Editor’s Note: This is an excerpt from Understanding Equine First Aid by Michael A. Ball, DVM. 

The ability to exercise and the metabolic responses to exercise differ in each individual horse and can vary greatly depending on many factors. The genetic potential for athleticism, the level of training, the degree of conditioning, the presence of underlying health problems, and environmental variables such as ambient temperature and humidity are only a few of the factors that determine overall exercise tolerance. As many a “weekend warrior-type” human athlete knows all too well, if you have trained to run a couple of miles once a week and then set out to do the 26-mile Boston Marathon, the outcome will be painful. The same is true for your horse.

Many exercise-induced problems could be avoided by applying common sense limits to performance stresses. If you plan to compete at eventing and endurance riding, make sure both you and your horse are appropriately prepared. Know your limits and don’t be afraid to pull up if those limits are going to be exceeded. During the training for such events, monitoring of the health and well being of your horse becomes your responsibility. Seek out advice on training. Learn what the danger signs are and gradually expand your horse’s individual limits.

Exercise exhaustion can occur after relatively brief maximal exercise or after prolonged submaximal exercise. The signs of exercise exhaustion are: 1) a persistently elevated body temperature, heart rate, and respiratory rate; 2) the presence of dehydration with a lack of thirst; 3) an increased capillary refill time observe