9.5 Types of Muscle Fibres

Two criteria to consider when classifying the types of muscle fibres are how fast some fibres contract relative to others, and how fibres produce ATP. Using these criteria, there are three main types of skeletal muscle fibres. Slow oxidative (SO) fibres contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. Fast oxidative (FO) fibres have fast contractions and primarily use aerobic respiration, but because they may switch to anaerobic respiration (glycolysis), can fatigue more quickly than SO fibres. Lastly, fast glycolytic (FG) fibres have fast contractions and primarily use anaerobic glycolysis. The FG fibres fatigue more quickly than the others. Most skeletal muscles in a human contain(s) all three types, although in varying proportions.

The speed of contraction is dependent on how quickly myosin’s ATPase hydrolyses ATP to produce cross-bridge action. Fast fibres hydrolyse ATP approximately twice as quickly as slow fibres, resulting in much quicker cross-bridge cycling (which pulls the thin filaments toward the centre of the sarcomeres at a faster rate). The primary metabolic pathway used by a muscle fibre determines whether the fibre is classified as oxidative or glycolytic. If a fibre primarily produces ATP through aerobic pathways it is oxidative. More ATP can be produced during each metabolic cycle, making the fibre more resistant to fatigue. Glycolytic fibres primarily create ATP through anaerobic glycolysis, which produces less ATP per cycle. As a result, glycolytic fibres fatigue at a quicker rate.

The oxidative fibres contain many more mitochondria than the glycolytic fibres, because aerobic metabolism, which uses oxygen (O2) in the metabolic pathway, occurs in the mitochondria. The SO fibres possess many mitochondria and can contract for longer periods because of the large amount of ATP they can produce, but they have a relatively small diameter and do not produce a large amount of tension. SO fibres are extensively supplied with blood capillaries to supply O₂ from the red blood cells in the bloodstream. The SO fibres also possess myoglobin, an O₂-carrying molecule like O₂-carrying haemoglobin in the red blood cells. The myoglobin stores some of the needed O₂ within the fibres themselves (and gives SO fibres their red colour). All these features allow SO fibres to produce large quantities of ATP, which can sustain muscle activity without fatiguing for long periods of time.

The fact that SO fibres can function for long periods without fatiguing makes them useful in maintaining posture, producing isometric contractions, stabilising bones and joints, and making small movements that happen often but do not require large amounts of energy. They do not produce high tension, and thus they are not used for powerful, fast movements that require high amounts of energy and rapid cross-bridge cycling.

FO fibres are sometimes called intermediate fibres because they possess characteristics that are intermediate between fast fibres and slow fibres. They produce ATP relatively quickly, more quickly than SO fibres, and thus can produce relatively high amounts of tension. They are oxidative because they produce ATP aerobically, possess high amounts of mitochondria, and do not fatigue quickly. However, FO fibres do not possess significant myoglobin, giving them a lighter colour than the red SO fibres. FO fibres are used primarily for movements, such as walking, that require more energy than postural control but less energy than an explosive movement, such as sprinting. FO fibres are useful for this type of movement because they produce more tension than SO fibres but they are more fatigue-resistant than FG fibres.

FG fibres primarily use anaerobic glycolysis as their ATP source. They have a large diameter and possess high amounts of glycogen, which is used in glycolysis to generate ATP quickly to produce high levels of tension. Because they do not primarily use aerobic metabolism, they do not possess substantial numbers of mitochondria or significant amounts of myoglobin and therefore have a white colour. FG fibres are used to produce rapid, forceful contractions to make quick, powerful movements. These fibres fatigue quickly, permitting them to only be used for short periods. Most muscles possess a mixture of each fibre type. The predominant fibre type in a muscle is determined by the primary function of the muscle.

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