Muscles are one of the most important components in the equine body. Without them, the horse would be unable to walk, chew and digest food, or even swish his tail. Muscles comprise the largest tissue mass in the horse's body. There are various types of muscles performing a wide variety of duties, but basically they all function in the same general way--a period of contraction or shortening of muscle fibers, followed by a period of muscle relaxation or lengthening of muscle fibers.
In this article, we'll take a look at how equine muscles function and are nourished, as well as examine some of the problems that have surfaced, such as hyperkalemic periodic paralysis (HYPP) and tying-up. As has been the case throughout this series, we will draw on a wide variety of sources, including textbooks. However, a couple of researchers deserve specific mention. Much of the information concerning description of muscle structure comes from a paper authored by Craig H. Wood, PhD, coordinator of Distance Learning at the University of Kentucky, and published in the Horse Industry Handbook. When we discuss HYPP, the work of Sharon Spier, DVM, Dipl ACVIM, PhD, associate professor at the University of California, Davis, becomes a source of information. No discussion on tying-up should be conducted without drawing on the research of Stephanie Valberg, DVM, PhD, professor of large animal medicine and director of the Equine Center at the University of Minnesota.
Types of Muscles
The three muscle classifications are smooth, cardiac, and skeletal. The first two are involuntary or automatic, which means they function as needed without having to be called into action for a specific need. Skeletal muscles are voluntary; the brain sends a signal to the muscles via nerves, and the muscles react accordingly.
Non-skeletal muscles are basically the same for all horses. Smooth muscles are located in such places as the digestive tract, respiratory and circulatory systems, as well as the urogenital system. These muscles are many and varied, and they respond to stimuli within the respective systems.
There are three major types of cardiac muscles: atrial muscle, ventricular muscle, and specialized excitatory and conductive muscle fibers. Cardiac muscle is controlled by the central nervous system.
The skeletal muscles are striated in design, with a single muscle being composed of numerous muscle bundles made up of muscle fibers. There are two general classifications for these muscles fibers: Type 1 and Type 2.
Generally speaking, the slow-contracting Type 1 fibers are utilized heavily by horses involved in non-strenuous activity, while the fast-contracting Type 2 fibers are more often used by equines athletes, such as a racing Thoroughbred or Quarter Horse.
Wood writes that the muscle fibers contain several hundred to several thousand myofibrils. Each myofibril, in turn, has many myosin and actin filaments. These filaments are large polymerized protein molecules that are responsible for muscle contraction.
He also tells us that skeletal muscle contains a sarcoplasmic reticulum (calcium pump) and a tubular system. The calcium pump, Wood reports, is extremely important in muscle contraction through the release and uptake of calcium ions, which are involved in the contraction process.
Muscles are signaled to contract via nerve impulses. Relaxation occurs when the nerve impulses cease.
The slow-contracting or slow-twitch Type 1 fibers do not oxidize the muscle fuels rapidly and, as a result, are fatigue resistant. They are aerobic in nature, which means they utilize fuel in the presence of oxygen. Primarily, these fibers are used for long-term, non-strenuous work.
By comparison, Type 2 fibers are fast-contracting (fast-twitch) and tend to burn up muscle fuel in a hurry. As a result, they fatigue rapidly when compared to their Type 1 counterparts. Type 2 fibers are further classified as Type 2A, Type 2B, and Type 2C. Type 2A fibers are recruited for speed or an increase in strenuous activity. Type 2B fibers are called on for sustained bursts of speed or effort that carry the horse to the point where anaerobic energy--the type provided without oxygen--is required as an energy source.
Type 2B fibers contract with a maximum velocity that is 10 times that of Type 1 fibers and three times that of Type 2A fibers.
Type 2C fibers are classified as transitional and can serve as either a Type 2A or Type 2B. Research has demonstrated that racing Thoroughbreds and Quarter Horses have more Type 2 fibers than Type 1.
It is estimated that 80-90% of the muscle fibers in Thoroughbreds and Quarter Horses are of the fast-twitch or Type 2 variety. Standardbreds and Arabians have an intermediate number, about 75%. A heavy hunter or a draft horse, by comparison, would have a preponderance of Type 1, or slow-twitch, fibers.
Training can have a bearing on muscle fiber composition. For example, the number of Type 2A muscles fibers will increase in Thoroughbreds and Standardbreds as they are trained to race at longer distances.
The amount of adenosine triphosphate (called ATP, supplier of energy to cells in the body) required by a working muscle is dependent on the effort being expended. For example, when a horse is just ambling across the pasture at a slow walk, very little energy is required, and Type 1 fibers are utilized. The fuel for these muscle fibers is a combination of glycogen (the main form of carbohydrate storage), glucose (sugar), and fat, with the emphasis on fat during non-strenuous activity. However, when the horse breaks into a trot, the scenario changes. Now, Type 2A fibers are recruited. If speed is increased to a run, Type 2B fibers might be brought into play.
When this happens, more glycogen and/or glucose is needed as fuel through a process known as glycolysis. This involves the breaking down of glucose or glycogen into energy (ATP) without oxygen and, thus, is an anaerobic reaction. Glucose is the end product of carbohydrate metabolism and is the chief source of energy for living organisms. Excess glucose is converted to glycogen and is stored in the liver and muscles for future use.
