Is the notion of central fatigue based on a solid foundation?

Abstract

-Exercise-induced muscle fatigue has been shown to be the consequence of peripheral factors that impair muscle fiber contractile mechanisms. Central factors arising within the central nervous system have also been hypothesized to induce muscle fatigue, but no direct empirical evidence that is causally associated to reduction of muscle forcegenerating capability has yet been reported. We developed a simulation model to investigate whether peripheral factors of muscle fatigue are sufficient to explain the muscle force behavior observed during empirical studies of fatiguing voluntary contractions, which is commonly attributed to central factors. Peripheral factors of muscle fatigue were included in the model as a time-dependent decrease in the amplitude of the motor unit force twitches. Our simulation study indicated that the force behavior commonly attributed to central fatigue could be explained solely by peripheral factors during simulated fatiguing submaximal voluntary contractions. It also revealed important flaws regarding the use of the interpolated twitch response from electrical stimulation of the muscle as a means for assessing central fatigue. Our analysis does not directly refute the concept of central fatigue. However, it raises important concerns about the manner in which it is measured and about the interpretation of the commonly accepted causes of central fatigue and questions the very need for the existence of central fatigue. central fatigue; motor units; interpolated twitch; voluntary drive THE SOURCES of exercise-induced muscle fatigue have been debated since the early 1900s. There is general agreement on the influence of peripheral factors on muscle fatigue, i.e., those that develop within the muscle and impair the muscle fiber contractile mechanism, such as metabolite accumulation during prolonged exercise (see, e.g., Bergström et al. 1967;Hermansen et al. 1967;Pernow and Saltin 1971). These contractile impairments can be measured as changes in the amplitude and duration of the elicited muscle force twitch De Luca 2003, 2005;Burke 1981;Macintosh et al. 1994; Vandervoort et al. 1983).In addition to the influence of peripheral factors of muscle fatigue, some studies have put forth the notion of "central fatigue," i.e., a limitation in muscle performance caused by central factors. These would arise within the central nervous system (CNS) and diminish the voluntary drive to the motoneuron pool of a muscle. Potential sources include failure of the motor cortex (see, e.g., Gandevia et al. 1996;Todd et al. 2007) and the influence of afferent inputs at the spinal level (see, e.g., Gandevia 2001). Mosso (1904) was the first to draw attention to this concept, but others have followed (e.g., Bellemare and Bigland-Ritchie 1987; Gandevia 2001; Reid 1927). Today, central fatigue is commonly accepted as a physiological phenomenon that plays a relevant role in muscle fatigue and, according to BiglandRitchie et al. (1983) and Gandevia (2001), protects muscles against excessive effort.However, unlike the dire...

Effects of a Finger Tapping Fatiguing Task on M1-Intracortical Inhibition and Central Drive to the Muscle

“…The support for this statement is twofold. First, VA levels in the un-fatigued conditions were the expected values with reference to the literature reports, either considering empirical data 31 or modelling 32 . Second, when fatigue developed by repetitions of MVC over time (without ft ), we showed a strong lineal relationship between MVC reduction and central drive waning.…”

“…This effect (only present in the ft session ) is likely related to temperature increases induced by the repetitive nature of ft 39 . Currently, there is a debate regarding the validity of the twitch-interpolation technique to test central fatigue in the presence of SMR 32 . Some reports indicate that SMR contributes to overestimating the level of VA 40 , while others exclude this possibility 41 .…”

Methodological issues with the assessment of voluntary activation using transcranial magnetic stimulation in the knee extensors

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“…There are limits to the ability to measure neuromuscular fatigue within the brain and central nervous system, including the gold standard of the interpolated twitch technique. The limits of the interpolated twitch technique have previously been raised (Contessa et al 2016) and debated (de Haan et al 2009). However, much of the focus of debate concerning this technique has centred on studies that have performed short-duration exercise bouts and/or isometric contractions.…”