Quantifying fatigueWhatismusclefatigue?Incontrasttothefatigueencounteredinclinicalsettings(Baileyetal.2007;Friedmanet al.2007; Hacker & Ferrans, 2007), the termmuscle fatigue is used to denote a transient decrease inthe capacity to perform physical actions. The followingexcerptscharacterizetherangeofeffectsascribedtomusclefatigue:‘Intensiveactivityofmusclescausesadeclineinperformance, known as fatigue...’ (Allen & Westerblad,2001).‘Performing a motor task for long periods of timeinduces motor fatigue, which is generally defined as adecline in a person’s ability to exert force.’ (Loristet al.2002).‘...CNS administration of caffeine increased treadmillrun time to fatigue...’ (Daviset al.2003).‘...a fatiguing task was performed with the muscles ofthe left hand until the muscles were exhausted.’ (Edgley& Winter, 2004).‘Fatigue is known to be reflected in the EMG signal as anincreaseofitsamplitudeandadecreaseofitscharacteristicspectralfrequencies.’(Kallenberget al.2007).‘...the sensation of fatigue is the conscious awareness ofchangesinsubconscioushomeostaticcontrolsystems...’ (St Clair Gibsonet al.2003).‘The primary purpose of the study was to use functionalmagneticresonanceimaging(fMRI)todetermineC2008 The Authors. Journal compilationC2008 The Physiological SocietyDOI: 10.1113/jphysiol.2007.139477
12R. M. Enoka and J. DuchateauJ Physiol586.1the association between feelings of mental fatigueandbloodoxygenleveldependent(BOLD)brainresponsesduringamentallyfatiguingcognitivetask.’ (Cooket al.2007).Muscle fatigue, it seems, can refer to a motor deficit, aperception or a decline in mental function, it can describethe gradual decrease in the force capacity of muscle or theendpoint of a sustained activity, and it can be measured asa reduction in muscle force, a change in electromyographicactivity or an exhaustion of contractile function. Suchbroadusageisproblematic,however,becausefatigueinthiscontext can encompass several phenomena that are eachthe consequence of different physiological mechanisms,which reduces the likelihood that the cause of musclefatigue can be identified. To circumvent this limitation,most investigators invoke a more focused definition ofmuscle fatigue as an exercise-induced reduction in theability of muscle to produce force or power whether ornot the task can be sustained (Bigland-Ritchie & Woods,1984; Søgaardet al.2006).A critical feature of this definition is the distinctionbetween muscle fatigue and the ability to continue thetask. Accordingly, muscle fatigue is not the point oftask failure or the moment when the muscles becomeexhausted. Rather, muscle fatigue is a decrease in themaximal force or power that the involved muscles canproduce, and it develops gradually soon after the onsetof the sustained physical activity. A common protocolused to quantify the development of muscle fatigue is tointerrupt the fatiguing exercise with brief maximal contra-ctions (voluntary or electrically evoked) to estimate thedecline in the maximal force capacity (Merton, 1954;Bigland-Ritchieet al.
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