Overtraining Syndrome in Horses

Overtraining syndrome is defined as chronic fatigue in horses in high-intensity training that results in poor performance. Clinical signs relate to disruption of the adrenal response to stress, evidenced by anorexia and increased plasma levels of muscle enzymes. Although measurement of CK or gamma-glutamyl transferase may support a diagnosis, diagnosis of overtraining is more commonly based on behavioral changes. Treatment focuses on rest and modification of the training schedule to enable adequate recovery between races or events.

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, clinical signs of overtraining syndrome should persist after > 2 weeks of rest or decreased physical activity. A less severe form of overtraining syndrome is termed “overreaching,” which is also a syndrome of poor performance and fatigue; however, athletic recovery in overreaching typically occurs within a few days to 2 weeks after decreasing the workload.

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