GLOSSARY

Type IIx fibres are the fastest and strongest muscle fibres in humans, also called fast glycolytic (FG) fibres; they produce great explosive force but fatigue very quickly.

Characteristics

• Myosin isoform: MHC IIx with the highest ATPase activity in humans.
• Metabolism: heavy reliance on the phosphagen system and glycolysis; few mitochondria, capillaries, and little myoglobin.
• Colour: white or pale.
• Motor unit: the largest units with large motor neurons; according to the size principle, they are recruited last and activated only for very high forces or very fast movements.

Terminology Note

Older textbooks called these fibres type IIb; molecular research showed that the predominant isoform in humans is actually IIx. In addition, hybrid fibres (such as IIa/IIx) containing more than one myosin type are common, especially during periods of training change.

Relevance to Sport

• Explosive sports: the 100 metres, jumps, throws, and weightlifting depend on these fibres.
• Training paradox: almost any type of training, even sprint training, reduces the proportion of IIx fibres and converts them to IIa; nevertheless power increases because fibres grow larger and the nervous system becomes more efficient.
• Tapering: a few days to two weeks of reduced volume before competition may partly restore IIx fibres and help achieve peak speed.