Assessment of Respiratory Muscle Fatigue

The purpose of this Section is to outline a potential diagnostic strategy to assess the development of respiratory muscle fatigue in humans. At the outset, it is important to note that there is evidence suggesting that human respiratory muscle fatigue may develop in pathophysiological states associated with the development of respiratory failure (i.e., respiratory loading induced by lung disease) (1). It is also important to recognize that fatigue is defined as a loss of the capability to generate skeletal muscle force and/or velocity that is accompanied by recovery during rest (2). As a result, a single measurement of force is inadequate to detect fatigue: rather, muscle force generating or shortening capability must be demonstrated to fall during serial measurements over time. It could further be argued that a demonstration that force subsequently rises if muscle contraction is stopped and a rest period is provided would be necessary to fully satisfy the definition of fatigue and to exclude the possibility that a given fall in force did not represent muscle injury (the latter condition, by definition, does not improve with short periods of rest). It follows that muscle "fatigue" can therefore be distinguished from muscle weakness (i.e., a reduction in the level of force generation at a given point in time) and muscle injury (i.e., a slowly reversible or irreversible decrement in muscle contractility).

This Section is divided into a brief discussion of (1) the present theories regarding the genesis of muscle fatigue and the different types of muscle fatigue, and (2) a review of the various tests available with the potential to detect the development of fatigue in normal subjects and patients.

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