Physical training can be used to increase the time until muscular fatigue and decrease its severity. A warm-up should be used, regardless of the sport, and the animal should have time to acclimate to the exertion expected as well as the environment. Feeding regimens can be used to help maintain hydration during exertion, and a high-fat diet may help improve the metabolic and thermoregulatory response to exercise. Providing electrolyte solutions during and after an event may help maintain hydration and prevent fatigue and heat-related illnesses.
Physical Training to Prevent Fatigue in Horses
Physical training is the most effective way to decrease fatigue and increase capacity for exercise. Physiological responses to training that contribute to increased exercise capacity include increases in the following:
maximal rate of oxygen transport
cardiac stroke volume
muscle capillary density
circulating blood volume
total hemoglobin content
Hypertrophy of muscle cells occurs, coupled with increases in concentrations of mitochondria, glycogen, and enzymes concerned with cellular energy production.
Sport-specific training can result in specific adaptions to the type of activity, although much of this research has been performed in racehorses. For example, sprint training can result in decreased proportions of slow-twitch muscle fibers, whereas endurance training can result in increased oxidative capacity of fast-twitch muscle fibers. Sprint training also modulates the electrolyte changes associated with intense exercise, including decreased potassium efflux from working muscles, resulting in a smaller increase in plasma potassium concentration and delayed onset of fatigue. Training also modulates the exercise-induced decline in both calcium reuptake by the sarcoplasmic reticulum and calcium-ATPase activity associated with fatigue.
Adaptations to training in skeletal muscle depend on the training intensity. Horses trained at intensities > 80% of their maximal oxygen uptake (VO2 max) had an increase in their percentage of fast-twitch muscle fibers and an 8% increase in the buffering capacity of exercised muscle. These responses did not occur in horses trained at 40% of VO2 max.
Heart rate monitors can be used to guide the intensity of training to improve the adaptive response to training. Heart rates that result in 80% of VO2 max are approximately 90% of maximal heart rate, which typically ranges from 210 to 240 bpm in horses. Heart rate monitors can also be used to measure an individual horse's heart rate during slow and fast exercise and to calculate the exercise velocities that result in heart rates of 90% of maximal heart rate. Blood lactate after exercise may be used to measure the appropriate training intensity. At an exercise intensity of 80% VO2 max, plasma lactate concentration during treadmill exercise is in the range of 4–10 mmol/L.
Warm-Up to Prevent Fatigue in Horses
Warm-up before exercise delays the onset of fatigue during intense exercise. Warm-up increases muscle temperature before exercise, increases the rate at which oxygen uptake increases, and decreases lactate accumulation by enhancing aerobic metabolism. The effect is similar whether the warm-up is low intensity (5–10 minutes at 50% VO2max), moderate intensity (1 minute at 70% VO2max), high intensity (1 minute at 115% VO2max), or a combination of low and high intensity.
The practical importance of this finding is that a warm-up before competition involving intense exercise is likely to increase the time to fatigue during Quarter Horse, Thoroughbred, and Standardbred races.
Manipulation of Acid-Base Balance, Diet, and Hydration to Prevent Fatigue in Horses
Bicarbonate Solutions
Some feel that fatigue during intense exercise may be delayed by manipulation of acid-base status before exercise to increase plasma-buffering capacity. Although some trainers have administered sodium bicarbonate before races, this practice(known as "milkshaking") is now banned by many racing administrations. The treatment does alter blood pH and lactate concentration during exercise; however, the effect of alkalinizing solutions on equine performance is equivocal.
Bicarbonate administered at a dosage of 0.6 g/kg has not been shown to delay fatigue. Although a metabolic alkalosis could be induced, giving sodium bicarbonate before intense treadmill exercise did not effect the metabolic response to exercise. However, there may be an ergogenic effect when sodium bicarbonate is administered at high dosages. Sodium bicarbonate at a dosage rate of 1 g/kg by nasogastric tube increased the time to fatigue in horses running on a treadmill, suggesting that treatment at this dose would affect performance.
Hydration Status
Energy supply and hydration are frequently manipulated in human athletes to limit fatigue during endurance exercise. Dehydration before exercise results in higher core temperatures during exercise in horses. Horses are more susceptible to hyperthermia during prolonged exercise than are humans because of their high body mass to surface area ratio, which inhibits heat loss. Equine thermoregulation also results in extreme changes to total body fluid status, and there is increasing interest in ways to limit excessive responses to exercise by pre-exercise fluid administration.
Hyperhydration by administration of electrolytes or saline solutions orally before exercise results in expansion of blood volume during the event. Studies suggest that hyperhydration before prolonged exercise helps conserve plasma volume during exercise but does not lower body temperature or improve arterial hypoxemia. Maintenance of euhydration with water or a carbohydrate-electrolyte solution during exercise improves perfusion parameters and sweating rates and decreases heat storage. Horses should also be acclimated to hot environments before competition.
