The force producing capability of skeletal muscle is regulated by a number of simultaneously occurring processes that provide real-time fine tuning of contractility. This allows for the locomotory demands of the individual to attempt to be met within functional confines of contracting muscle that change as exercise progresses. Fatigue occurs in response to high-intensity exercise, as well as during prolonged low-intensity exercise, although the ionic and metabolic factors contributing to these two types of fatigue may differ. Fatigue of high-intensity exercise is recognized to occur primarily by depression and failure of excitability, either at the sarcolemmal or perhaps within the t-tubule system, due to altered electrico-chemical gradients for K+, Cl- and Na+. There is increasing evidence that increased intra- and extracellular [H+] and [lactate-] help to stabilize the sarcolemma and help to maintain excitability. The fatigue of prolonged, low-intensity exercise may be characterized by depletion of energy substrate, primarily glycogen, and a decreased intracellular [K+] with ensuing depressed membrane excitability may also contribute. Both types of fatigue are also associated with increased concentrations of reactive oxygen species (ROS), and increased [ROS] appears to downregulate the activities of key ion transporting and metabolic enzymes, as well as contributing to muscle damage in the long term. The purpose of this chapter is to provide an overview of our current knowledge and speculations regarding these main contributors to skeletal muscle fatigue.
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