Muscle fatigue
Initially normal, then declining ability of a muscle to generate force / From Wikipedia, the free encyclopedia
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Muscle fatigue is when muscles that were initially generating a normal amount of force, then experience a declining ability 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).
Muscle fatigue is not the same as muscle weakness, though weakness is an initial symptom. Despite a normal amount of force being generated at the start of activity, once muscle fatigue has set in and progressively worsens, if the individual persists in the exercise they will eventually lose their hand grip, or become unable to lift or push with their arms or legs, or become unable to maintain an isometric position (such as plank). Other symptoms may accompany such as myalgia (muscle pain), shortness of breath, fasciculations (muscle twitching), myokymia (muscle trembling), and muscle cramps during exercise; muscle soreness may occur afterwards.[1] An inappropriate rapid heart rate response to exercise may be seen, such as in the metabolic myopathy of McArdle disease (GSD-V), where the heart tries to compensate for the deficit of ATP in the skeletal muscle cells (metabolic fatigue) by increasing heart rate to maximize delivery of oxygen and blood borne fuels to the muscles for oxidative phosphorylation.[2] The combination of an inappropriate rapid heart rate response to exercise with heavy or rapid breathing is known as an exaggerated cardiorespiratory response to exercise.[3]
Due to the confusion between muscle fatigue and muscle weakness, there have been instances of abnormal muscle fatigue being described as exercise-induced muscle weakness.[4][5][6]