Skeletal muscles consist of fibres that can be categorized into distinct types in terms of how they contract and generate force. The simplest classification is into slow twitch, e.g., type I and fast twitch, type II fibres, which describes the strength and performance of a muscle. For example, “postural” muscles are primarily slow twitch type I.
Fibres can be further categorized for example based on their use of energy, such as “FOG” fast oxidative glycolytic and their speed of contraction, force production and resistance to fatigue. Finally they can be categorized by staining with different ATPase at different pH, and at the molecular level by their myosin heavy chain (MHC) isoforms, MHC-I, MHC-IIA and MHC-IIB. An additional type is called type IIX muscle fibre. Note that fibres may be in transition from one type to another, so there’s a I-IIA and a IIA-IIB “hybrid” type.
The fibre types differ in activation and maximum power output and velocity. The conventional theory is that so-called “motor unit” fibres are recruited in order of their capacity. The fatigue resistant Type I fibres are always recruited while the fast powerful fibres are reserved for when more power or speed are needed. So the motor units containing slow fibres are recruited first and followed by motor units composed of faster muscle fibres in sequence from type I to IIA to IIB. The “fast” IIB fibres can generate about twice the power, up to four times as quickly as the slow type I fibres.
Isolated type IIB fibres can generate about 8x the power, while type IIA generate about 5x times the power of slow type I fibres. In terms of velocity, type IIB fibres shorten almost 4x as quickly and type IIA about 2.5x as quickly as type I.
How does this work in a muscle that is comprised of mixed fibre types? The makeup of each muscle is different in terms of proportion of fibre types, so that the muscle as a whole has optimal power and speed of contraction for its task. The “slow” type I fibres which better resist fatigue always contribute power, and do not simply “give up” when faster fibres create more power while shortening at their optimal speed for power generation. So they keep the tension in the muscle.
Our muscle fibres “automagically” shorten at a speed at which maximum power is developed. Our training “automagically” makes this happen!