Electromyography

The number of fibres comprising the individual motor units varies widely within a muscle and between muscles. As a general rule, smaller muscles that control fine movements, such as the muscles around the eyes, are composed of smaller motor units containing a smaller number of muscle fibres. In contrast, muscles that are used to generate strong forces and act on large body masses, such as limb muscles, are composed of larger motor units containing a greater number of fibres per motor unit. Muscle fibre types can be characterized as slow twitch (type I) or 'slow twitch oxidative' and fast twitch (type II). Type I fibres are characterized by a slower speed of force production than type II fibres and they produce smaller amplitude and longer duration force twitches. Fast twitch fibres can be further categorized into type IIA and type IIB fibres, which are also known as 'fast twitch oxidative' and 'fast twitch glycolytie, respectively. Type IIB fibres are characterized by a higher force and speed production but are fast to fatigue, whereas type HA fibres are more resistant to fatigue. Type I fibres have a reddish appearance due to the rich vascularization and blood supply providing oxygen, allowing them to be more resistant to fatigue. Type IIA have larger-diameter muscle fibres and have a paler appearance due to lower vascularization and blood supply. The limited amount of energy stored in their mitochondria renders them fast fatiguing. Although muscle fibres are classified into distinct groups (type I, IIA and JIB), each human muscle generally contains motor units of varying size and contractile properties that create a continuum among these groups. The proportion of slow to fast fibres in skeletal muscles varies depending on the type of muscle usage.