Slow and Fast muscle fibers

The fibers of the skeletal muscles of the entire body are not same. There are two primary variants, which differ from one another significantly. These variants are:

Slow (Type I) muscle fibers:

These type of fibers show a slow ‘tonic’ contraction characteristic of postural muscles. These are red in color because of large amounts of myoglobin. The fibers arc rich in mitochondria and oxidative enzymes, but poor in phosphorylases.  Because of a well-developed aerobic metabolism, slow fibers are highly resistant to fatigue. There features can be summarized as:

  • 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.

Fast (Type II) muscle fibers:

Type II muscle fibers are further divided into two categories: Type IIa and Type IIb.

Type IIa fibers:

They represent a variant of type II (fast) fibers which are relatively resistant to fatigue, although less than type I fibers. They are red in color because of presence of significant amount of myoglobin in them. Oxidative phosphorylation in this type of fibers is more developed that Type I fibers. Their features can be summarized as:

  • Large amounts of myoglobin.
  • Many mitochondria.
  • Many blood capillaries.
  • 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 IIb fibers:

They show a fast ‘phasic’ contraction, required for large-scale movements of body segments. These are paler (white) in color because of small amounts of myoglobin. The fibers arc rich in glycogen and phosphorylases, but poor in mitochondria and oxidative enzymes. Because of a glycolytic respiration, the fast fibers are quite easily fatigued. There features can be summarized as:

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

Muscles of the body are mixture of fiber types:

In man, most of the skeletal muscles show a mixture of above 3 fiber types, however, a specific muscle may show a dominance of one fiber type. This depends on the function of the muscle in body. Muscle required for fast phasic contractions have a dominance of type II fibers. On the other hand muscles required for tonic long term contractions have a dominance of type I fibers. It should however be kept in mind that a single motor unit of a skeletal muscle contains only one type of fibers.

Muscles of the body can be modified to contain specific types of fibers in relative abundance. It depends on the type of work performed by the muscles. For instance, muscle performing tonic work with long term contractions will eventually develop a lot of type I fibers.