory muscles may be defined as an inability to continue to generate sufficient pressure to maintain alveolar ventilation. Fatigue should be distinguished from weakness, in which reduced force generation is fixed and not reversed by rest, although the presence of muscle weakness may itself predispose to muscle fatigue (Table 1).
