Skeletal Striated Muscle - Muscle Fibers

Muscle Fibers

Individual muscle fibers are formed during development from the fusion of several undifferentiated immature cells known as myoblasts into long, cylindrical, multi-nucleated cells. Differentiation into this state is primarily completed before birth with the cells continuing to grow in size thereafter. Skeletal muscle exhibits a distinctive banding pattern when viewed under the microscope due to the arrangement of cytoskeletal elements in the cytoplasm of the muscle fibers. The principal cytoplasmic proteins are myosin and actin (also known as "thick" and "thin" filaments, respectively) which are arranged in a repeating unit called a sarcomere. The interaction of myosin and actin is responsible for muscle contraction.

There are two principal ways to categorize muscle fibers: the type of myosin (fast or slow) present, and the degree of oxidative phosphorylation that the fiber undergoes. Skeletal muscle can thus be broken down into two broad categories: Type I and Type II. Type I fibers appear red due to the presence of the oxygen binding protein myoglobin. These fibers are suited for endurance and are slow to fatigue because they use oxidative metabolism to generate ATP. Type II fibers are white due to the absence of myoglobin and a reliance on glycolytic enzymes. These fibers are efficient for short bursts of speed and power and use both oxidative metabolism and anaerobic metabolism depending on the particular sub-type. These fibers are quicker to fatigue.

Type I fibers (red) Type II a fibers (red) Type II x fibers Type II b fibers (white)
Contraction time Slow Moderately Fast Fast Very fast
Size of motor neuron Small Medium Large Very large
Resistance to fatigue High Fairly high Intermediate Low
Activity Used for Aerobic Long-term anaerobic Short-term anaerobic Short-term anaerobic
Maximum duration of use Hours <30 minutes <5 minutes <1 minute
Power produced Low Medium High Very high
Mitochondrial density Very High High Medium Low
Capillary density High Intermediate Low Low
Oxidative capacity High High Intermediate Low
Glycolytic capacity Low High High High
Major storage fuel Triglycerides Creatine phosphate, glycogen ATP, Creatine phosphate, glycogen (little) ATP, Creatine phosphate
Note Consume lactic acid Produce lactic acid and Creatine phosphate Consume Creatine phosphate Consume Creatine phosphate
Myosin heavy chain,
human genes
MYH7 MYH2 MYH1 MYH4

Skeletal muscle fibers are not all the same. Traditionally, they were categorized depending on their varying color, which is a reflection of myoglobin content.

Red Fibers: Those containing high levels of myoglobin and oxygen storing proteins had a red appearance. Red muscle fibers tend to have more mitochondria and greater local capillary density.

White Fibers: Those with a low content had a white appearance.

As more was learned about the functional differences between skeletal muscle fibers, they were also classified, depending on their twitch capabilities, into fast and slow twitch, traits that largely, but not completely, overlap the previous classification based on color.

Fast Twitch: Some authors define a fast twitch fiber as one in which the myosin can split ATP very quickly.

However, fast twitch fibers also demonstrate a higher capability for electrochemical transmission of action potentials and a rapid level of calcium release and uptake by the sarcoplasmic reticulum. The fast twitch fibers rely on a well-developed, short term, glycolytic system for energy transfer and can contract and develop tension at 2-3 times the rate of slow twitch fibers.

Slow Twitch: The slow twitch fibers generate energy for ATP re-synthesis by means of a long term system of aerobic energy transfer. They tend to have a low activity level of ATPase, a slower speed of contraction with a less well developed glycolytic capacity. They contain large and numerous mitochondria and with the high levels of myoglobin that gives them a red pigmentation. They have been demonstrated to have high concentration of mitochondrial enzymes, thus they are fatigue resistant.

The 2 main categories of muscle fibers become several, when further differentiating type II into several subtypes, based on myosin isoforms and denoted with letters of the alphabet. In humans the two subtypes are IIa and IIx; IIx is often referred to as IIb because earlier classification had clumped together two different types. Later on, further research recognized these subtypes as distinct, but the use of the name IIb remained entrenched. Non human fiber types include true IIb fibers, IIc, IId and so on.

Type I Red fibers. Slow oxidative (also called slow twitch or fatigue resistant fibers). Contain:

  • Large amounts of myoglobin.
  • Many mitochondria.
  • Many blood capillaries.
  • Generate ATP by the aerobic system, hence the term oxidative fibers.
  • Split ATP at a slow rate.
  • Slow contraction velocity.
  • Resistant to fatigue.
  • Found in large numbers in postural muscles.
  • Needed for aerobic activities like long distance running.

Type IIa Red fibers. Fast oxidative (also called fast twitch A or fatigue resistant fibers). Contain:

  • Large amounts of myoglobin.
  • Many mitochondria.
  • Many blood capillaries.
  • Large amount of glycogen.
  • High capacity for generating ATP by oxidation. Split ATP at a very rapid rate and, hence, high contraction velocity
  • Resistant to fatigue but not as much as slow oxidative fibers.
  • Needed for sports such as middle distance running and swimming.

Type IIx / IIb (dependent upon species) White. Fast glycolytic (also called fast twitch B or fatigable fibers). Contain:

  • Low myoglobin content.
  • Few mitochondria.
  • Few blood capillaries.
  • Large amount of Creatine phosphate.
  • Split ATP very quickly.
  • Fatigue easily.
  • Needed for sports like sprinting.

Individual muscles are a mixture of 3 types of muscle fibers (type 1, type 2a and type 2b), but their proportions vary depending on the action of that muscle. It must be remembered that skeletal muscles, although a mixture, can only have one type of muscle fiber within a motor unit. This is demonstrated if we look at contractions. E.g. If a weak contraction is needed only the type 1 motor units will be activated. These fibers are used mainly for endurance activities. If a stronger contraction is required the type 2a fibers will be activated or used to assist the type 1 fibers. Maximal contractions facilitate the use of type 2b fibers which are always activated last. These fibers are used during ballistic activities but tire easily. With advanced EMG techniques it is possible to look at which muscle fibers are recruited when performing an exercise/test. The total number of skeletal muscle fibers has traditionally been thought not to change. It is believed there are no sex or age differences in fiber distribution, however, relative fiber types vary considerably from muscle to muscle and person to person. Sedentary men and women (as well as young children) have 45% type 2 and 55% type 1 fibers. People at the higher end of any sport tend to demonstrate patterns of fiber distribution e.g. endurance athletes show a higher level of type 1 fibers. Sprint athletes, on the other hand, require large numbers of type 2 b fibers. Middle distance event athletes show approximately equal distribution of the 2 types. This is also often the case for power athletes such as throwers and jumpers. It has been suggested that various types of exercise can induce changes in the fibers of a skeletal muscle. It is thought that if you perform endurance type events for a sustained period of time, some of the type 2b fibers transform into type 2a fibers. However, there is no consensus on the subject. It may well be that the type 2b fibers show enhancements of the oxidative capacity after high intensity endurance training which brings them to a level at which they are able to perform oxidative metabolism as effectively as slow twitch fibers of untrained subjects. This would be brought about by an increase in mitochondrial size and number and the associated related changes not a change in fiber type.

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