Physical training is the most effective way of reducing fatigue and increasing the capacity for exercise. Many physiologic responses to training contribute to increased exercise capacity. The maximal rate of oxygen transport increases. There are increases in stroke volume, capillary density in muscle, blood volume, and total blood hemoglobin content. Hypertrophy of muscle cells occurs, coupled with increases in concentrations of mitochondria, glycogen, and enzymes concerned with energy production.
Sport-specific training can result in specific adaptations. For example, sprint training can result in decreased proportions of slow-twitch fibers, and endurance training can result in increased oxidative capacity of fast-twitch fibers. Sprint training also modulates the ionic changes associated with intense exercise, including a reduction in the rise in plasma potassium and delayed onset of fatigue. Training also modulates the decreased calcium uptake rate and Ca2+-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, high oxidative fibers and an 8% increase in the buffering capacity of exercised muscle, but these responses did not occur in horses trained at 40% VO2max. Heart rate meters can be used to guide the intensity of training. Heart rates that result in 80% VO2max are ~90% of maximal heart rate. Maximal heart rates range from ~210 to 240 bpm in horses. To promote greater muscle adaptation to training, it is appropriate to use heart rate meters 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 the 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 in racehorses. Warm-up increases muscle temperature before exercise, increasing the rate at which oxygen uptake increases (kinetics of oxygen uptake) and the peak oxygen uptake during exercise. The oxygen deficit at the beginning of intense exercise and the rate of glycogen metabolism during exercise are also lower after prior warm-up. The effect is the same whether the warm-up is low-intensity (5 or 10 min at 50% of VO2max), moderate intensity (1 min at 70% VO2max), high intensity (1 min 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 reduce fatigue in Quarter horse, Thoroughbred, and Standardbred races.
Manipulation of Acid-Base Balance, Diet, and Hydration
It has been suggested that fatigue during intense exercise may be delayed by manipulation of acid-base status before exercise. 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, and recent treadmill studies using sodium bicarbonate at a dosage of 0.6 g/kg have not shown an effect on the time at which fatigue occurs. In addition, administration of sodium bicarbonate before intense treadmill exercise did not have any significant effect on the muscle metabolic response to exercise. In Greyhounds, a dose of sodium bicarbonate at 0.4 g/kg did not have a significant effect on race times in races >400 m long. However, there may be an ergogenic effect when sodium bicarbonate is used at high dosages. Sodium bicarbonate at a dosage rate of 1 g/kg (by nasogastric tube) increased the time to fatigue in horses, and it was concluded that the treatment affected performance.
Manipulation of energy supply and hydration are frequently used by 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 endurance exercise with suboptimal hydration or glycogen concentrations in liver and muscle. Horses are more susceptible to hyperthermia during prolonged exercise than people because of their high body mass to surface area ratio. Equine thermoregulation results in extreme perturbations of body fluid status, and there is increasing interest in ways of limiting these responses to exercise by pre-exercise fluid administration. Hyperhydration by administration of saline solutions orally before exercise results in expansion of the blood volume during exercise. Studies suggest that hyperhydration before prolonged exercise does help conserve plasma volume during exercise but does not result in lower body temperatures. Horses should be acclimated to hot environments before competition. Horses that are not acclimated could be expected to have higher body temperatures during exercise and earlier fatigue.
Horses should not be given large meals 1–2 hr before intense exercise, because plasma volume is decreased for at least 1 hr after a large meal. Feeding small portions every 4 hr does not result in changes to plasma volume. A decreased volume of water in the GI tract could decrease fatigue during intense exercise because it would decrease the oxygen needed for locomotion. Reduced fiber intake before racing is a strategy to reduce water volume and, hence, weight in the hindgut. For prolonged 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 equine endurance exercise. Endurance time during treadmill running was prolonged by the IV infusion of glucose solution during exercise. Plasma glucose was higher during exercise in the horses receiving glucose, and plasma lactate and body core temperatures were lower at fatigue. Plasma volume decreased more slowly in treated horses. 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.
Glycogen concentration in skeletal muscle before performance is relevant to fatigue during both short-term/intense and prolonged exercise. Depletion of muscle glycogen causes a decrease in anaerobic power generation and capacity for high-intensity exercise. Horses should not be depleted of glycogen before short-term or endurance events. Intense or prolonged exercise depletes the muscle glycogen stores, and it is sensible to allow at least 48 hr for adequate postexercise glycogen resynthesis in horses. No method of glycogen loading using adjustments to normal feeding has been described in horses. Use of glucose or other carbohydrate solutions before racing to promote performance in Standardbred and Thoroughbred racehorses has no scientific basis.
Feeding fat can increase performance during prolonged exercise. Elevation of free fatty acid concentrations in blood before endurance exercise results in an increased use of fat as an energy source during low-intensity exercise, and in higher blood glucose concentrations during exercise. The shift to greater use of fat as a fuel results in lower respiratory demands for the 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 affects the metabolic and thermoregulatory responses to exercise. Feeding vegetable oil at a rate of 10–12% of the total diet has been suggested.
Creatine has been used in horses as an ergogenic aid, but there is no evidence for its efficacy. Horses receiving 25 g creatine monohydrate twice daily for 6.5 days did not have significantly different run times to fatigue compared with controls. The supplementation also had no significant effect on muscle or blood creatine concentrations at rest or after exercise.
An association between vitamin E concentration and performance has been described in sled dogs. Dogs with higher prerace vitamin E concentrations were more likely to finish the race and were less likely to be withdrawn during the race for poor health, fatigue, or other reasons. Additional studies are needed to investigate whether the reduced fatigue is caused by higher vitamin E concentrations in blood.
Recovery of horses after endurance rides is influenced by the rehydration strategy used. After prolonged treadmill exercise and furosemide-induced dehydration, horses offered a saline solution (NaCl, 0.9%) as the initial rehydration fluid maintained an elevated plasma sodium concentration, and recovery of body weight was more rapid than in horses offered water. Horses may need to be trained to drink a saline solution. Use of saline solutions should be encouraged, especially in horses that are required to compete in events on consecutive days, such as endurance and 3-day events.
Last full review/revision July 2011 by Catherine McGowan, BVSc, MACVSc, DEIM, DECEIM, PhD, FHEA, MRCVS