Quantifying fatigue What is muscle fatigue In contrast to the fatigue

Quantifying fatigue what is muscle fatigue in

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Quantifying fatigue What is muscle fatigue? In contrast to the fatigue encountered in clinical settings (Bailey et al. 2007; Friedman et al. 2007; Hacker & Ferrans, 2007), the term muscle fatigue is used to denote a transient decrease in the capacity to perform physical actions. The following excerptscharacterizetherangeofeffectsascribedtomuscle fatigue: ‘Intensive activity of muscles causes a decline in performance, known as fatigue ... ’ (Allen & Westerblad, 2001). ‘Performing a motor task for long periods of time induces motor fatigue, which is generally defined as a decline in a person’s ability to exert force.’ (Lorist et al. 2002). ... CNS administration of caffeine increased treadmill run time to fatigue ... ’ (Davis et al. 2003). ... a fatiguing task was performed with the muscles of the left hand until the muscles were exhausted.’ (Edgley & Winter, 2004). ‘Fatigue is known to be reflected in the EMG signal as an increase of its amplitude and a decrease of its characteristic spectral frequencies.’ (Kallenberg et al. 2007). ... the sensation of fatigue is the conscious awareness of changes in subconscious homeostatic control systems ... ’ (St Clair Gibson et al. 2003). ‘The primary purpose of the study was to use functional magnetic resonance imaging (fMRI) to determine C 2008 The Authors. Journal compilation C 2008 The Physiological Society DOI: 10.1113/jphysiol.2007.139477
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12 R. M. Enoka and J. Duchateau J Physiol 586.1 the association between feelings of mental fatigue and blood oxygen level dependent (BOLD) brain responses during a mentally fatiguing cognitive task.’ (Cook et al. 2007). Muscle fatigue, it seems, can refer to a motor deficit, a perception or a decline in mental function, it can describe the gradual decrease in the force capacity of muscle or the endpoint of a sustained activity, and it can be measured as a reduction in muscle force, a change in electromyographic activity or an exhaustion of contractile function. Such broadusageisproblematic,however,becausefatigueinthis context can encompass several phenomena that are each the consequence of different physiological mechanisms, which reduces the likelihood that the cause of muscle fatigue can be identified. To circumvent this limitation, most investigators invoke a more focused definition of muscle fatigue as an exercise-induced reduction in the ability of muscle to produce force or power whether or not the task can be sustained (Bigland-Ritchie & Woods, 1984; Søgaard et al. 2006). A critical feature of this definition is the distinction between muscle fatigue and the ability to continue the task. Accordingly, muscle fatigue is not the point of task failure or the moment when the muscles become exhausted. Rather, muscle fatigue is a decrease in the maximal force or power that the involved muscles can produce, and it develops gradually soon after the onset of the sustained physical activity. A common protocol used to quantify the development of muscle fatigue is to interrupt the fatiguing exercise with brief maximal contra- ctions (voluntary or electrically evoked) to estimate the decline in the maximal force capacity (Merton, 1954; Bigland-Ritchie et al.
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