Muscle fibers, their types and functions

Reading another post has inspired me! If only it happened more often when I had the time to splurge the thoughts whirring around in my head.

In a horses muscle strength, endurance and power have distinctly different properties. These properties affect a horses performance in different sports the rider chooses to pursuit.
In a single effort strength is the greatest amount of force a muscle can produce. This strength varies depending on where the joint is in it's range of motion. There are advantages in relation to joint angle, for example in humans our elbow is the strongest point of our body. When you make a fist and pull it towards your shoulder you have the greatest amount of leverage, this is why we ride like we do with our arms and hands in this position. It gives us the most leverage on the reins.
Anyway. Endurance is the muscles ability to perform over and over again before fatigue prevents further contractions. Power is the rate of contractions and force the muscle has.

I would explain the different types of movement but that isn't incredibly important here. What is important is what type of muscles makes the breed of horse do what.

Muscle fibers all differ in their metabolic and contractile properties. Contraction properties determine the speed the muscles can contract at and metabolic properties determine the muscles ability to use different metabolic routes.

You all still with me? Or have I lost you? Good ok let's continue.

A single contraction and relaxation of a muscle is called a twitch. Each type of muscle fiber completes a twitch within different time limits. Some contract and relax slowly, others contract and relax quickly. These muscles do not change, at least not normally within a horses lifetime often.
The difference between contractile and metabolic properties allow us to determine the difference between muscle fibers. There are three types:

Slow oxidative fibers (SO)
Slow oxidative fibers contract and relax slowly and rely on aerobic metabolism. SO muscles have a high concentration of oxidative enzymes and store large amounts of fats called triglicerides which are used as an energy substitute. Because a well developed capillary bed is nessesary to deliver oxygen the SO fibers are known as the "red" muscle due to it's rich blood supply.

Fast glycolytic fibers (FG)
The complete opposite of SO muscle fibers, Fast glycolytic fibers contract and relax very rapidly. They are the largest in diameter of all muscle types. FG muscles rely primarily on anaerobic, lactic metabolism for their energy. To be able to keep up their fast pace FG muscles must store large amounts of oxidative enzymes. The capillary network surrounding FG muscles is not as dense as SO muscle fibers so they deliver oxygen slower. Because of the lack of blood flowing to the muscle it appears paler and is known as the "white" muscle.

Fast Oxidative/Glycotic fibers (FOG)
Fast oxidative/glycotic fibers have fast twitch properties yet store large ammounts of glycogen but are an intermediate between SO and FG fibers. FOG muscles are very responsive to conditioning and depending on the type of work being done they can become more oxidative or glycotic. This is a particularly desireable type of fiber because it offers the fast contractile rates with oxidative or glycotic metabolism depending on the work being done.

Muscles specializing in aerobic metabolism have a smaller mass to allow faster diffusion of oxygen from the surrounding capillaries. Muscles with a high mass of oxidative fibers appear to be lean instead of bulky. Muscles used to generate great force rely on glycotic metabolism and appear larger.

Horses in events that require speed or sheer power, a high precentage of fast twitch fibers (FOG or FG) is desireable. Horses required to complete endurance events should have a high percentage of aerobic (SO and some FOG) fibers.

Most breeds muscle types can indicate what type of sports the horse is best suited for. For instance on two ends of the spectrum. Arabians have a very large percentage of SO fibers and few FG so this allows them to have the endurance they are reknown for in endurance races.
Quarterhorses on the other hand have a very low percentage of SO fibers and a very high precentage of FG fibers. This allows them to complete sprints at high speeds and rely on the anaerobic metabolism of these muscles.

Now I'll briefly touch on thoroughbreds and Standardbreds. They also have similar muscle types as Quarterhorses but the difference is Thoroughbreds and Standardbreds have a high percentage of Oxidative Enzymes. Oxidative enzymes allow for fast, long distances. Quarterhorses are known for their great speed over short distances and this is because of the high rate of glycotic enzymes. Glycotic enzymes, unlike oxidative enzymes allow for quick, short distances.

Short but sweet, maybe another time I'll touch on types of contraction and conditioning on types of muscles. I hope you enjoyed, feel free to shoot questions my way.