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
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
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
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
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
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
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
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.
