You can tell that a quarter horse and a thoroughbred are two distinct breeds merely by looking at them. What is it that really catches your eye? Muscles! What we're talking about here are "skeletal muscles," muscles that surround the limbs. These are also called voluntary muscles because they are consciously and actively used to move.
Whether in horses or in humans, muscles are really just a giant bundle of stringy fibers gathered together at their ends and attached by tendons to bone. But it's not that simple because there are more than one "type" of fibers in each bundle. Animals have actually evolved to have different amounts of each fiber type in their bundles, depending on what they need to do. In this respect, horses differ from goats and kangaroos, and quarter horses differ from thoroughbreds.
Some muscles are made to contract quickly and explosively, resulting in power, strength, and immediate speed. These are called the "fast twitch" or Type IIb fibers.
Other muscles are meant to work steadily, without fatigue, for many hours, resulting in great endurance. These are classified as Type I, or slow-twitch
A third type of muscle fiber in the body is an intermediate, able to generate both rapid power and endurance ("Type IIa). Humans have the same mix of fiber types.
Horses that are meant for quick power and speed, such as quarter horses, will possess a greater proportion of Type IIb fast-twitch muscle fibers. Horses that are meant for endurance, such as Arabians, possess a greater proportion of Type I muscle fibers. Horses that are meant for both great speed and (relative) endurance, such as thoroughbreds, possess a greater proportion of Type I and IIa fibers, the slow and intermediate types.
The cellular makeup of the muscle fibers themselves, and the energy source that the muscle cell uses, that determine the type of contraction that they produce. The type of energy source that the muscle uses, in turn, determines the amount of blood supply to the muscle. Type I muscles are absolutely reliant on the presence of oxygen in order to work. They are necessary for aerobic exercise. Type II muscles can function without the presence of oxygen, and are necessary for anaerobic exercise
Muscle fiber types do not change with the type of training that the horse receives – it is actually the nerve supply during development of the muscle that determines its fiber type. However, work will enlarge or "hypertophy" certain types of muscles. For example, Type II fibers will respond to power work, like pumping iron in the gym, and Type I by long slow work analogous to running.
Each muscle consists of thousands of cells that are bundled together to form one functional unit. Skeletal muscles are covered with a protective sheath that eventually comes together to form tendons and ligament. Muscles have a plentiful blood supply, because they require constant delivery of oxygen and nutrients. The blood supply also takes away the toxic substances that muscles build up because of their high activity level.
Muscle Energy Sources
All cells are reliant upon high-energy phosphate bonds for energy. When these powerful phosphate chemical bonds break down, they release energy which is used by the body to power things, like pumps, enzymes, and revolving doors for proteins to come and go from cells.
The fastest method of producing energy comes from a molecule that stores phosphate, known as creatine phosphate. However, this hot fuel called in creatine phosphate lasts only 5-10 seconds. That's a quick burnout. For more continuous function, e.g. to cross a river, the muscles will be required to make and store a more reliable energy source.
Most of this reliable energy is found in a molecule called ATP. It's so important it's often called "energy currency." The P in ATP stands for phosphate – and, once again, it is in the P bonds that energy is stored. There are three of these high-energy bonds in ATP, which makes it a particularly valuable energy source.
ATP itself must come from the breakdown of nutrients – sugar (glucose, or glycogen in its stored form), fats, and protein. ATP is continually used, remade, and reused. It's such important stuff, only a very small amount is actually stored in the muscle cells. But there has to be this stuff around, because if a muscle cell runs out of ATP, it can no longer function. Rigor mortis, the stiffness seen in death is due to a final lack of ATP.
Type I muscles break down sugars by a process that requires oxygen, much like the gas burning engire that requires oxygen from air. The form of sugar breakdown that occurs in the presence of oxygen is known as aerobic glycolysis. Aerobic glycolysis is well suited to endurance type muscles, because it produces a very large amount of ATP (36 in total) for every molecule of glucose that is broken down.
Type I muscles also use fats for production of ATP, producing an astounding 460 ATP for every molecule that it broken down. Therefore, fat is an extremely dense source of energy production. Both aerobic glycolysis and the breakdown of fats are relatively slow processes. Anaerobic glycolysis is the process of breaking down glucose without the presence of oxygen. The net production of ATP from one sugar is only two ATP. So it we only get two dollars (ATPs) versus 460 from a molecule of fat, what is the advantage of anaerobic glycolysis? Actually it's great because it is extremely fast, and speed is the name of the game with muscles.
The down side to anaerobic glycolysis is a greater production of by-products, especially lactate. Lactate accumulation blocks anaerobic glycolysis itself, and thus muscle function.
Your quarter horse, is about to begin running the barrels. He starts to gallop. When a horse starts to gallop, he needs an immediate source of energy. This energy is found in the very small amounts of stored ATP and phosphocreatine. However, these supplies are quickly exhausted (within seconds). Your quarter horse's powerful, predominately Type IIb muscles, have by now started the process of producing energy by anaerobic glycolysis. This process is in at its peak within 60 seconds – with the job that your horse is doing now, this is all the energy that he needs. Because your quarter horse has a high proportion of fast-twitch muscles that are meant to move into gear quickly and anaerobically, and produce great strength and power, he is innately suited for many of his jobs, such as the quarter-run, barrel-racing, and calf-roping. However, these muscles can't sustain this process for more than a half a mile, so your quarter horse can't keep up the pace over a longer distance.
Imagine a different scenario – your thoroughbred racehorse is in the starting gates, facing a mile and a half course, instead of several hundred yards of sprinting. What happens? Well, he starts the same way that the quarter horse did under full throttle. But now, he must sustain his effort for longer than is possible with anaerobic glycolysis. The thoroughbred, by birth, has a higher proportion of slow twitch, oxidative, Type I and Type IIa muscles. Within one minute, the slower, but more efficient process of aerobic glycolysis has begun to supplement his efforts. Although aerobic glycolysis is much more efficient, it is not as fast a method of producing energy, so at this point, the pace starts to slow. Although the thoroughbred still keeps up an amazing speed over the last 3/4 of the race, he is physically incapable, no matter what his training regimen, of completing the entire race at a sprint.
Now, to stretch your mind a little further, imagine an Arab competing in a 100 mile endurance race. He needs more energy than even aerobic glycolysis can afford, but he doesn't need the powerful, short-term speed of the quarter horse, or even the pace that the thoroughbred can maintain for a mile to a mile and a half. Instead, he needs to be steady and sure for a truly impressive distance. The endurance horse needs a fuel that is in plentiful supply, but he doesn't need instant delivery of fuel – and this is found in the form of oxidation of fats. This process is slow, but extremely efficient. Even in a fit, muscular looking horse, there are enough body stores of fat to last for a very long time. Thus, the endurance horse will rely primarily on his Type I muscles to (relatively) slowly but very steadily power him through his grueling task.
Different breeds are intrinsically suited to the type of work that they do. They are not limited by their will or their desire, but rather by their physiological make-up, which is breed (i.e. genetically) determined to a large extent.
Appropriate training can bring each type of horse to its peak level of fitness, but will not change the type of muscle that the horse has by birth. A good example of how this poses a dilemma is the difficulty in training for 3-day eventing, which requires a horse to perform aerobic (cross-country, dressage) and anaerobic (jumping) exercises. These horses are truly impressive that can do it all.
Despite the limitations imposed by nature, training can enable each horse to use his muscular strength, power, speed, and endurance to its fullest effect.