We used to apply the notions of Red (type I) and White (type II) muscle fibers based on how the actual fibre would look in certain animals. An average person consists of around 50% of each fibre type, but the distribution hereof might vary with each person and is genetically determined.

 

The fibre composition varies from muscle to muscle, e.g. back and shank muscles contain more type I fibres than arms, which is of advantage when it comes to making our body standing up all day without collapsing.

 

Each person contains a different amount of each type of fibre, and these amounts may be identified by means of a muscle biopsy. This method involves removing a piece of muscle for microscope study purposes. It is a slightly painful procedure for the patient and it is only carried out at specialised laboratories.

 

Muscle fibre composition is genetically determined, meaning that certain people have a better predisposition for long distance races, as an example, whereas others are better at spurting 100 meters. 

 

 

Muscle fibre type I   

 

This type of fibre is also known as ”red” or ST (Slow Twitch) muscle fibres because they contract slowly and are very enduring. These muscle fibres are mainly used during long-term aerobic work (involving oxygen), e.g. for marathons or bike races.

 

 

Muscle fibre types IIx and IIb 

 

Contrary to type I, type II contracts particularly fast but lacks persistency. These fibres mainly work in an anaerobic manner (without oxygen) and are put to use for power or strength based work, e.g. spurts.

 

Type II fibres are divided into two subcategories: type IIx and type IIb, IIx being the fastest and IIb slightly slower but more enduring. By means of targeted training, the IIb fibre characteristics may be changed towards IIx or type I.

 

Type II is also known as ”white” or FT (Fast Twitch) fibres; however, these terms are rarely used these days.