Fatigue may occur during both aerobic and anaerobic exercise and at submaximal effort. Factors that can affect the onset of fatigue include:
As muscular effort increases, glycogen depletion, intracellular acidosis, and accumulation of metabolic byproducts will contribute to the onset of fatigue. Fatigue during exercise can also be the result of pathologic conditions, including diseases that affect oxygen uptake, energy metabolism, or neuromuscular function. This discussion focuses on muscular fatigue in healthy animals.
Fatigue is considered a normal consequence of exercise of prolonged duration or high intensity, and it is regarded as an intrinsic safety mechanism. Without the onset of fatigue, or if fatigue is delayed, structural damage to the myocytes and supportive tissues may occur. There are two types of fatigue: peripheral and central.
Peripheral fatigue is fatigue secondary to altered muscle function. The primary cause is failure of ATP to resynthesize, with accumulation of ADP and inorganic phosphate ions. Studies of muscle metabolism after exercise to identify peripheral fatigue have relied mainly on muscle biopsies and direct measurement of muscle glycogen, creatine phosphate, ATP, ADP, inosine monophosphate, inorganic phosphate, glycolytic intermediary products, pH, and other metabolites. Other studies have investigated the expression of mRNA in muscle tissue to monitor adaptations in gene expression of proteins that regulate oxygen-dependent metabolism, glucose metabolism, and fatty acid utilization.
Indirect serum biomarkers associated with peripheral fatigue that could be used clinically include:
Central fatigue is defined as an alteration in the signals arising from the CNS, directly decreasing performance by modifying the frequency of the action potential in the motor neurons. Central fatigue may occur secondary to pain, dyspnea, perceptions of exertion, hypoglycemia, hyperthermia, ammonia accumulation, increases in serotonin, altered amino acid metabolism, and changes in extracellular ions. Central fatigue is associated with:
However, the cause of central fatigue is multifactorial, and the response to these stimuli is highly variable. For example, some horses can continue endurance exercise at speed despite severe hyperthermia, dehydration, and plasma electrolyte disturbances.
Fatigue results from the inability of the muscle to generate maximal force.
Fatigue is a physiologic protective mechanism activated to prevent injury.
Clinical signs of fatigue result from changes at the cellular level of the muscle tissues as well as alterations in nerve signaling pathways from the CNS.