What is central fatigue?
Neuromuscular fatigue can be defined as a decline in performance that is usually determined by power generation capacity. During a static maximal contraction, force will decrease steadily and fatigue would be observed from the beginning of the exercise. Contrarily, in submaximal contractions, the target force is maintained for a long time. In this situation, the fatigue is defined as the inability in maintaining the force, even if the capacity of maximal force generation is impaired earlier during contraction. Neuromuscular fatigue is usually defined as a reduction of the capacity of maximal force generation . Velstad (1997), defined neuromuscular fatigue as any ...view middle of the document...
Thus, excitation factors, integrating sensory information and motor signal transmission affect the central fatigue. However, in healthy human, broadcasting motor signals for physiological excitations in normal levels seems to be enough. In Tables 1 and 2, we review the mechanisms proposed for occurring central fatigue.
Protecting the organism and central fatigue
Why is the brain unable to recruit additional motor units to keep up the activity during long-term exercise?
Prediction of the central governor model during endurance exercises is that brain does not use extra motor units because such a use could threaten the capacity of maintaining the homeostasis which potentially causes to an early ending of exercise in the best case, and organ damage or even death in the worst cases . Findings of isolated muscles suggest that CNS monitor the peripheral situation of active limb muscles through the sensory feedback and use this information to adjust muscle activation in order to protect muscle storage capacity. Alternatively, muscle afferents probably disturb slowly the willingness or ability of maintaining the high Central Motor Output (CMO) and therefore, the high-intensity exercises and performance in order to prevent more development of peripheral fatigue over than the tolerable level which causes exercise to be painful (figure 1). This model, known as the fatigue nonlinear system is an optimized model which by that fatigue will be perceived as unconscious perception of afferent feedback through the different linear model. Brain integrates these afferent signals (which are received as a result of a severe disruption in homeostasis), in order to protect the organism against damage or death . Studies have also shown that by increasing the running time, reduction in force may eventually reach a plateau. This could be due to the influence of the central protective mechanism that attempts to limit the muscular work during prolonged running to prevent large homeostasis disorders, muscle damage, and biological damages . Gandevia (2001) stated that, CMO reduction as a result of the inhibitory afferent feedback costs to the really maximal performance. This regulatory mechanism is suggested as a limiting factor in endurance performance . However, this model predicts that the rising feeling of pain increasingly decreases the individual's conscious tendency to stop this mechanism that may cause more motor unit utilization. Thus, the existence of this canceling system is undesirable because it can maintain or increase the intensity of activities that may threaten homeostasis .
Figure 1. Visual view of the supraspinal inhibition reflex model in endurance exercise. The red line represents the efferent nerve activity (central...