What are muscle fibers

The light (white) muscle fiber type (fast twitch or FT fiber / FTG fiber) can react very quickly and enables strong contractions. But he tires quickly.

The dark (red) muscle fiber type (slow twitch or ST fiber) responds more slowly to stimuli, has a longer contraction time, but tires much more slowly.

The third type of muscle fiber is that Intermediate type (FTO), which is located in its properties between light and dark muscles. It seems that this type of person can be influenced in one direction or another through training. However, there are still too few scientific studies on this.

There is much to suggest that the proportion of different muscle fiber types is hereditary. The American jumper and sprinter Carl Lewis is said to have had 90% FT fibers.
In contrast to the fast muscle fiber type, which can hardly be increased, there are many arguments in favor of converting FT fibers into ST fibers or FTO fibers through endurance training.
Diagram: Fiber changes through special training.
Text: muscles, genes and competitive sports

Differences in metabolism:
The FT fibers impress with their abundance of energy-rich phosphates and glycogen and the fact that they are appropriately equipped with enzymes for anaerobic energy production. In addition to the abundance of glycogen, the ST fibers also have enzymes of the aerobic metabolism.

Differences in innervation
The ST fibers are innervated via slowly conducting nerve tracts, FT fibers via fast conducting tracts.