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 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.

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:

Slow-contracting muscle fibre (Type I) Main characteristics: Low myosin ATPase activity (compared with Type II fibres) High capacity for ATP production via oxidative phosphorylation Very dense capillary network High levels of intracellular myoglobin Therefore ... Due to (a), (b) and (c), the main pathway for ATP production is aerobic cellular respiration, the final stage of which is oxidative phosphorylation Due to (c) and (d), their predominant colour is red, as in 'red muscle'. Fast-contracting muscle fibre (Type IIa) Main characteristics: Higher myosin ATPase activity than Type I fibres High capacity for ATP production via oxidative phosphorylation Dense capillary network High levels of intracellular myoglobin Therefore ... Due to (a), (b) and (c), the main pathway for ATP production is aerobic cellular respiration, the final stage of which is oxidative phosphorylation Due to (c) and (d), their predominant colour is red, as in 'red muscle'. Fast-contracting muscle fibre (Type IIb) Main characteristics: Higher myosin ATPase activity than Type I fibres Lower capacity for ATP production via oxidative phosphorylation than 'red' fibres Sparser capillary network No intracellular myoglobin Therefore ... Due to (a), (b) and (c), the main pathway for ATP production is anaerobic glycolysis, which is fast but not sustainable for as long as aerobic respiration, hence muscle fatigue occurs sooner (i.e. these fibres have the least resistance to muscle fatigue) Due to (c) and (d), their predominant colour is white .

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 way is to compare and contrast their structures and properties in short note form as in the following table. The table form is more concise whereas descriptions of each type of skeletal muscle fibre allow for fuller explanations than fit easily into a single comparison table.

Table comparing the 3 main types of skeletal muscle fibres:

Property: Red / Slow (Type I) Red / Fast (Type IIa) White / Fast (Type IIb) also called ... Slow twitch fibers Fast oxidative fibers Fast glycolytic fibers Colour Red - due to the respiratory pigment myoglobin which, in common with the haemoglobin (Am.Sp. hemoglobin) in red blood cells, stores O2 by loosely binding it. Red - due to the respiratory pigment myoglobin which, in common with the haemoglobin (Am.Sp. hemoglobin) in red blood cells, stores O2 by loosely binding it. White - due to the absence of pigmentation, e.g. the respiratory pigment myoglobin or hemoglobin (as present in red blood cells). Contraction time Slow Fast Very Fast Oxidative capacity High High Low Resistance to fatigue High Medium (Intermediate) Low Diameter (of muscle fibre) Small Medium (Intermediate) Large Capillary density High Medium (Intermediate) Low Mitochondrial density High High Low Glycogen reserves Low Intermediate High Main (metabolic) pathway for production of ATP Aerobic cellular respiration - final stage: oxidative phosphorylation Both aerobic and anaerobic metabolic pathways Only anaerobic metabolism, esp. anaerobic glycosis Force production (i.e. force produced by muscle) Low Medium-High Very High Example of typical use Repeated low-level contractions e.g. walking or low intensity cycling for 30 mins. Activities involving speed, strength and power, e.g. moderately weight training and fast running e.g. 400 metres. Short, fast, bursts of power (but rapid fatigue) e.g. heavy weight training, power lifting, and 100 metre sprints *Examples of muscles or locations of skeletal muscles with this type of fibre Postural muscles, e.g. of the neck and spine, & leg muscles (which have Type I & Type IIa fibres). Leg muscles have large quantities of both Type I and Type IIa fibres. Arm muscles. N.B. Type IIb fibres can be converted into IIa fibres by resistance training.

*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.