The Dangers of Overtraining, Part 2

Previously at Equine Health Labs, we have touched on the dangers of overtraining and its effects on the horse's cardiovascular system and overall athletic performance. Many trainers give careful consideration to the risks of overtraining and, over the years, have developed special training regimens designed to reduce the risks of overtraining. One such regimen is high-intensity intermittent training (HIIT). Because this type of training is often used for conditioning before races, the potential dangers of HIIT should be considered.

High-intensity intermittent training involves running the horse at maximum or near maximum speed for brief periods and allowing the horse to reduce intensity and gait in between sprints. A 2011 study published in the Journal of Equine Veterinary Science demonstrated that HIIT had a negative impact on athletic performance in thoroughbreds between 4 and 5 years old. The study, led by Arno Lindner, DVM, PhD, was featured in an article at

As part of the HIIT training study, the horses were conditioned with two 100-meter runs at near maximal intensity, separated by 10-minute periods of walking. Horses were divided into groups conditioned with this regimen once, twice, or three times a week during six week conditioning periods. The horses were worked lightly on the days not involving HIIT. Standard exercises were initiated before, every 2 weeks during, and for 2 weeks after the horses' respective conditioning periods. Blood samples were drawn during these times to determine V4, the running velocity at which a blood lactate concentration reaches 4 millimoles per liter [mmol/L].

The number of HIIT regimens per week did not affect V4 but taken as a whole, V4 decreased among all training groups [1]. Lower V4 values indicate poorer training results, clearly demonstrating that the horse exhausts its aerobic capacity and that the delivery of oxygen to the tissues becomes insufficient for metabolic needs. These results suggest that the intensity of exercise has a more profound effect on racing performance than the duration of exercise. Trainers should consider the drawbacks of HIIT because the excess maximal or near-maximal exercises may contribute to decreased athletic performance and may potentially impair the health and lifespan of the horse.

Increasing exercise intensity forces the spleen to release more red blood cells into circulation, a necessary accommodation for respiratory and metabolic needs [2]. This adaptive response results in concentration of the blood, making it thicker and stickier. Horses trained at high intensity and speed have higher packed cell volume (PCV) than those trained for endurance [3]. Higher PCV elevates blood viscosity, ultimately hindering blood flow and nutrient exchange when horses need it most.

Measuring blood viscosity helps a trainer determine how well his or her horse adapts to exercise at varying degrees of exercise. Optimizing blood viscosity allows the trainer to prevent overtraining, reducing performance and cardiovascular health issues.


1. Lindner, A., et al., Effect of Conditioning Horses Once, Twice, or Thrice a Week with High-Intensity Intermittent Exercise on V4. Journal of Equine Veterinary Science, 2011.

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

3. 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.