The Makeup of a Muscle
The picture below indicates the three fibre types identified using antibodies specific to the type I and type IIa MHC isoforms. The fibre labeled with an ‘I’ is a pure type I fibre, ‘A’ is a pure type IIa fibre, and ‘X’ is a pure type IIx fibre. All the fibres will be labeled accordingly and counted. To calculate the fibre type composition, the amount of fibres from each type is then divided by the total number of fibres counted and multiplied by 100 to obtain percentage.
It is therefore no wonder that muscle fibre type composition is an important part of muscle function (see the graph on the right). Studies have shown that the proportion of type I fibres is the predominant fibre type in endurance trained athletes (>60% in their Vastus lateralis).
The muscles from athletes that take part in explosive, high intensity type sports, like sprinting, seem to have more type II fibres, particularly type IIA fibres.