Abstract
Neuromuscular fatigue is a complex and multi-determined phenomenon. Its assessment usually requires the accomplishment of maximal voluntary contractions. More recently, it has been proposed that certain non-linear analyses (providing measures of complexity), applied to force signals acquired during submaximal contractions, could be considered as new indicators to quantify this fatigue. The aim of this PhD thesis project was to identify the interest of non-linear measures to quantify neuromuscular fatigue in different populations, using fatiguing cognitivo-motordual tasks, representative of daily living activities. Our first study highlighted a detrimental effect of the dual task on muscular endurance in a population of young adults, providing an interesting experimental framework to assess the ability of some measures of complexity to detect differences in fatigability. The second study, based on the use of sample entropy, revealed that the non-stationarity of force signals could lead to misinterpretation of fatigue-induced changes in neuromuscular complexity. In this study, we propose a methodological approach to limit this phenomenon. In our third study, we demonstrated the ability of recurrence quantification analysis to detect fatigue-induced changes in the complexity of force signals. Our results also show that this approach enables the identification of different fatigability profiles (i.e. male vs. female). These methods are currently being used in a clinical study conducted in people with chronic obstructive pulmonary disease (COPD). Considering the systemic nature of this disease, complexitymeasurements seem particularly relevant to detect alterations in neuromuscular dynamics caused by prolonged solicitation of the cognitivo-motor system. Our preliminary results from traditional methods of quantifying fatigue do not yet allow us to conclude whether there is an increase of the detrimental effect of dual tasks in COPD patients. Nevertheless, some measures of complexity suggest that fatigue-induced changes in neuromuscular dynamics are increased by the addition of cognitive stress. Overall, our work shows that complexity measures are appropriate to characterise neuromuscular fatigue in different contexts. The completion of the current study will allow to better define the use of these approaches in the clinical context.