Thoroughbreds travel at a high rate of speed during a race, with Type 2 muscle fibers being recruited along the way. Because this relatively high rate of speed requires a continued burst of energy, it isn't long before the utilization of fat and glycogen stores by the muscles is unable to supply all of the energy required and anaerobic glycolysis (without the presence of oxygen) occurs with its more rapid burning of glycogen. The lactic acid that accumulates as the result of glycolysis can bring with it an early onset of fatigue.
We can compare lactic acid to exhaust from a gasoline engine. Both lactic acid and engine exhaust are by-products of fuels being burned to provide energy. The gasoline engine must emit the exhaust in order to continue functioning properly. Likewise, the horse's muscles must rid themselves of lactic acid for optimal functionality. When lactic acid accumulates faster than it can be dissipated, the working muscles are compromised, and the horse is fatigued.
It is different when an endurance horse competes. It normally travels at slower speeds where muscles can rely heavily on aerobic energy generation. As a result, when an endurance horse becomes fatigued, it is often the result of glycogen depletion--running out of fuel--rather than a build-up of lactic acid.
Oxygen for Fuel
It quickly becomes apparent that the most important commodity for the equine muscles to function appropriately is oxygen, even though a portion of its energy might be produced anaerobically during strenuous exercise. Oxygen is inspired into the lungs and is conveyed by the bloodstream to the working muscles.
The average, healthy horse at rest has no trouble inspiring an appropriate amount of oxygen to fuel the muscles, but things can change rapidly when the horse is performing at speed. Researchers have conducted countless experiments using treadmills to measure the amount of oxygen the exercising horse inspires.
These studies have revealed that the strenuously exercising horse might be required to inspire up to 90 liters of oxygen per minute.
To achieve this, the horse's respiratory rate might exceed 150 breaths per minute (the normal range is 8-12 breaths per minute).
Form and Movement
Conformation is involved in the way that muscles function and also is involved in determining what disciplines are right for a particular individual. We have already indicated that the thickness and massiveness of the draft horse's muscle structure adapt it for power at the walk.
When walking, the draft horse is capable of moving heavy loads. However, its muscle structure is not designed for it to travel at a high rate of speed. Conversely, the angular form, long legs, and well-muscled rear quarters of the Thoroughbred allow it to move fluidly at speed, but it does not have the muscular strength to move heavy loads at the walk.
Within these respective groupings, however, conformational differences can have a strong bearing on how efficiently the muscles function.
For example, a racehorse with straight shoulders and pasterns will have a short stride. This means that the muscles must work harder via more frequent strides for this horse to reach a particular rate of speed. Its counterpart, with appropriate conformation, will have a fluid stride that will cover more ground and reach the same rate of speed with much less effort.
Various disciplines also bring with them certain muscle conformation requirements. The cutting horse, for example, needs a heavily muscled rear quarters because it is required to stop and pivot on its rear legs when preventing a cow from returning to the herd. The same is true of a reining horse, which is required to slide to a stop and spin. A roping horse that is used by a header in team roping, on the other hand, needs powerful muscles in the shoulders to turn a steer so that it is positioned for the heeler to throw his loop.
Injury and Disease
Injury to muscles can limit their ability to function properly. The same is true of some afflictions that have surfaced. One of them is HYPP. This is a muscular affliction that has been traced to a genetic defect in the Quarter Horse stallion Impressive.
Earlier it was noted that muscles receive signals from nerves to react. As part of this process, a membrane "pore" or channel in the muscle opens and closes, allowing for exchange of the electrolyte sodium from outside to inside of the muscle cell. Proper functioning of the sodium channel is vital for electrical stimulation and contraction of the muscle fibers.
In horses with HYPP, this channel does not function properly, and horses so afflicted often are characterized by intermittent episodes of muscle tremors, manifested by generalized or localized shaking, trembling, and weakness. In severe cases, the horse might suffer an episode of paralysis that could cause its death through cardiac arrest or respiratory failure.
Tying-up (know as recurrent exertional rhabdomyolysis) is another affliction of the muscles. It generally is manifested in one of two forms. One form is called polysaccharide storage myopathy (PSSM) and has been noted in Quarter Horses, Paints, Appaloosas, draft horses, draft crosses, warmbloods, and a few Thoroughbreds.
With PSSM, glycogen accumulates in the muscle, but the muscle is unable to utilize it. In essence, the muscle has an abundance of fuel, but is unable to burn it to produce energy. Minus the energy that the glycogen is designed to supply, these horses develop muscle cramps and stiffness.
Treatment of horses with this affliction involves supplying them with alternative sources of energy, such as a diet high in fat rather than high in carbohydrates.
There are more types of tying-up, some of which show up in excitable young racing Thoroughbred fillies and might be genetic in origin.
More is being learned about muscles and how they function. The good news about equine muscles is that they are adaptable. Through exercise and conditioning, they can be prepared and conditioned for diverse activities in such a way that they function efficiently and stave off fatigue in order to avoid injury.
About the Author
Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.
POLL: University Equine Hospitals