Types of Skeletal Muscle Fibres

There are three main types of skeletal muscle cells.
Muscle cells are also called muscle fibres, (Am Sp. muscle fibers).

The 3 types of skeletal muscle cells ( = 'muscle fibres') are:

• Red / Slow (Type I fibres, 'slow twitch fibres')
• Red / Fast (Type IIa fibres, 'fast oxidative fibres')
• White / Fast (Type IIb fibres, 'fast glycolytic fibres')

More about these 3 types follows further down this page. To understand the differences between the 3 types of skeletal muscle fibres it is also useful to know a bit about the 3 processes of ATP production in muscle cells.

Why are there different types of skeletal muscle fibres?

The body's diverse requirements of its skeletal muscles, e.g. to generate rapid movements in some cases but to maintain high levels of tension (without fatigue) in the cases of other muscles, are such that the muscle fibres forming some muscles have different properties than muscle fibres forming other muscles whose main function and activity is significantly different.

Consider two extremes:

The above examples are of 'extreme' muscles whose functions are such that they are composed mainly of either slow- or fast-contracting muscle fibres. However, many muscles have to perform a combination of endurance functions (e.g. contributing to the maintenance of body posture) and rapid actions. Muscles that need to be able to perform effectively in both respects consist of a combination of both slow- and fast-contracting muscle fibres.

Slow-contracting muscle fibres are called Type I.
Fast-contracting muscle fibres are called Type II.

There are two main types of fast-contracting muscle fibres (that is, Type II muscle fibres are divided into Type IIa and Type IIb), which have different resistance to fatigue. This relates to how effectively the muscle fibres can access the energy they need to contract. As explained on supply of energy for muscle contraction, the immediate source of (chemical) energy for muscle contraction is the molecule adenosine triphosphate (ATP), which releases energy when it breaks down: ATP ADP + P_i + Energy. As ATP is used-up by muscles as they contract, an important aspect of the supply of energy for muscle contraction is how the muscle fibres produce ATP.

In order to describe and compare the different types of skeletal muscle fibres it is useful to know that:

• In general, the myosin heads (within the thick filaments of muscle fibres) include an enzyme called ATPase that catalyzes, i.e. it acts as a catalyst for (=increases the rate of) the reaction ATP ADP + P_i + Energy, in which ATP decomposes into ADP and a free phosphate ion.
• Oxidative phosphorylation is the final series of chemical reactions in the synthesis of ATP by aerobic cellular respiration, which is a very efficient method of production of ATP and a method of ATP production that can be sustained for long periods of time, e.g. when running a race that takes several hourse to complete. However, this process of ATP synthesis requires oxygen.
• Oxidative phosphorylation takes place in the mitochondria within cells, so cells that contain many mitochondria are better adapted for production of ATP via aerobic cellular respiration, of which the final steps that yield most of the ATP are known as oxidative phosphorylation, than cells that contain fewer mitochondria.

Short descriptions of each of the 3 types of skeletal muscle fibres:

There are two ways to describe and compare the different types of skeletal muscle fibres. One way is to describe each type (as above), another is to compare and contrast their structures and properties in short note form as in the following tables. On larger screens and some smart phones in 'landscape' orientation, all the information appears in one table, enabling easier comparison. The comparison in the form of the table below is more concise whereas descriptions of each type of skeletal muscle fibre allow for fuller explanations than fit easily into a single table.

Tables comparing the 3 main types of skeletal muscle fibres:

*Most skeletal muscles contain a mixture of all three types of skeletal muscle fibres. However, the proportions of the different types of fibres that form specific muscles vary according to how the muscle is used - and, to a certain extent, also on genetic factors.

Note: General terms such as 'High', 'Medium' and 'Low' are shown in this table for ease of comparison of the fibre types I, IIa and IIb. Advanced courses may require examples of specific values e.g. of typical muscle fibre contraction times.

The general properties of each of the types of skeletal muscle fibres listed in the table above can be explained using a combination of other properties listed in the table supported by the descriptions in the section above.

For example, compare the resistance to fatigue of the 3 skeletal fibre types and explain the relative differences.

Why Type IIb has the least resistance to fatigue (= highest rate of fatigue):
Type I and Type IIa have higher resistance to fatigue than Type IIb because in the case of Type IIb, the main pathway for ATP production is anaerobic glycosis which cannot be sustained for as long a period of time as aerobic cellular respiration (the main pathway for ATP production in Type I and Type IIa).

Why Type I has the highest resistance to fatigue (= slowest rate of fatigue):
Aerobic cellular respiration requires oxygen, which is supplied via the capillary network. As the capillary density for Type I fibres is higher (i.e. their capillaries are more dense) than for Type IIa fibres, the aerobic cellular respiration (metabolic pathway) is likely to be better supplied with oxygen in the case of Type I fibres, giving them the highest resistance to fatigue.

See also what is metabolism?, simple explanation of lactic acid formation in muscle tissue, 3 ways ATP is produced in muscles and types of muscle contractions (isotonic, isometric, isokinetic).

Note: Fibres or Fibers ? There are two different spellings of the word fibres, as in 'muscle fibres'. Fibres (with 're' at the end) is a common British English spelling whire fibers (with 'er' at the end) is the usual American English spelling. Both versions appear online and are used in internet searches.