Muscle fibres come in two forms: type I slow twitch (ST) fibres and type II fast twitch (FT) fibres. Type II fibres can be further broken down into two categories: type IIa and type IIb (also referred to as IIx in some texts). A “twitch” refers to the amount of time required to stimulate contraction of a fibre. A “fast twitch” fibre, therefore, contracts quickly and is beneficial to sprinting and explosive-type movements. A “slow twitch” fibre is optimal for endurance sports, as they are highly resistant to fatigue and because endurance events occur at a submaximal workload.
Source: Plowman & Smith, Exercise physiology: for health, fitness, and performance, 2nd Ed - Tab 19-2
Slow twitch fibres: As evident in the above table, structurally ST fibres contain a high proportion of mitochondrial and capillary density, and myoglobin content compared to FT fibres. Cast your minds back to our article discussing improving VO₂ max and you will remember that these three components play massive roles in improving your aerobic capacity.
Functionally, ST fibres produce low amounts of force and have a slow contraction velocity. This is fine, however, as explosive movements are a rarity in endurance events. Importantly, ST fibres are highly resistant to fatigue, so they can contract for long periods of time without reducing force output. What do you think of when you picture an elite marathon runner? Probably a very thin and light athlete without an ounce of fat on them. ST fibres have a small fibre diameter, which is why elite marathoners appear to have very little muscle. They actually have a similar overall number of muscle fibres to everyone else, ST fibres just don’t experience much hypertrophy (size gain).
Metabolically, ST fibres have high triglyceride (fat) stores and oxidative enzyme capacity, assisting them to specialise in aerobic energy production. They are not required to have high phosphocreatine stores or a huge glycolytic enzyme activity, because their contraction velocity and energy demand is submaximal.
Fast twitch IIb muscle fibres are polar opposites to ST fibres. They have characteristics which favour high intensity, explosive, sprint-type work, and a very low oxidative capacity. They contract quickly and forcefully, but fatigue in a short period of time. FT fibres are prone to hypertrophy, which is why you see elite sprinters having a heap of muscle mass.
FT type IIa fibres fall in between ST type I and FT type IIb fibres, and elicit moderate characteristics of both. As you can see in the table, most of their characteristics are “intermediate”.
Can We Manipulate The Type of Muscle Fibre We Have Through Training?
Yes. Around 50% is genetic, and 50% is influenced through the training environment. Every individual will be born with a certain percentage of each muscle fibre type. Elite sprinters have been shown to express up to 70-75% of FT type II fibres in their muscles, and endurance athletes 70-80% ST fibre. Therefore, there is a genetic limit to what you are born with.
Source: Powers & Howley, 8th Ed - Tab 8.2
Source: Simoneau JA & Bouchard C. (1995). Genetic determinism of fiber type proportion in human skeletal muscle. FASEB J 9, 1091-1095
You can, however, modify fibre type to an extent through training. FT type IIa fibres (the ‘intermediate’ fibres) are highly influenced by the training environment. If you complete a lot of endurance training, they will shift towards eliciting ST fibre characteristics. If you complete high intensity sprinting, they shift towards becoming explosive FT IIb fibres.
The take home message? Your body adapts to the stressors you place on it. If you complete a heap of endurance work, you will both strengthen your current ST fibres, as well as manipulate your FT IIa (intermediate) fibres into expressing and/or transforming into ST fibre characteristics. Ever wonder why those Masters endurance athletes can hold a steady pace literally all day long? Their muscle fibres have adapted to many, many years of endurance training.
That wraps up the physiological determinants of successful endurance performance. The last three articles will begin to focus on the nutritional and behavioural determinants of performance enhancement.
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Written by Luke McIlroy – Director of Sport Science at METS Performance Consulting
BEx&SpSci, ESSAM, AES