Owners commonly report muscular fatigue of horses. Muscular fatigue can be caused by numerous disorders of several body systems, which are discussed in other chapters.

Fatigue is a normal consequence of exercise that is continued at high intensity or for prolonged periods of time. The decreased ability of the muscle to produce force is actually a safety mechanism for the body. If fatigue did not occur and force the animal to stop, the intense exercise could cause structural damage to muscle cells and supportive tissues.

Most knowledge concerning fatigue in animals has been described in horses because horses can easily be trained to exercise on high-speed treadmills that allow investigation of respiratory, cardiovascular, and metabolic responses. When the horse becomes fatigued, it is generally unable or unwilling to maintain the same speed as the treadmill when the treadmill speed is increased. Changes in the gait and joint movements are seen, and these changes may be an important factor contributing to musculoskeletal injuries of racehorses. Because of these changes, training of racehorses should avoid fatiguing the animal.

Fatigue during High-intensity Exercise

In general, the cause of fatigue during exercise depends greatly on the duration and energy demands of the event. Endurance events can last many hours or days, while many Quarter Horse, Standardbred, and Thoroughbred races require intense exercise at maximal speeds lasting 3 minutes or less. Generally, exercise at an individual animal's highest attainable speed cannot be maintained for more than about 30 to 40 seconds. After that, fatigue sets in and the animal slows down.

Synchronous diaphragmatic flutter, a condition in which the diaphragm contracts at the same time as the heart to produce loud thumping noises (heard with a stethoscope) and usually visible contractions in the flank area, is sometimes seen after races in Thoroughbreds and Standardbreds. It is more often associated with fatigue during prolonged exercise. The condition occurs primarily when there is a significant electrolyte or acid-base imbalance. It often resolves within 30 to 60 minutes without treatment, but close monitoring is needed.

Very intense training over many weeks can result in a form of longterm fatigue referred to as overtraining. This condition causes weight loss and decreased performance during intense exercise that is not reversed by 1 to 2 weeks of rest. Horses must be given many weeks of rest from training and racing to recover from this condition.

Fatigue during Prolonged Exercise

During exercise lasting many hours, heat is generated within the body. The body tries to cool off by sweating, which can result in dehydration and metabolic disturbances. These factors are usually implicated in the fatigue, exhaustion, and even death that can occur after prolonged exercise in horses. Environmental temperature and humidity also have a major impact on the degree of disturbance to body fluids during prolonged exercise.

Washing the horse repeatedly with water that is very cold (near ice temperature) is the most effective way of cooling a horse.

Horses competing in 3-day events or endurance rides may show signs of exhaustion, despite current practices of evaluation of recovery at rest stops. Horses can lose large amounts of fluid by sweating, and show signs of depression, fatigue, dehydration, increased heart and breathing rates, and high body temperature. These horses need urgent treatment. Horses with a high body temperature should be continuously hosed with very cold water, stood in the shade (in a cooling breeze if possible), and given fluid therapy (both oral and intravenous).

Prevention of Fatigue

Physical training is the most effective way of reducing fatigue and increasing the capacity for exercise. Physical training not only builds up muscle size and strength, it also transports more oxygen to and produces more energy within the muscle. Skeletal muscle responds to training depending on the training intensity, which can be guided by heart rate meters. In horses, maximal heart rates range from about 210 to 240 beats per minute. Heart rates that are about 60 to 90% of the horse's maximal heart rate are ideal during training of Standardbred and Thoroughbred horses. If the maximal heart rate is not known, training should be kept at heart rates of 120 to 190 beats per minute, with the addition of occasional high speed exercise later in the training program.

Warmup before competition at high speeds is likely to reduce fatigue in Quarter Horse, Thoroughbred, and Standardbred races. A 10-minute warmup with trot and canter should be used whenever possible.

Horses should be well hydrated before any intense or prolonged exercise and given access to fluids during and after events. Horses that are dehydrated or that have not been acclimated to a hot environment before exercise have higher body temperatures and earlier fatigue. Horses should not be given large meals 1 to 2 hours before competition. Feeding small portions every 4 hours is best. Reduced fiber intake before racing can also decrease fatigue during intense exercise. Horses that are overweight also fatigue earlier.

During high-intensity exercise, energy comes from the metabolic conversion of various compounds, including glycogen, a compound stored in the liver and muscles and converted to glucose (sugar) as a source of energy. Glycogen concentration in skeletal muscle before performance has an effect on fatigue during both short-term/intense and prolonged 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 hours for glycogen levels in horses to return to normal after exercise. 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 improve performance in Standardbred and Thoroughbred racehorses has no scientific basis.

Feeding fat can increase performance during prolonged exercise. Adding fat to the diet affects various metabolic responses to exercise, including body temperature. Feeding vegetable oil at a rate of 100 to 120 grams per kilogram of weight has been suggested as ample.

Recovery

Recovery of horses after endurance rides is influenced by the rehydration strategy used. Horses offered a saline solution as initial rehydration recover more rapidly than horses offered water. However, horses may need to be trained to drink a saline solution. Use of saline solutions should be encouraged, especially for horses that compete in endurance events or other events over multiple days. Owners should consult with their veterinarian concerning the most suitable saline solution to use.

Blood lactate concentrations do decrease more quickly after racing and intense training if the horse is trotted for 30 minutes, but there is no obvious benefit for the horse of this so-called “warm-down.”

Last full review/revision July 2011 by George M. Barrington, DVM, PhD, DACVIM; David L. Evans, BVSc, PhD; Katharine F. Lunn, BVMS, MS, PhD, MRCVS, DACVIM; Donald C. Sawyer, DVM, PhD; Ivan W. Caple, BVSc, PhD, MACVSc, MRCVS; Sharon J. Spier, DVM, PhD, DACVIM