To get a racehorse to run fast, you have to increase the oxidative capacity of the horse's musclesContrary to popular opinion on the backstretch, the racehorse's engine is not his heart. The heart is the carburetor. It is muscle that is the engine that makes the Thoroughbred go.
Horsepower is muscle at work. The next time you hear a car salesman bragging about 300 horsepower under the hood, you might ask him, "What kind of horse?" Personally, I'd prefer Arabians in the family car, Thoroughbreds in a fine sports car, Quarter horses in a dragster, and Belgian draft horses in a one-ton pickup. In fact, while there is a definite difference in the qualities of muscle fiber by breed, you also might want to ask, "How were the horses trained?," in order to get a true perspective of the capabilities, because muscle cells are elastic.
Let's look at some words that are commonly used when discussing muscle. "Strength" is a matter of how many muscle cells are contracting at one time. "Power" takes the concept a step further by describing how fast the muscle cells are contracting. "Endurance" is a matter of how long the muscle cells can keep contracting. And "stamina" is a matter of how long the muscle cells can keep contracting at a high power output. Stamina, then, is the word we want to use when describing Thoroughbred muscles, because stamina describes Secretariat's capabilities-sustained firepower. Sustained firepower wins wars, business markets, the Daytona 500, and horse races.
There are two kinds of skeletal muscle cells in all mammals: slow contracting and fast contracting, also known as slow twitch and fast twitch, or Type I and Type II. Slow twitch muscle cells (muscle cells are actually long fibers that contract when triggered) contract slowly and are used when the animal is moving slowly. Fast twitch muscle cells contract quickly to produce power and speed. Every horse begins life with a certain percentage of slow twitch versus fast twitch muscle cells. Arabians have more slow twitch cells than Thoroughbreds, while Quarter horses have more fast twitch muscle cells. In fact, in the propulsive muscles, Quarter horses have more than 90% fast twitch muscle cells.
Training changes muscle cells. Not only can the ratio between slow twitch and fast twitch muscle cells be changed, but the internal composition of muscle cells can be changed through conditioning. That is what exercise is all about-changing muscle cells. The possible internal changes include the types and quantities of available fuels, the size (thickness) of muscle cells, and the oxidative capacity.
Increases in fuel supply and oxidative capacity in fast twitch muscle cells are what turns raw power into stamina. Muscle cells will fatigue and stop contracting if they run low on available fuel or if lactic acid builds to a point where it interferes with the internal chemistry of the cell and paralyzes it.
Lactic acid is produced because fast twitch muscle cells generally burn muscle fuel-glycogen-without using oxygen. This anaerobic fuel burning is inefficient, leaving behind partially burned fuels in the form of lactic acid. Lactic acid can be further burned in muscle cells that have oxidative capacity. And this leads us to the key muscle cell of the racehorse, the fast twitch, high oxidative, or Type IIB. This is a fast contracting cell that is more difficult to fatigue. In other words, it is the stamina muscle cell. It is the missing link between ordinary cheap speed and Secretariat. It is the muscle cell that turns lactic acid from a deterrent to sustaining speed into a rich source of additional racing fuel.

Oxidation of fuel
One more fat word to learn: mitochondria. Mitochondria is a bug, a virus-like cell that exists in all living things and enables fuels to be oxidized. It is mitochondrial density that changes fast twitch muscle to fast twitch, high oxidative muscle. The more mitochondria, the more oxidation of lactic acid takes place, and the more stamina the muscles exhibit. It is that simple.
The key to mitochondria is that they show up where lactic acid is, just like flies and bees show up where sugar is. If a horse exercises his fast twitch muscle cells hard enough and long enough, lactic acid is produced in large quantities. When this happens, mitochondria are attracted to the site of lactic acid production. The next time lactic acid is produced, some of it is processed by these mitochondria, and even more mitochondria are attracted to the site. Over time, with the right kind of exercise, the stamina of any racehorse can be increased dramatically.
While local muscle oxidative capacity is the most important characteristic we can train into muscle cells, there are other important considerations, such as muscle cell fueling. Remember, physical and chemical adaptation takes place in all living things faced with environmental stress. If you deplete a muscle cell of fuel in a workout or a race, that muscle cell will refuel in about 48 hours and will overstore some fuel, just in case you try to drain that cell dry again. That's because a fuel-depleted muscle cell will burn itself, or neighboring cells, for fuel. The body is always trying to prevent this kind of damage by adapting quickly. Thus, fuel-depleted muscle cells will get more attention and extra supplies.
Now, let's say you walk a horse continuously for four hours one day. You're using slow twitch muscle cells, and since slow twitch cells amount to only about 20% of the horse's propulsive musculature, many of those cells might be depleted by four hours of walking. Certainly, four hours of trotting would do the job. The next day, then, all those depleted slow twitch cells, the ones that have survived, will swell up with extra fuel. After a couple of days of recovery, you may actually have ten pounds of extra fuel available to those slow twitch muscle cells. That is an extra ten pounds of baggage in a race, because fast twitch cells, not slow twitch cells, are the racing muscle cells.
Just the opposite happens with extended, faster work. Then, fast twitch cells are depleted of fuel and subsequently supercompensated in the following days of recovery. This is one reason why races "do a horse good." The racing exercise is specifically challenging the fuel capacity and oxidative capabilities of fast twitch muscle cells. Short three-eighths-mile breezes do not do much good for fuel storage and oxidative capacity because they do not last long enough to deplete much in the way of muscle fuel or to build lactic acid. To build stamina, you have to train for stamina.
If you want speed-that is, the horse is lacking speed-then you have to train for it. What you are training for is neuromuscular coordination. But that is another story that will be addressed in the next column. For now, just remember that mitochondrial density is the key to victories in the classics.
If you want a practical example of preparations that increase mitochondrial density and fast twitch muscle fuels, look to Woody Stephens's training schedules on his way to winning five straight Belmont Stakes (G1). What you will notice is a series of races and exercises that start long and strong and, workout by workout, taper back. Challenge the appropriate systems, then let them recover, rebound, and supercompensate. Woody ain't "just lucky."

Types of muscles

• Slow twitch-Contract slowly and are used when the animal is moving slowly
  
  
• Fast twitch-Contract quickly to produce power and speed