The Fibre Type Concept
The fibres of muscle differ from one another in various aspects. Historically, the fibres were divided into red slow-twitch fibres and white fast-twitch fibres. This classification system has since evolved and is currently known as type I, type IIA and type IIX fibres. Most large animals, including humans, have these three fibre types in their muscles.
Accordingly, these three fibre types derive their names from the MHC isoform each fibre expresses. These are known as MHC I, MHC IIa and MHC IIx. In small mammals (like rodents), an extra fibre type is found in conjunction with the previous three, namely type IIB (MHC IIb). Although rare, a fibre can have both the MHC I and the MHC IIa isoform, and is classified as a hybrid type I/IIa fibre. Similarly, some may have MHC IIa and MHC IIx isoforms, and then would be termed a hybrid type IIa/IIx fibre.The table below summarises the properties of the three muscle fibre types found in human skeletal muscle.
Each of these fibre types has unique contractile properties (force, shortening velocity and power) derived primarily from the type of heavy chain the myosin is made up of, as well as the size of the muscle fibre (cross-sectional area). To keep up with the demand in contraction, each fibre type specialises in a particular way of producing energy (ATP) from the various fuel sources (fat, glycogen, phosphocreatine), which is derived anaerobically (producing creatine or lactate) or aerobically via mitochondria.
The fibre type composition of muscle contains a number of components that work in synergy to achieve muscle contraction and to resist fatigue. Fuel sources and the metabolism thereof to provide ATP will try to sustain the demand for ATP required for contraction. The table below summarises the characteristics of the three major fibre types found in larger mammals.
The next section focusses on the performance of single muscle fibres.