THE MERCK VETERINARY MANUAL
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Prevention of Fatigue

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Physical training is the most effective way to reduce fatigue and increase capacity for exercise. Physiologic responses to training that contribute to increased exercise capacity include increases in the maximal rate of oxygen transport, cardiac stroke volume, muscle capillary density, circulating blood volume, and 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. 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 ionic changes associated with intense exercise, including decreased potassium efflux from working muscles, resulting in a smaller increase in plasma potassium 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 (VO2max) 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 VO2max.

Heart rate meters can be used to guide the intensity of training to improve the adaptive response to training. Heart rates that result in 80% of VO2max are ~90% of maximal heart rate, which typically ranges from 210 to 240 bpm in horses. Heart rate meters can be used to measure an individual horse's heart rates 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 can also be used to measure the appropriate training intensity. At an exercise intensity of 80% VO2max, plasma lactate concentrations during treadmill exercise are in the range of 4–10 mmol/L.

Warm-up before exercise significantly increases the time to fatigue during intense exercise. Warm-up increases muscle temperature before exercise, increases the rate at which oxygen uptake increases, and reduces lactate accumulation by enhancing aerobic metabolism. The effect is similar whether the warm-up is low intensity (5–10 min at 50% VO2max), moderate intensity (1 min at 70% VO2max), high intensity (1 min at 115% VO2 max), 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.

It has been suggested that fatigue during intense exercise may be delayed by manipulation of acid-base status before exercise to increase the plasma-buffering capacity. Although some trainers have administered sodium bicarbonate before races, this practice is now banned by many racing administrations. The treatment does alter blood pH and lactate concentrations during exercise. However, the effect of alkalinizing solutions on equine performance is equivocal. Studies using bicarbonate at a dosage of 0.6 g/kg have not shown an effect on the time at which fatigue occurs. Although a metabolic alkalosis could be induced, the administration of sodium bicarbonate before intense treadmill exercise did not have any significant effect on metabolic response to exercise. In Greyhounds, a dose of sodium bicarbonate at 0.4 g/kg did not have a significant effect on race time in races >400 m long. 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, and it was concluded that the treatment at this dose affected performance.

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. It would be inappropriate for an animal to begin an endurance exercise with suboptimal hydration. Horse are more susceptible to hyperthermia during prolonged exercise than people 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 the way of limiting these excessive responses to exercise by preexercise fluid administration.

Hyperhydration by administration of electrolytes or saline solutions orally before exercise will result in expansion of the 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 reduces heat storage. Horses should be acclimated to hot environments before competition.

Horses should not be given large meals (equal to half the ration) 1–2 hr before intense exercise, because plasma volume is decreased for at least 1 hr after a large meal. Large meals may also shift fluid to the GI tract, reducing cardiovascular and thermoregulatory function during exercise. Feeding smaller rations every 4 hr does not result in changes in plasma volume. A short-term reduction in fiber intake before high-intensity racing (fed as ~1% of body wt in hay for 3 days before high-intensity exercise) is a strategy to reduce 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.

Glucose supplementation may be important in limiting fatigue in endurance exercise in horses. Endurance time during treadmill running was prolonged by the IV infusion of a glucose solution during exercise. Plasma glucose was higher than controls, and plasma lactate and body temperature were lower at the point of fatigue. These results suggest that supplemental glucose during exercise prolongs performance time in horses. This increase may be due to increased glucose availability, reduced reliance on anaerobic energy production, lower core temperature, and better maintenance of plasma volume. In endurance horses, hydrolysable starches and sugars fed within 3 hr of a race may increase glucose utilization in the short term but inhibit lipid oxidation, which could be detrimental for energy production.

Glycogen concentration in skeletal muscle before performance is relevant to fatigue during both short-term/intense 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 hr for glycogen to be replenished in the horse. While modest increases in glycogen stores may be obtained using high-starch diets in the horse, no benefit on performance has been shown. On the contrary, high-carbohydrate diets have increased heart rates and blood lactate concentrations during intense exercise. Use of glucose or other carbohydrate solutions before racing to promote performance in Standardbred and Thoroughbred racehorses has no scientific basis.

Fat supplementation is now a widespread practice in diets for athletic horses and can increase performance during endurance exercise. Increased free fatty acid concentrations in the bloodstream before prolonged exercise results in an increased use of fat as an energy source and in higher blood glucose and muscle glycogen concentrations during exercise. The shift to increased use of fat as a fuel results in lower respiratory demands for exercise, because less carbon dioxide must be expired when more fat is used for energy. 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, but horses must be acclimated to high-fat diets. In horses not acclimated, fat supplementation can slow the rate of muscle glycogen repletion.

Creatine has been used in horses as an ergogenic aid, but there is no evidence of its efficacy. Horses receiving 25 g of creatine monohydrate bid for 6.5 days did not have significantly different run times until fatigue on a treadmill than control horses. Supplementation with creatine also had no significant effect on muscle or blood creatine concentrations at rest or after exercise until fatigue.

An association between plasma vitamin E concentration and performance has been described in sled dogs. Dogs with a higher prerace vitamin E concentration were more likely to finish the race and were less likely to be withdrawn during the race for poor health, fatigue, or other reasons. Vitamin E concentrations for the dog team overall were not associated with speed during the race. Additional studies are needed to investigate whether the reduced signs of fatigue are directly linked to higher vitamin E concentrations in the bloodstream.

Recovery of horses after endurance exercise is influenced by the rehydration strategy used. After prolonged treadmill exercise and furosemide-induced dehydration, horses offered a saline solution (0.9% NaCl) as the initial rehydration fluid maintained a plasma sodium concentration higher than that of control horses. The recovery of body weight was more rapid than in horses offered plain water. A similar study noted ambient temperature fluids were more palatable (20°C [68°F]) and increased voluntary intake. Water intake can also be increased by providing a saline solution (0.9% NaCl) both during and after endurance exercise. Sodium chloride solutions may increase plasma chloride and accentuate a metabolic acidosis; as a result, isotonic polyionic electrolyte solutions formulated specifically for horses should be selected. 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.

Last full review/revision January 2014 by Amelia S. Munsterman, DVM, MS, DACVS, DACVECC

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