5.3.3 Types of muscle fibres and muscles
Meat & Livestock Australia March 5, 2021
Muscle fibres are not all identical. The myosin heads that drive contraction can be expressed in different isoforms within the same muscle. The key difference between these isoforms of myosin is the rate at which they can undertake a complete contraction cycle, with the “fast twitch” isoforms doing so rapidly, and the “slow twitch” isoforms doing so more slowly. This leads to variation in the rate of ATP usage, thus the fast twitch isoforms become more reliant on glycolysis to replenish this ATP because it is more readily accelerated, where-as the slow twitch isoforms are more reliant on oxidative metabolism because it is more efficient. Therefore muscle tissue can be classified by several methods:
red, white and intermediate (appearance)
fast or slow (physiological)
oxidative or glycolytic (metabolic)
In general, these can be combined into the following classifications:
Type I muscle fibres (red muscle) are also called slow twitch or slow oxidative fibres and have a slower contraction velocity, and an associated slower rate of ATP usage. They rely upon oxidative metabolism (ie TCA cycle) to generate ATP and therefore contain many mitochondria, and high concentrations of myoglobin which give them a red colour. Type I fibres are also highly vascularised (many blood capilaries) which enables blood to carry oxygen to them efficiently. Due to their reliance on oxidative metabolism these fibres tend to be quite resistant to fatigue. Postural muscles (i.e. the legs, neck and back muscles) tend to comprise a high propotion of Type 1 fibres.
Type IIA fibres (intermediate) are also called fast twitch or fast oxidative fibres, and in contrast to Type I fibres they have a faster contraction velocity, and an associated faster rate of ATP usage. However, similar to Type I fibres they rely on oxidative metabolism to generate ATP, and therefore contain many mitochondria, and high concentrations of myoglobin which give them a red colour.
Type IIX fibres (white muscle) are also called fast twitch or fast glycolytic fibres. These fibres have the fastest contraction velocity, and therefore the fastest rate of ATP usage. For this reason they are heavily reliant on glycolysis for ATP production, as this pathway can be massively accelerated when required. Therefore they have few mitochrondria, relatively few blood capillaries (for delivering oxygen), and lower concentrations of myoglobin which results in a whiter colour. Muscles of the limbs often contain higher proportions of these fibres.
The classification of fibres using Type I, IIA and IIX is only one of several nomenclature systems. There are other classification systems based on different histochemical methods of staining, but these are essentially equivalent to the one given above, which is the most commonly used.
It should be noted that muscle tissue is a heterogenous mixture of Type I, IIA, and IIX fibres. Muscle ‘types’ result from a predominance of one or other of the types of fibres. The breast meat of chicken is a good example. Although it has a relatively white appearance, this does not mean that chicken breast muscle is devoid of red Type I fibres, only that it contains a predominance of white Type IIX fibres, and therefore has a low myoglobin content.
The difference in frequency of muscle fibre types within a muscle, with associated physical and biochemical differences, result in characteristic differences between muscle ‘types’.
Factors affecting muscle fibre type
There are a range of different factors affecting muscle fibre type. These include:
Age – young animals tend to have more fast glycolytic fibres. This gradually changes as they mature with an increase in the proportion of type I and type IIA fibres, hence the meat becomes more red (Brandstetter et al 1998).
Nutrition – animals maintained on poor nutrition for extended periods express greater proportions of type I and type IIA fibres making their meat more red. This is likely to be an efficiency adaption, with the greater oxidative metabolism in type I and type IIA fibres producing more ATP per molecule of glucose than gycolytic metabolism (Brandstetter et al 1998).
Selection for muscling and leaness – animals selected for increased muscularity and leanness tend to have greater proportions of fast glycolytic type IIX fibres which have a greater cross-sectional area. This reduced reliance on oxidative metabolism is associated with less myoglobin concentration which makes their meat whiter. This has important consumer implications, particularly for the red meat industries where consumers purchase red meat in part for its high iron content which may be diminished due to the reduced myoglobin concentration. The impact of selection is particularly evident in breeds that have been selected for muscling and leaness such as the Belgian Blue cattle and Texel sheep, but it can also be seen within breeds across the extremes in their muscling and leanness breeding values.
Exercise – There is also evidence to suggest that exercise (or lack of it) can increase the proportions of fast glycolytic white fibres, though the change may not be permanent. This may have implications for the feedlot industries where animals are maintained in confined pens and therefore a relatively sedentary state.
The proportions of muscle fibre types within particular muscles can also vary with sex, and castration.
Wild and feral animals often have a larger number of oxidative fibres, with greater blood supply, than comparable muscles in their domestic equivalents. This is likely to be a reflection of a number of the effects described above, including their more active state, no selection for growth and muscling, poorer nutrition, and generally older age when harvested.
Post mortem characteristics of fibres
White fibres have a more highly developed sarcoplasmic reticulum which influences the response of muscle to changes in temperature and pH early post mortem. White muscles, because of their predominantly glycolytic nature, exhibit much more rapid lactic acid accumulation early post mortem. For this reason they are more susceptible to problems associated with rapid post mortem glycolysis. Alternatively, muscles with higher proportions of white fibres are more resistant to pre-rigor shortening than those composed of red.