ABSTRACT
Muscle fatigue (hereafter referred to as fatigue) could be defined as an exercise-induced reduction in muscle performance. Two different measurement models are generally used to quantify such impairment. The first consists of quantifying the reduction of power output during real exercise such as cycling or running (generally at a maximal intensity), where dynamic performance decline could be attributed to reduced force and/or velocity. The second model is based on the assessment of maximal isometric muscle
force-generating capacity before and immediately after real or simulated exercise, to describe the decline in static muscle performance. Therefore, measurement methods of fatigue mainly consist of maximal force or power recordings obtained before, during or after exercise. Because fatigue is a decline in muscle force or power induced by exercise, all fatigue models (Cairns et al., 2005) entail the two following components: fatigue induction and fatigue quantification (Table 2.1). Accordingly, the two important factors that should be determined when designing a fatigue study are the type of exercise inducing fatigue and the measurement methods of fatigue, which include type and timing of the measurements. Depending on the study population and research question, investigation of the neuromuscular and metabolic mechanisms underlying fatigue (i.e. fatigue mechanisms) could also be considered (Table 2.1). Fatigue is an important functional parameter for physical work and daily activities, and its assessment is relevant to practitioners in sport, occupational, geriatric and orthopaedic medicine. In all these settings, it is essential to respect the specificity between the activity of interest and the type of exercise inducing fatigue. The best option is represented by fatigue quantification during real exercise in specific conditions (e.g. cycling power loss for a road cyclist). However, it is quite challenging to assess accurately muscle force/power during the majority of sport, work and daily conditions, because of the acyclic nature of these activities. There is indeed the
necessity to select/conceive a fatiguing simulated exercise, which would mimic the type of muscle actions realised during real exercise (Table 2.2). It is worth noting that such specificity should also be respected when determining the characteristics of muscle actions for fatigue quantification (cf. next section). In this chapter, we will enumerate the different methods allowing quantification of muscle fatigue, which mainly include the tools for the assessment of force-time and power-time curves (part I) and the techniques for the evaluation of neuromuscular (part II) and metabolic (part III) mechanisms of fatigue.
