Muscle Fiber Types

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Slow-twitch and fast-twitch fibers are two general types of muscle fibers that make up the skeletal muscles, and each has its own advantages given certain situations. Type I is a slow-twitch muscle fiber that is used predominately in low intensity, long duration situations that require the oxidative pathway for ATP production. The best activity to demonstrate the use of Type I muscle fibers is an endurance activity such as running for an extended distance. Lance Armstrong resembles the ideal body type for Type I muscle fibers, because he bikes at a relatively low intensity throughout France for several weeks. His body resembles a leaner, less bulky build than most. In order to train your body to better perform such activities is to train aerobically, meaning that you must utilize oxygen. The most effective way to do this is running or biking for 30 to 60 minutes at a time.

Type IIa is a fast-twitch muscle fiber that uses the glycolytic pathway for ATP production. These fibers have a much higher force output than Type I muscle fibers and are larger in size as well. As a result they are more likely to become fatigued. Any activity that lasts from around 40 to 85 seconds will use Type IIa fibers to a great extent. An example would be a 200 to 400 meter dash. A typical body type would be placed under a mesomorph who has a muscular build. The best way to train in order to develop your Type IIa fibers would be using a rep range of 8 to 12 reps and practicing sprinting about 300 meters.

Type IIb is a second kind of fast-twitch fiber. This particular fiber is extremely strong, however because of this it is very susceptible to fatigue and injury. In order to effectively train these fibers you must utilize the ATP/CP energy pathway or they will be destroyed. Such an activity would last 0 to 3 seconds. A shot put is a good example. Endomorphs have a lot of Type IIb because of their sedentary lifestyles. Performing maximum lifts with a lot of rest, about 5 minutes, is the only way to train these fibers.

Type IIc is a third kind of fast-twitch fiber. It is produced when Type IIb fuses with surrounding satellite cells. Type IIb fibers are destroyed during physical activity and when cortisol is blocked they fuse together with satellite cells. Insulinlike growth factor-1 is responsible for the fusion process.

Source: Paul, PhD, FASCM, Davis O. Fitness: The Complete Guide. 8.1.5th ed. Santa Barbara: International Sports and Sciences, 2004. 111-113.

The size principal states that muscle fibers will be recruited from low-threshold and high-reliability units (Type 1) to high-threshold and low reliability units (Type II). Using a repetition range of 6-12 reps will recruit Type I fibers first and Type II muscle fibers secondly. Using a repetition range of 1-2 reps will only recruit Type II fibers, because Type I fibers are inhibited. A repetition range of 15-20 will only recruit Type I fibers, because the force is not great enough to recruit the high-threshold motor units.

Source: Source: Hatfield, Phd, F. C. (2004). Determining Training Loads. Fitness: The Complete Guide (pp. 426-445). Santa Barbara, CA: International Sports and Sciences Association.

Distribution of Muscle Fiber Types

The proportion of type 1 and type II muscle fibers differ from muscle to muscle, even though almost all muscles contain type 1 and type II muscle fibers. The distribution of fiber types usually depends on the individual’s level and type of physical activity and on the individual’s genetic makeup.

*Fast-twitch muscle fibers are also known as fast-glycolytic, white, or type II muscle fibers.

*Slow-twitch muscle fibers are also known as slow-oxidative, red, or type 1 muscle fibers.

Source: Saladin, Kenneth S. Anatomy and Physiology : The Unity of Form and Function. 4th ed. New York: McGraw-Hill Education, 2006. 429.

Changes in fiber type distribution

Type II fiber can adapt to resemble Type I fiber and Type I fiber cannot change to Type II fiber. Many scientists argue whether they morph into one another, or if they just take any certain characteristics that resemble one another.

Source: Davis, Phd, Fascm, P. O., & Hatfield, Phd, F. C. (2004). Muscle Anatomy and Physiology. Fitness: The Complete Guide (pp. 73-121). Santa Barbara, CA: International Sports and Sciences Association.

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