Within the body, there are three types of muscles — smooth, cardiac and skeletal muscles. Muscles are placed into one of the three categories based on their structure and function. Cardiac muscle is found solely within the heart, while smooth — or involuntary — muscle is found around organs and is not under conscious control. Skeletal muscles are so named because they connect to the skeleton. They are under voluntary control, but there are different types of skeletal muscles.
All skeletal muscles are attached to the bones and are striated due to alternating bands of light and dark cells. Skeletal muscles support the skeleton as well as move it through contracting and relaxing of different muscles. The different types of skeletal muscles are often grouped based on whether they contract quickly or slowly.
Two categories — Type I and Type II fibers — are used to classify the different types of skeletal muscles. Type I fibers are also referred to as slow twitch or slow oxidative muscles. These types of muscles contract at a slower rate than Type II muscles. They are red in color, and contain a large number of mitochondria within their cells. Type I fibers are mainly made up of myoglobin and have many capillaries throughout the muscle.
In order for a muscle contraction to occur, energy is needed by the cells. This energy is produced by splitting molecules of adenosine triphosphate (ATP). The rate at which ATP is split will affect how quickly or slowly a muscle can contract. Type I fibers split ATP at a much slower rate than Type II fibers. The process of splitting and regenerating ATP in Type I fibers uses oxygen, and the muscles take longer to become fatigued.
Type II fibers can be subdivided further into different types of skeletal muscles. Both groups of Type II fibers are fast twitch fibers, but they differ in other functional and structural ways. Type II A fibers look similar to Type I fibers in that they are red in color, contain a great deal of myoglobin, and have many mitochondria and capillaries. Unlike Type 1 fibers, however, Type II A fibers split ATP very quickly, so they contract quickly. These types of skeletal muscles are rarely found in humans.
Type II B fibers are the other category of fast twitch skeletal muscles. Unlike Type I and Type II A fibers, they contain little myoglobin and few mitochondria and have few capillaries throughout. They do contain high levels of glycogen. These fibers appear white in color split ATP quickly so that they contract quickly. The process of splitting ATP in these muscle fibers is anaerobic, or does not require oxygen, so it cannot be prolonged and the muscles become fatigued relatively quickly.