The Dangers of Overtraining

A horse’s reserve blood stored in the spleen is usually ‘bad blood’ because it has a higher viscosity than the blood in normal circulation. This is because the stored red blood cells are stiffer and spikier than normal blood cells. Like bits of sand flowing through tubes, the stiff and spiky blood cells can damage the inner lining of blood vessels, and because of their higher viscosity, they can also increase systemic blood pressure.

How many of a horse’s reserve blood cells are released into circulation actually depends on how hard the horse is exercising. Researchers have shown that packed cell volume (PCV, or hematocrit, Hct), which is the percentage of blood made up of red blood cells, increases together with the speed of the exercising horse. This usually occurs until the PCV or Hct reaches about 60-65% during exercise [1].

In other words, during the time a horse is trained (the exposure time), how much thicker and stickier the horse’s blood gets actually depends on how hard the horse is trained. The intensity of training plays a major role in determining how much damage the blood causes to the horse’s cardiovascular system. Additionally, horses undergoing high-intensity exercise can lose up to 10% of the plasma volume in their blood through sweat and internal fluid shifts, which concentrates the blood even further [2, 3].

Maximal Training Can Maximize Damage

How hard the spleen contracts and how thick the horse’s blood gets depend on the level of exertion, suggesting that there is a profound risk to overtraining. Even thoroughbred racehorses should have their maximal training carefully limited, using more gradual and less intense workouts. The use of high-intensity, interval training regimens increases the exposure time of ‘bad blood’ to the horse.

Studies show that speed-trained horses at rest tend to have much higher Hct than endurance-trained horses at rest [4]. Interestingly, in Spain, horses with Hct (or PCV) higher than 50% (not unusual for short and maximally exercising horses) are disqualified from endurance races, and many of these horses need to be treated for dehydration [5].

Despite a wide spectrum of approaches to training in the equine community, it is clear from a blood flow perspective that overtraining can be detrimental to a horse's racing performance and overall cardiovascular health.

It may be dangerous to use high-intensity interval training even for thoroughbreds. Until a horse’s blood viscosity is tested and optimized, workouts of maximal intensity should be used on a limited basis only. Blood viscosity testing is a simple and intuitive approach to monitoring changes in blood flow and preventing overtraining induced performance issues before they occur.


1. Persson, S., Evaluation of exercise tolerance and fitness in the performance horse. Equine Exercise Physiology, 1983. 1: p. 441-457.

2. McKeever, K.H., et al., Role of decreased plasma volume in hematocrit alterations during incremental treadmill exercise in horses. Am J Physiol, 1993. 265(2 Pt 2): p. R404-8.

3. Muñoz, A., et al., Locomotor, cardiocirculatory and metabolic adaptations to training in Andalusian and Anglo-Arabian horses. Res Vet Sci, 1999. 66(1): p. 25-31.

4. Satué, K., A. Hernández, and A. Muñoz, Physiological Factors in the Interpretation of Equine Hematological Profile, in Hematology – Science and Practice, C.H. Lawrie, Editor 2012, InTech: Janeza Trdine 9, 51000 Rijeka, Croatia. p. 596.

5. Muñoz, A., et al., Dehydration, electrolyte imbalances and renin‐angiotensin‐aldosterone‐vasopressin axis in successful and unsuccessful endurance horses. Equine Vet J, 2010. 42: p. 83-90.