9.15.2008

Muscle Fibers

Introduction
Not all muscles are equal. They have different colors, sizes, contraction rates and metabolic processes used. The focus of this entry will be on the two classifcation of skeletal muscle types: slow twitch and fast twitch.

Muscle Composition
Skeletal muscle is comprised of bundles of individual muscle fibers called myocytes. Each myocyte contains many myofibrils, which are strands of proteins (actin and myosin) that can grab onto each other and pull. This process shortens the muscle and causes contraction. It is largely accepted that muscle fibers can be broken down into two main types: slow twitch (Type I) muscle fibers and fast twitch (Type II) muscle fibers. Fast twitch fibers can be further categorized into Type IIa and Type IIb fibers.

These varying muscle fiber types influence how muscles respond to training and physical activity, and each fiber type is unique in its ability to contract in a certain way. The average human muscle composition contains roughly 50% slow twitch muscle fiber and 50% fast twitch muscle fiber in those muscles used for movement. The "athletic freaks" possess bodies that are finely-tuned and customized for their sport. Below is an image of muscle compostion.
Slow Twitch (Type I)
The slow twitch muscle fibers are more efficient at using oxygen to generate more fuel (known as ATP) for continuous, extended muscle contractions over a long time. They fire more slowly than fast twitch muscle fibers and can go for a long time before they fatigue. This makes slow twitch muscles ideal for long distance athletes such as those competing in marathons or Ironman events. Olympic marathon athletes have been shown to have 80% slow twitch fibers.

Fast Twitch (Type II)
Because fast twitch muscle fibers use anaerobic metabolism to create fuel, they are much better at generating short bursts of strength or speed than slow muscles. However, they fatigue more quickly. Fast twitch fibers generally produce the same amount of force per contraction as slow twitch muscle fibers, but they are named such because of their ability to fire more rapidly. The possession of more fast twitch muscle fibers allows an athlete the opportunity to be an excellent sprinter since such athletes need to quickly generate a lot of force. Olympic sprinters have been shown to have 80% fast twitch fibers.

Type IIa Fibers
These fast twitch muscle fibers are also known as intermediate fast twitch fibers. They can use both aerobic and anaerobic metabolism almost equally to create energy. In this way, these are a combination of Type I and Type II muscle fibers.

Type IIb Fibers
These fast twitch fibers use anaerobic metabolism to create energy and are the "classic" fast twitch muscles fibers that excel at producing quick, powerful bursts of speed. This muscle fiber has the highest contraction rate (rapid firing) of all muscle fiber types, but it also has a much faster rate of fatigue and cannot last long before it needs rest. 

Summary
It is not proven whether or not training can convert slow twitch muscle fibers to fast twitch, or vice versa. At this time, there does not seem to be enough hard evidence to back this up.

Sources
About.com: Sports Medicine, Elizabeth Quinn, 

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