Nutritional Status and Supplements
Horses should not consume large meals (recommendations are equal to half the ration) 1–2 hours before intense exercise because plasma volume is decreased for at least 1 hour after a large meal. Large meals may also shift fluid to the GI tract, decreasing cardiovascular and thermoregulatory function during exercise. Feeding smaller rations every 4 hours does not result in changes in plasma volume.
A short-term decrease in fiber intake before high-intensity racing (fed as approximately 1% of body weight in hay for 3 days before high-intensity exercise) is a strategy to decrease GI water volume and, hence, body weight. For endurance exercise, feeding before exercise, especially high-fiber feeds, is likely to be beneficial, because the increased water in the GI tract can be an important reservoir for water and electrolytes to replace sweat losses. Feeding high-fiber feeds also increases voluntary water intake and may have a positive effect on performance in both high-intensity exercise and endurance trials.
Glucose supplementation may help limit fatigue during endurance exercise in horses. Endurance time during treadmill running was prolonged by IV infusion of a glucose solution. Plasma glucose concentration was higher than in control subjects, and plasma lactate concentration and body temperature were lower at the point of fatigue (1). These results suggest that supplemental glucose during exercise may benefit endurance horses by the following means:
increasing glucose availability
decreasing reliance on anaerobic energy production
lowering core temperature
better maintaining plasma volume
Glycogen concentration in skeletal muscle before performance is relevant to fatigue during both short-term intense exercise and prolonged endurance exercise. Depletion of muscle glycogen before exercise causes a decrease in anaerobic power generation and capacity for high-intensity work. Intense or prolonged exercise depletes the muscle glycogen stores, and it may take 48 hours for glycogen to be replenished in a horse. Although modest increases in glycogen stores may be obtained using high-starch diets in a horse, no benefit to performance has been shown. On the contrary, high-carbohydrate diets have increased heart rate and blood lactate concentration during intense exercise.
Fat supplementation is now a widespread practice in diets for athletic horses and can improve performance during endurance exercise. Increased free fatty acid concentrations in the bloodstream before prolonged exercise result in an increased use of fat as an energy source and higher blood glucose and muscle glycogen concentrations during exercise.
The increased use of fat as a fuel results in lower respiratory demands for exercise because less carbon dioxide must be expired.
Fat adaptation appears to facilitate the metabolic regulation of glycolysis by sparing glucose and glycogen at low-intensity work and by promoting glycolysis when power is needed for high-intensity exercise.
Adding fat to the diet also affects the metabolic and thermoregulatory response to exercise.
Feeding vegetable oil at a rate of 10%–12% of the total diet on a dry-matter basis has been suggested. Horses must be acclimated to high-fat diets because fat supplementation can slow the rate of muscle glycogen repletion.
Recovery to Prevent Fatigue in Horses
Recovery of horses after endurance exercise is influenced by the rehydration strategy used. Horses tend to preferentially drink water of ambient to cool temperature rather than cold water, so the temperature of the water provided is important.
Water intake can also be increased by offering or administering a saline solution (0.9% NaCl) both during and after endurance exercise, or by ingestion of a solution of balanced electrolytes (formulated for horses) in water. When providing electrolyte solutions, horses may need to be trained to drink these fluids, and plain water should always be available. However, use of electrolyte solutions should be encouraged, especially in horses required to compete on consecutive days, such as in endurance rides and 3-day events.
Key Points
Regardless of the sport, animals must be trained to develop the fitness needed to complete the task at hand.
Both high-intensity and endurance sports benefit from an adequate warm-up before exertion to improve aerobic metabolism.
Animals should be well hydrated before an event and provided opportunities to rehydrate in endurance sports or during periods of rest between activities.
For More Information
Fielding CL, Meier CA, Balch OK, Kass PH. Risk factors for the elimination of endurance horses from competition. J Am Vet Med Assoc. 2011;239(4):493-498. doi:10.2460/javma.239.4.493
Munsters CCBM, van Iwaarden A, van Weeren R, Sloet van Oldruitenborgh-Oosterbaan MM. Exercise testing in Warmblood sport horses under field conditions. Vet J. 2014;202(1):11-19. doi:10.1016/j.tvjl.2014.07.019
Fédération Équestre Internationale, FEI Endurance Rules. Accessed October 16, 2023.
References
Farris JW, Hinchcliff KW, McKeever KH, Lamb DR, Thompson DL. Effect of tryptophan and of glucose on exercise capacity of horses. J Appl Physiol (1985). 1998;85(3):807-816. doi:10.1152/jappl.1998.85.3.807
