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Muscle fibre types: What does it all mean?

Updated: Dec 11, 2019

Fast twitch, slow twitch, fast glycolytic, type I, Type II, Type IIB, Red fibres, white fibres. Why isn’t a muscle just muscle? To understand how and why a muscle works, its first important to understand the biological make up of a muscle; Bear with me it’s not as scientific as it sounds.

Anatomy of a muscle

People often refer to the term muscle fibre, this is where the muscle contraction is causes through a process known as the sliding filament theory. The fibre is the inner most part of a muscle, which in turn is surrounded by multiple layers. Without boring you with lots of complex words Fig 1 shows how a muscle is made up. It’s within the Muscle Fibre where contraction speed and strength of the muscle is determined, in simple terms are you a sprinter, middle distance runner or marathon runner. It has always been accepted that there are 3 different types of muscle fibre.

Muscle Fibre Types

Slow Twitch Fibres:

Also known as type I fibres, these allow us to work for long periods of time at a steady state. Slow twitch fibres are rich in Mitochondria and myoglobin which utilise and store oxygen, the key chemical element needed for Adenosine Triphosphate production. You can find out more about ATP and energy system here. These fibres can continue repeated contractions for long periods of time without fatigue, great for endurance events and are primarily used in day to day tasks which involve low intensity effort for long periods. The downside to Slow twitch fibres is in the name, they contract slowly. We only utilise this fibre type for activities that have very little resistant or effort require.

Fast Twitch Fibres:

Fast twitch fibres are broken down further into Type IIA and Type IIB fibres. Type IIA fibres allow us to perform explosive, high energy movements. Although these fibres do contain Mitochondria and Myoglobin their concentration is a lot lower compared to the slow twitch fibres and therefore the presence of Oxygen is very limited. Fast twitch fibres rely on short-lived glycogen reserve meaning they can contract rapidly with high force however are not able to repeat this over long periods as fatigue sets in early.

Intermediate fast-twitch Fibres:

Also known as Type IIB fibres, these are a compromise between the two aforementioned fibre types. They have a moderate concentration of Mitochondria and Myoglobin allowing for reproduction of ATP to avoid fatiguing quickly but also can use short lived glycogen to contract with high force.

What Fibre Type am I? Nature Vs Nurture

Naturally the first question people will want answer is what fibre type am I? Unfortunately, the answer is all of them (which is actually a great thing), genetics is a big thing here as we are born with our make up of Muscle fibre types and in most cases, humans have a 50-50 split between slow and fast/intermediate fast twitch fibres. This allows us to enjoy both high energy, short term exercise such as 100M sprinting but also to be able to operate over endurance type sports such as marathon running. Often people find they are naturally better at one compared to the other (School PE or club sport at a young age is usually where preference are made).

Although no hard evidence is present it has been suggested that Usain Bolts Fibre type mix might have been as high as 80-20 in favour of fast twitch fibres.

So can you change your muscle fibre type?

The simple answer is no, if you are born with a 50-50 split between fast and slow this will not change; however, you can change the dominance or surface area of one muscle type over the other. Endurance or aerobic training will increase the capacity of mitochondria and myoglobin within the Slow twitch fibres, therefore increasing the bodies ability to perform low intensity, long duration events. Similarly, anaerobic or speed/strength training will increase the size and strength of Fast Twitch muscle fibres improving the body’s ability to perform fast, explosive movements.

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