Evaluation of central and peripheral fatigue using fractal dimension and conduction velocity.

Beretta Piccoli, Matteo and Barbero, Marco and D'Antona, Giuseppe and Fisher, Beth and Dieli-Conwright, Christina and Clijsen, Ron and Cescon, Corrado (2014) Evaluation of central and peripheral fatigue using fractal dimension and conduction velocity. In: XX Congress of the International Society of Electrophysiology and Kinesiology, 16-18 July 2014, Roma.

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Background and aim: Over the past decade, linear and non-linear surface EMG descriptors for central and peripheral fatigue have been developed. We tested fractal dimension (FD) as an indicator of central fatigue, and conduction velocity (CV) as an indicator of peripheral fatigue. FD was initially used to characterize patterns of MU recruitment, but subsequently, Mesin and colleagues (2009) suggested that FD is also related to MU synchronization during muscle fatigue. Hence, the objectives of this study were: (1) to describe myoelectric manifestations of fatigue using FD and CV slopes as indexes of central and peripheral fatigue, respectively, during isometric contractions in vastus medialis (VM) muscle in healthy subjects; and (2) to analyze the relationship between FD and CV slopes and the correlation of the FD slope with another indicator of central fatigue (i.e., CAF). Methods: A total of 29 recreationally active women (mean age±SD: 24±4 years) performed two knee extensions: (1) at 20% maximal voluntary contraction (MVC) for 30 s, and (2) at 60% MVC held until exhaustion, interspersed with five minutes of rest. Surface EMG signals were detected from the VM using bidimensional arrays. FD was computed with a numerical algorithm using non-overlapping signal epochs of 1s (Gitter and Czerniecki, 1995), whereas CV was estimated with a multichannel algorithm on single differential signals (Farina and Merletti, 2003). The evaluation of CAF was obtained by means of electrically elicited contractions using the twitch interpolation method (Gandevia et al., 1998). Results: The average time course of FD and CV, during the isometric contractions at 20% MVC and 60% MVC is depicted in Figure 1. Both FD and CV showed a significant decrease during the sustained 60% MVC contraction. A significant positive correlation was observed between FD and CV (R=0.52; p<0.01) and between FD and CAF (R=0.49; p<0.01) during the sustained 60% MVC (Figure 2). Conclusions: This study investigated, for the first time, FD and CV slopes as indexes of central and peripheral muscle fatigue during isometric contractions in recreationally active women. The negative slopes of FD suggest that there is an increase of MU synchronization during the endurance 60% MVC contraction, probably due to an adaptation to muscle fatigue by the central nervous system. Interestingly, at 20% MVC the level of synchronization remains almost constant, just as with the muscle fiber CV. The correlation observed between the normalized slopes of FD and CV could be explained by the fact that the fatiguing task induced both central and peripheral fatigue, as a result of mutual interactions between central and peripheral mechanisms. FD slope and CAF showed a direct correlation which is counterintuitive considering that the slope of FD is inversely proportional to central fatigue. Therefore FD and CAF may deal with different aspects of central fatigue. Future studies are needed to confirm FD and CV as universally acceptable indexes of central and peripheral fatigue and their roles in unraveling the impact of both these aspects of muscle fatigue in sport sciences and in several diseases.

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