The relationship between a horse’s speed and his cardiovascular system.
By Dr. William E. Jones publisher and editor of the Journal of Equine Veterinary Science and the Journal of Equine Veterinary Data
Research in equine exercise physiology was first aimed at basic studies, which revealed many of the unique physiological characteristics that make the horse such a superb athlete. In the last decade, equine exercise research has taken a more practical turn, and knowledge has been gained that can help trainers understand exactly how training changes a horse’s physiology to make it a better performer. Research is also hinting at the type of training that produces the changes your horses need for the particular type of performance you ask of them.
Perhaps the most important physiological change for a racehorse is in the cardiovascular system — the heart and blood vessels. Proper training can produce a variety of cardiovascular changes that make the racehorse faster.
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Researchers have learned how to measure the oxygen uptake in the muscles of the horse. This differs from oxygen taken in by the lungs. A horse with a healthy set of lungs can take in more oxygen than can be distributed to the muscles, a concept known as maximal oxygen uptake. When all other systems are in order, the level of performance for a racehorse is directly determined by maximal oxygen uptake, which can increase by 35 times in a horse from rest to intensive exercise. This oxygen is measured as milliliters per minute per kilogram (ml/min/kg) of body weight. A human runner can muster around 70 ml/min/kg, while a high-class racehorse has a maximal oxygen uptake of twice that amount. This difference provides much more power for a longer time for the athletic horse than for the athletic human.
As horses train, they experience increases in maximal oxygen uptake. This increase comes from changes in various physiological parameters of the cardiovascular system. Because oxygen is carried to the muscles by the blood, an increase in blood flow increases the amount of oxygen moving to the muscles, and both the heart and blood vessels respond to exercise in ways that increase the amount of oxygen coming to the muscles. In addition, the spleen of the horse improves its function to supply more red blood cells, which carry oxygen.
The right kind of training tends to enlarge the capillary network within the muscles, allowing more blood to be delivered in a shorter period of time. With proper training, the capacity of the spleen to hold a reserve of blood cells increases, and there is evidence that the spleen becomes more efficient at contracting during exercise, forcing more blood cells into circulation. But perhaps the most important changes that occur with proper training involve the heart itself.
Improvement in the heart is measured by cardiac output. The more blood that pumps through the heart, the more oxygen arrives at the muscles. Cardiac output is measured by liters per minute. A good racehorse at maximal exercise can have a cardiac output exceeding 240 liters/min. Cardiac output is a product of heart rate and stroke volume, both of which increase with proper training. Horses with little athletic ability generally do not have a heart rate above 200 beats per minute at intense exercise, while the best horses with proper training will approach 250 bpm.
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Stroke volume increases due to hypertrophy of the heart muscles. Overall heart size is a factor in cardiac output, and research has shown that heart mass in 2-year-olds can increase by up to one-third with proper training. There are large variations in heart size among untrained horses, and there is evidence that heart size is a major genetic factor that determines high-class horses from the rest of the population.