Muscular fatigue during exercise is the decline in the ability of a muscle to generate force of contraction, resulting in the inability of the horse to continue to perform at the same level of intensity. Alterations in cardiovascular parameters, serum electrolytes, and muscle tissue may be observed if physiological compensatory mechanisms are exhausted. Treatment involves rest, rehydration, restoration of normal serum electrolyte concentrations, and cooling strategies.
Fatigue may occur during both aerobic and anaerobic exercise and at submaximal effort. Factors that can affect the onset of fatigue include the following:
ambient environmental temperature
hydration status and serum electrolyte concentrations
external motivators
the horse's desire to work
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 pathological conditions, including diseases that affect oxygen uptake, energy metabolism, or neuromuscular function. This discussion focuses on muscular fatigue in healthy horses.
Pathophysiology of Fatigue in Horses
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 messenger RNA (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 the following:
lactate
ammonia
hypoxanthine and xanthine
markers of oxidative damage (thiobarbituric acid reactive substances, glutathione, and glutathione peroxidase)
inflammatory mediators (IL-1, tumor necrosis factor alpha)
lymphocytes
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 (1, 2, 3, 4). Mass spectrometry has been used to identify changes in the metabolome of endurance horses and has found distinct differences in plasma levels of certain metabolites between horses that completed the race and those that did not (5).
Central fatigue is associated with the following:
decreased motivation
lethargy
loss of muscle coordination
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.
Key Points
Fatigue results from the inability of the muscle to generate maximal force.
Fatigue is a physiological protective mechanism activated to prevent injury.
Clinical signs of fatigue result from changes at the cellular level of the muscle, as well as alterations in nerve signaling pathways from the CNS.
For More Information
References
Halama A, Oliveira JM, Filho SA, et al. Metabolic predictors of equine performance in endurance racing. Metabolites. 2021;11(2):82. doi:10.3390/metabo11020082
Serteyn D, Sandersen C, Lejeune JP, et al. Effect of a 120 km endurance race on plasma and muscular neutrophil elastase and myeloperoxidase concentrations in horses. Equine Vet J. 2010;38:275-279. doi:10.1111/j.2042-3306.2010.00269.x
Mach N, Plancade S, Pacholewska A, et al. Integrated mRNA and miRNA expression profiling in blood reveals candidate biomarkers associated with endurance exercise in the horse. Sci Rep. 2016;6:22932. doi:10.1038/srep22932
Barrey E, Mucher E, Robert C, Amiot F, Gidrol X. Gene expression profiling in blood cells of endurance horses completing competition or disqualified due to metabolic disorder. Equine Vet J. 2010;38(S36):43-49. doi:10.1111/j.2042-3306.2006.tb05511.x
Le Moyec L, Robert C, Triba MN, et al. A first step toward unraveling the energy metabolism in endurance horses:comparison of plasma nuclear magnetic resonance metabolomic profiles before and after different endurance race distances. Front Mol Biosci. 2019;6:45. doi:10.3389/fmolb.2019.00045
