There is a fine line between striving to maximize a horse’s performance and pushing the animal beyond his physical limits. Standardbred racehorses experience a decrease in body weight and an abnormal decrease in blood cortisol when they are overtrained. Researchers from New Zealand recently conducted a study to see if there are any other physiologic changes associated with overtraining. The ability to recognize such changes would allow trainers to adjust each horse’s regimen to maximize his potential.

Ten Standardbred racehorses not in training for six months were started on a four-phase, 34-week training schedule. Blood and urine samples were collected throughout the study. For the first 14 weeks, horses were trained in a routine manner. Scheduled time trials were run to monitor performance. From 14-24 weeks, training entered a taper phase, which is also routine. Weeks 24-32 involved purposeful overtraining, including high-intensity galloping sessions, long hobbled workouts, and frequent sprints. Horses were then allowed to rest during weeks 32-34.

Physiologic parameters that changed in these Standardbreds with overtraining included increased blood lactate (causing oxygen debt), decreased blood cortisol (which raises blood sugar and stimulates conversion of proteins to carbohydrates), and decreased packed blood cell volume after exercise, as well as substantial loss of body weight. In addition, the speed achieved (kg/hr) by overtrained horses at a heart rate of 200 beats/minute was significantly reduced. The authors concluded that Standardbred trainers might use these five parameters as reliable indicators of acute overtraining.

Hamlin, M.J.; Shearman, J.P.; Hopkins, W.G. Equine Veterinary Journal, 34(4): 383-388, 2002