Muscle fibers

Muscle fibers or muscle cells are the individual contractile units within muscle. A single muscle such as the biceps brachii contains many muscle fibers.

Types of Muscle Fibers

There are three types of muscle fibers found in humans: Type I, Type IIa, and Type IIb. Each type has unique characteristics that determine its functional role.

Type I

Type I muscle fibers, also known as slow-twitch fibers, are red due to the presence of large volumes of myoglobin and high numbers of mitochondria. They can sustain force for extended periods, but they are not able to generate a significant amount of force.

Type IIa

Type IIa muscle fibers, also known as fast-twitch oxidative fibers, are a hybrid of Type I and Type IIb fibers. They are capable of using both aerobic and anaerobic metabolism almost equally to create energy, thus they are a combination of the characteristics of both Type I and Type IIb fibers.

Type IIb

Type IIb muscle fibers, also known as fast-twitch glycolytic fibers, are white due to a low level of myoglobin and fewer mitochondria. They generate energy primarily through anaerobic metabolic processes, thus they fatigue more quickly than Type I and Type IIa fibers.

Function

Muscle fibers are responsible for the contraction of muscles, which allows for movement in the body. The type of muscle fiber activated and the order of recruitment depend on the force required for the movement.

See Also

• Muscle tissue
• Myocyte
• Skeletal muscle

References

Muscle fibers Resources
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