Equine exercise physiology is a relatively new scientific field. While not commonly used as yet, advancements in this area are beginning to creep into the daily routine of many horse trainers. However, others seem to be skeptical of researchers mumbling to themselves about VO2 Max, fiber type distributions, metabolic specificity, and scientific jargon. These uncertainties are understandable, but the modern horse trainer will be forced to learn and use the basics of this new science to remain competitive. This is not to say that the way horses are trained now is wrong. In fact, many training techniques turn out to be very effective in terms of effects on a horse’s body, but an understanding of exercise physiology can eliminate some of the hit and miss approach so common in the horse industry.
The purpose of this paper is to familiarize horsemen with some of the terminology appearing in horse publications, concepts involved in equine exercise physiology, and to suggest methods in which these concepts can be applied to physical training of a wide variety of horses. Before one can understand the concepts a brief explanation of some terminology is necessary. The exercise physiologist is mainly concerned with the healthy individual and ways to improve performance. Some of the terms commonly used and a brief explanation of their meaning follow. An understanding of these terms is necessary to understand the concepts to be presented later.
EXPLANATION OF TERMINOLOGY
Exercise physiology simply means the study of the horse’s body in response to exercise. That includes the study of skeletal muscle, the blood and its circulation, and the cardiorespiratory system, or more simply, the heart and lungs.
Metabolic Specificity of Exercise
Metabolic specificity of exercise is a concept which is of extreme importance. Put simply this concept says that if you want to be a fast runner you must train by running fast or if you want to be a good long distance swimmer you must train by swimming long distances. You cannot become a fast runner by swimming long distances nor a swimmer by running. Although that sounds very simplistic on the surface, the consequences are far reaching. For example, if a horse runs a mile today at a 4 minute pace his body will respond by storing fuel and rebuilding tissue so that he can run a 4 minute mile tomorrow with greater ease. However, he will not be fit to run a mile at a 2 minute pace. The exercise he performed in training specifically geared him for running a 4 minute mile. While there are some benefits of long slow distance (LSD) work for the fast runner, the runner still must work fast to become fast.
Glycogen is a form of stored fuel for exercise that is high intensity and relatively short in duration (30 seconds to 5 minutes). Glycogen is made up of many molecules of glucose tied together. Muscle glycogen is used for a localized fuel supply and liver glycogen is used to maintain the level of glucose in the blood. Glycogen does not require oxygen to be utilized as a fuel source.
Fatty acids are the form of stored fuel used in exercise of low intensity and long duration. Utilization of fatty acids as a fuel source requires oxygen.
Aerobic work is exercise of relatively low intensity during which glucose and fatty acids are the primary fuel. Walking, trotting, and slow loping are all aerobic types of work. Typically the horse’s heart rate remains below 150 beats per minute.
Anerobic work is exercise of high intensity or long duration during which glycogen, ATP, and CP are the primary fuel sources. Short sprints or a long fast gallop are examples of anaerobic work which typically cause a heart rate of above 150 beats per minute.
Adenosine Triphosphate and Creatine Phosphate
Adenosine triphosphate (ATP) and Creatine Phosphate (CP) are high energy fuels which are used in short term, high intensity exercise for 30 seconds or less. These compounds are sometimes collectively referred to as the Phosphagen System. ATP is the ultimate source of all energy for muscular contraction and can be synthesized from fats, carbohydrates (glucose or glycogen) and protein.
Lactic acid, or lactate as it is sometimes referred to, is a by-product of anaerobic work. Lactate s thought to be one of the causes of muscular soreness and stiffness 24 to 48 hours after intense exercise. Lactic acid is sent from the muscle to blood and is then removed from the blood by the liver. The process requires oxygen and is hastened by light work during recovery. Lactate was once thought to be a major cause of muscle fatigue, but recent evidence has cast doubt on that concept.
Anaerobic threshold is the term used to describe the point in exercise at which lactate begins to accumulate in the muscle and spill over into the blood stream. In horses, a workload that results in a heart rate of 150 is typically quoted as the anaerobic threshold, although it varies from 120 to 180 in individual horses.
Slow Twitch Muscle Fiber
Slow twitch muscle fibers are fibers characterized by a slow contraction speed and a high ability to utilize oxygen. They store a large amount of fatty acids as their primary fuel source. They are sometimes referred to as red fibers. These fibers are best suited for exercise of low intensity and long duration such as a 50 - mile competitive trail ride.
Fast Twitch Muscle Fiber
Fast twitch muscle fibers are characterized by a fast contraction speed. There are two categories of fast twitch fibers; those that have a high capability to use oxygen and those that cannot use oxygen to a large extent. These are sometimes called intermediate and white fibers respectively. Intermediate fibers are capable of using fatty acids as a fuel source and also utilize glycogen to a greater extent than do red fibers. White fibers use glycogen as their primary fuel source and do not use fatty acids to a great degree. Fast twitch fibers are best suited to high intensity exercise of a relatively short duration such as a 440 - yard sprint up to a 2 - mile race.
Printed with permission from Buckeye Horse Nutrition www.buckeyenutrition.com