Describe the types of skeletal muscle fibers
By the end of this section, you will be able to:
- Differentiate between slow oxidative fibers, fast oxidative fibers, and fast glycolytic fibers
Skeletal muscle fibers can be classified based on two criteria: 1) how fast do fibers contract relative to others, and 2) how do fibers regenerate ATP. Using these criteria, there are three main types of skeletal muscle fibers recognized (Table 1). Slow oxidative (SO) fibers contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. Fast oxidative (FO) fibers have relatively fast contractions and primarily use aerobic respiration to generate ATP. Lastly, fast glycolytic (FG) fibers have relatively fast contractions and primarily use anaerobic glycolysis. Most skeletal muscles in a human body contain 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 rapidly 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, then it is classified as 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.
Slow oxidative fibers have structural elements that maximize their ability to generate ATP through aerobic metabolism. These fibers contain many more mitochondria than the glycolytic fibers, as aerobic metabolism, which uses oxygen (O2) in the metabolic pathway, occurs in the mitochondria. This allows slow oxidative fibers to contract for longer periods because of the large amount of ATP they can produce, but they have a relatively small diameter and thus do not produce a large amount of tension.
In addition to increased numbers of mitochondria, slow oxidative fibers are extensively supplied with blood capillaries to supply O2 from the bloodstream. They also possess myoglobin, an O2-binding molecule similar to hemoglobin in the red blood cells. The myoglobin stores some of the needed O2 within the fibers themselves and is partially responsible for giving oxidative fibers a dark red color.
The ability of slow oxidative fibers to function for long periods without fatiguing makes them useful in maintaining posture, producing isometric contractions, and stabilizing bones and joints. Because they do not produce high tension, they are not used for powerful, fast movements that require high amounts of energy and rapid cross-bridge cycling.
Fast glycolytic fibers primarily use anaerobic glycolysis as their ATP source. They have a large diameter and possess large volumes of glycogen which is used in glycolysis to generate ATP quickly. Because of their reliance on anaerobic metabolism, these fibers do not possess substantial numbers of mitochondria, a limited capillary supply, or significant amounts of myoglobin, resulting in a white coloration for muscles containing large numbers of these fibers.
Fast glycolytic fibers fatigue quickly, permitting them to only be used for short periods. However, during these short periods, the fibers are able to produce rapid, forceful contractions associated with quick, powerful movements.
Fast oxidative fibers are sometimes called intermediate fibers because they possess characteristics that are intermediate between slow oxidative fibers and fast glycolytic fibers. These fibers produce ATP relatively quickly, and thus can produce relatively high amounts of tension, but because they are oxidative, they do not fatigue quickly. Fast oxidative fibers are used primarily for movements, such as walking, that require more energy than postural control but less energy than an explosive movement.
The three types of muscle fibers are slow oxidative (SO), fast oxidative (FO) and fast glycolytic (FG). Slow oxidative fibers use aerobic metabolism to produce low power contractions over long periods and are slow to fatigue. Fast oxidative fibers use aerobic metabolism to produce ATP but produce higher tension contractions than slow oxidative fibers. Fast glycolytic fibers use anaerobic metabolism to produce powerful, high-tension contractions but fatigue quickly.
Critical Thinking Questions
1. Why do muscle cells use creatine phosphate instead of glycolysis to supply ATP for the first few seconds of muscle contraction?
2. Is aerobic respiration more or less efficient than glycolysis? Explain your answer.
- fast glycolytic fiber(FG)
- muscle fiber that primarily uses anaerobic glycolysis
- fast oxidative fiber (FO)
- intermediate muscle fiber that is between slow oxidative and fast glycolytic fibers
- slow oxidative fiber (SO)
- muscle fiber that primarily uses aerobic respiration
Answers for Critical Thinking Questions
- Creatine phosphate is used because creatine phosphate and ADP are converted very quickly into ATP by creatine kinase. Glycolysis cannot generate ATP as quickly as creatine phosphate.
- Aerobic respiration is much more efficient than anaerobic glycolysis, yielding 36 ATP per molecule of glucose, as opposed to two ATP produced by glycolysis.