9.5: Types of Muscle Fibers
By the end of this section, you will be able to:
- Describe the types of skeletal muscle fibers
- Explain fast and slow muscle fibers
Two criteria to consider when classifying the types of muscle fibers are how fast some fibers contract relative to others, and how fibers produce ATP. Using these criteria, there are three main types of skeletal muscle fibers. Slow oxidative (SO) fibers contract relatively slowly and use aerobic respiration (using oxygen) to produce ATP. Fast oxidative (FO) fibers have fast contractions and primarily use aerobic respiration, but because they may switch to anaerobic respiration (a process called glycolysis that does not require oxygen), can fatigue more quickly than SO fibers. Lastly, fast glycolytic (FG) fibers have fast contractions and primarily use anaerobic glycolysis. The FG fibers fatigue more quickly than the others. Most skeletal muscles in a human contain(s) all three types, although in varying proportions.
The speed of contraction is dependent on how quickly myosin’s ATPase hydrolyzes ATP to produce cross-bridge action. Fast fibers hydrolyze ATP approximately twice as quickly as slow fibers, resulting in much quicker cross-bridge cycling (which pulls the thin filaments toward the center of the sarcomeres at a faster rate). The primary metabolic pathway used by a muscle fiber determines whether the fiber is classified as oxidative or glycolytic. If a fiber primarily produces ATP through aerobic pathways it is oxidative. More ATP can be produced during each metabolic cycle, making the fiber more resistant to fatigue. Glycolytic fibers primarily create ATP through anaerobic glycolysis, which produces less ATP per cycle. As a result, glycolytic fibers fatigue at a quicker rate.
Anatomical distinctions in oxidative fibers reflect the fact that they use and need more oxygen. The oxidative fibers contain many more mitochondria than the glycolytic fibers, because aerobic metabolism, which uses oxygen (O 2 ) in the metabolic pathway, occurs in the mitochondria. The SO fibers possess a large number of mitochondria and are capable of contracting for longer periods because of the large amount of ATP they can produce, but they have a relatively small diameter to facilitate oxygen delivery and thus have fewer myofibrils and do not produce a large amount of tension or power. SO fibers are extensively supplied with blood capillaries to supply O 2 from the red blood cells in the bloodstream. The SO fibers also possess myoglobin, an O 2 -carrying molecule similar to O 2 -carrying hemoglobin in the red blood cells. The myoglobin stores some of the needed O 2 within the fibers themselves (and gives SO fibers their red color). All of these features allow SO fibers to produce large quantities of ATP, which can sustain muscle activity without fatiguing for long periods of time.
The fact that SO fibers can function for long periods without fatiguing makes them useful in maintaining posture, producing isometric contractions, stabilizing bones and joints, and making small movements that happen often but do not require large amounts of energy. They do not produce high tension, and thus they are not used for powerful, fast movements that require high amounts of energy and rapid cross-bridge cycling.
FG fibers primarily use anaerobic glycolysis as their ATP source. They have a large diameter and possess high amounts of glycogen, which is used in glycolysis to generate ATP quickly to produce high levels of tension. Because they do not primarily use aerobic metabolism, they do not possess substantial numbers of mitochondria or significant amounts of myoglobin and therefore have a white color. FG fibers are used to produce rapid, forceful contractions to make quick, powerful movements. These fibers fatigue quickly, permitting them to only be used for short periods. Most muscles possess a mixture of each fiber type. The predominant fiber type in a muscle is determined by the primary function of the muscle.
FO fibers are sometimes called intermediate fibers because they possess characteristics that are intermediate between fast fibers and slow fibers. They hydrolyze ATP relatively quickly, more quickly than SO fibers, and thus can produce relatively high amounts of tension. They are oxidative because they produce ATP aerobically, possess high amounts of mitochondria, and do not fatigue quickly. However, FO fibers do not possess significant myoglobin, giving them a lighter color than the red SO fibers. FO fibers are used primarily for movements, such as walking, that require more energy than postural control but less energy than an explosive movement, such as sprinting. FO fibers are useful for this type of movement because they produce more tension than SO fibers but they are more fatigue-resistant than FG fibers.
Concept Review
The three types of muscle fiber are slow oxidative (SO), fast oxidative (FO) and fast glycolytic (FG). SO fibers use aerobic metabolism to produce low power contractions over long periods and are slow to fatigue. FO fibers use aerobic metabolism to produce ATP but produce higher tension contractions than SO fibers. FG fibers use anaerobic metabolism to produce powerful, high-tension contractions but fatigue quickly.
Review Questions
Q. A sprinter would experience muscle fatigue sooner than a marathon runner due to ________.
A. anaerobic metabolism in the muscles of the sprinter
B. anaerobic metabolism in the muscles of the marathon runner
C. aerobic metabolism in the muscles of the sprinter
D. glycolysis in the muscles of the marathon runner
- Answer
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Answer: A
Q. Drug X blocks ATP regeneration from ADP and phosphate. How will muscle cells respond to this drug?
A. by absorbing ATP from the bloodstream
B. by using ADP as an energy source
C. by using glycogen as an energy source
D. none of the above
- Answer
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Answer: D
Critical Thinking Questions
Q. Compare the anatomical differences between SO and FG muscle fibers. How do these differences relate to their function?
- Answer
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A. Anatomical Differences.
Glossary
- fast glycolytic (FG)
- muscle fiber that primarily uses anaerobic glycolysis
- fast oxidative (FO)
- intermediate muscle fiber that is between slow oxidative and fast glycolytic fibers
- slow oxidative (SO)
- muscle fiber that primarily uses aerobic respiration
Contributors and Attributions
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OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology