<P> Muscle fatigue is the decline in ability of a muscle to generate force . It can be a result of vigorous exercise but abnormal fatigue may be caused by barriers to or interference with the different stages of muscle contraction . There are two main causes of muscle fatigue: the limitations of a nerve's ability to generate a sustained signal (neural fatigue) and the reduced ability of the muscle fiber to contract (metabolic fatigue). </P> <P> Muscle cells work by detecting a flow of electrical impulses from the brain which signals them to contract through the release of calcium by the sarcoplasmic reticulum . Fatigue (reduced ability to generate force) may occur due to the nerve, or within the muscle cells themselves . </P> <P> Nerves are responsible for controlling the contraction of muscles, determining the number, sequence and force of muscular contraction . Most movements require a force far below what a muscle could potentially generate, and barring pathological nervous fatigue, is seldom an issue . But in extremely powerful contractions that are close to the upper limit of a muscle's ability to generate force, nervous fatigue (enervation), in which the nerve signal weakens, can be a limiting factor in untrained individuals . </P>

Where would muscle c most likely be found