Exercise physiology research

Abstract

Research in the field of equine exercise physiology was first aimed at basic studies that produced little useful information that could make a difference to a racehorse trainer. Scientists learned about the unique physiologic characteristics that make the horse such a superb athlete. Within the last decade, however, equine exercise research has taken a more productive turn, and as a result, knowledge has emerged that can help a trainer realize exactly how training changes a horse’s physiology to make it a better performer. Research is also hinting at what type of training produces the changes their horses need for the particular type of performance they ask of it. Perhaps the most important physiologic change for a racehorse is in the cardiovascular system. Proper training can produce a variety of cardiovascular changes that make the racehorse faster. Researchers have learned how to measure the oxygen uptake in the muscles of the horse. This differs from the 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. This leads to the concept of “maximal oxygen uptake.” The level of performance for a racehorse is directly determined by maximal oxygen uptake when all other systems are in order. The maximal oxygen uptake can increase by 35 times in a horse from rest to intensive exercise. This oxygen is measured as milliliters per minute per kilogram of body weight. A human runner can muster around 70 mL/min/kg, and 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 human athlete. As horses train, they experience an increase in maximal oxygen uptake. This increase comes from a change in various physiologic parameters of the cardiovascular system. Because oxygen is carried to the muscles by the blood, an increase in blood flow will increase the amount of oxygen moving to the muscles. Both the heart and blood vessels respond to exercise in a way that increases the amount of oxygen coming to the muscles. Also, the spleen of the horse improves its function to supply more red blood cells for the carrying of oxygen. The right kind of training tends to enlarge the capillary network within the muscles, allowing more blood to be delivered in a shorter amount of time. With proper training, the capacity of the spleen to hold a reserve of blood cells increases, and evidence exists that the spleen becomes more efficient at contracting during exercise, forcing more blood cells into the circulation. Perhaps the most important changes that occur with proper training involve the heart itself. Improvement in the heart is measured by the cardiac output. The more blood that pumps through the heart, the more oxygen that 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 L/min. Cardiac output is a product of heart rate and stroke volume. Both of these can increase with proper training. Horses with little athletic ability generally do not have a heart rate above 200 bpm at intense exercise. However, the best horses with proper training will approach 250 bpm. Stroke volume increases over a period of proper training from hypertrophy of the heart muscles. The overall heart size is a factor in cardiac output. Research has shown that heart mass can increase in 2-year-olds up to one third with proper training, although it is known that there is a large variation in heart size among untrained horses. Some evidence exists that heart size is genetic, and that is a major trait that determines the high-class horse from the rest of the population. This knowledge gleaned from equine exercise physiology research is very interesting. However, there is much more to learn about the proper training techniques required to maximize the best physiologic changes to make a horse compete at its highest possible level. Trainers should keep an open mind to possible changes in their traditional training regimen as more information comes from scientific studies. Copyright 2003, Elsevier Inc. All rights reserved 0737-0806/03/2303-0014 $30.00/0 doi: 1053/jevs.2003.39