Muscle 101

Muscle Fibre Size

Each of the three fibre types differ in their size. The type I fibres are small, relative to the type IIA fibres, whereas the type IIX fibres are the largest. Normally, we measure the size of the fibres using histological techniques (as depicted below). In our lab, we use the cross-sectional area (CSA) of the fibre. As in a car engine, the larger the piston, the more force it can produce. The same applies to muscle fibres: the larger the fibres, the greater the amount of force each can production. Body builders, for example, have very large fibres compared to inactive individuals (controls), whereas elite endurance runners have smaller fibres than controls. However, some internal factors of a single muscle fibre, not yet identified, can increase or decrease the amount of force, irrespective of the muscle CSA.

The final section will summarise the composition of a whole muscle, primarily the muscle fibre type composition and relate these to various sporting codes.