Merck Manual

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Professional Version

Overtraining Syndrome in Animals


Amelia S. Munsterman

, DVM, PhD, DACVS, DACVECC, Department of Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University

Reviewed/Revised Apr 2019 | Modified Nov 2022

Highly intense exercise training over many weeks can result in a form of chronic fatigue referred to as overtraining syndrome. Racehorse trainers have long used the terms “overtraining,” “staleness,” or “sourness” to describe a syndrome of poor performance, failure to recover from exercise, and prolonged fatigue that does not resolve for weeks or months. By definition, signs of overtraining syndrome should persist after >2 weeks of rest or reduced physical activity. A less severe form of overtraining syndrome is termed “overreaching,” which is also a syndrome of poor performance and fatigue, but athletic recovery in overreaching typically occurs from a few days to 2 weeks after a reduced workload.

Overtraining syndrome was first reported in Swedish Standardbred trotters based on observations of horses with signs of fatigue and poor performance combined with weight loss, inappetence, and signs of stress, including tachycardia, nervousness, muscle tremors, sweating, and diarrhea. The severity of clinical signs of overtraining was associated with the degree of red cell hypervolemia, and these horses exhibited adrenal exhaustion that may be similar to parasympathetic overtraining reported in people.

Pathophysiology of Overtraining Syndrome in Animals

In experimental studies, a milder form of overtraining has been reproduced, without any evidence of red cell hypervolemia, inappetence, or adrenal gland exhaustion. However, this syndrome was associated with a decrease in the plasma cortisol response to intense exercise, suggesting that overtraining is associated with dysfunction of the hypothalamic-pituitary-adrenal axis. Recent research has shown that overtrained horses have altered growth hormone activity levels, with an increase in the normal pulsatile growth hormone activity overnight, in addition to altered glucose metabolism. Biomarkers of skeletal muscle metabolism are currently being investigated as a means to identify overtraining syndrome but require invasive muscle biopsies for measurement.

Clinical Signs of Overtraining Syndrome in Animals

Overtraining syndrome should be suspected in horses with evidence of sustained decreased performance in association with one or more physiologic or behavioral signs. Although no single physiologic marker is able to identify the syndrome, clinical signs in horses may include:

  • decreased body weight

  • increased heart rate during exercise

  • decreased plasma cortisol response to exercise

  • increased plasma concentrations of muscle enzymes or gamma glutamyl transferase

Behavioral signs are a consistent and early marker of overtraining syndrome, and development of a behavioral score to assist in early detection is warranted.

Key Points

  • Prolonged exercise relies on aerobic metabolism, which is inherently inefficient, producing large amounts of heat that must be removed by the body.

  • Exhausted horse syndrome, characterized by dehydration, myopathies, abdominal pain, and changes in mentation, has been observed in horses competing in endurance sports or 3-day events.

  • Treatment of exhausted horse syndrome should focus on rehydration strategies and rapid external cooling with cool hosing and sweat scraping.

  • Additional therapies may include sedation, NSAIDs, and glucocorticoids.

For More Information

  • Foreman JH. The exhausted horse syndrome. Vet Clin North Am Equine Pract 1998 Apr;14(1):205-19. Review.

  • Flaminio MJ1, Rush BR. Fluid and electrolyte balance in endurance horses. Vet Clin North Am Equine Pract 1998 Apr;14(1):147-58.

  • White SL. Fluid, electrolyte, and acid-base balances in three-day, combined-training horses. Vet Clin North Am Equine Pract 1998 Apr;14(1):137-45.

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