Muscle fibres

A closer look at skeletal muscles: muscle fibres

Skeletal muscles are made up of two groups of muscle fibres, adapted for different functions

There are many skeletal muscles, large and small. They all pull on bones for leverage, but are adapted for different uses through the muscle fibres they contain.

‘Slow-twitch’ muscle fibres (type I) have a richer blood supply and use oxygen to help release energy through aerobic respiration, which makes them resistant to fatigue. ‘Fast-twitch’ fibres (type II) contract more rapidly, but respire anaerobically and fatigue more quickly.

In fact, there are actually two subcategories of type II fibres – ‘fast’ (type IIa) and ‘very fast’ (type IIb). Type IIa fibres can be thought of as a hybrid of type I and type II: they contain myoglobin and many blood capillaries and mitochondria – meaning they can respire aerobically, making them relatively resistant to fatigue. But they are also able to hydrolyse ATP quickly, using both aerobic and anaerobic metabolism, and so contract faster than type I.

On the other hand, type IIb fibres use hardly any oxygen directly – they are anaerobic, extracting energy from organic molecules stored in their cells. They produce lots of energy very rapidly, and so are the fastest, but fatigue quickly too. Because they don’t rely on aerobic respiration, they contain little myoglobin and fewer capillaries and mitochondria.

Most muscles have a mixture of fibre types, and specialised muscles will have more of one than the other. The muscles that move your eyes, for example, contain mostly fast-twitch fibres.

Sports training and muscle fibres

Naturally, people have different proportions of fibre types in their muscles, and different sports make use of different types. For example, sprinters, throwers and weightlifters rely on bursts of power from muscles with a high proportion of fast-twitch fibres. Someone with many slow-twitch fibres in their muscles is unlikely to break records over 100 metres, but someone with a higher proportion of fast-twitch muscle fibres will have a harder time getting in shape for a marathon.

Because of this, there has been much debate surrounding how to train specific fibre types, and even whether it’s possible to change a fibre’s type altogether. It’s widely accepted that fibres can’t be changed from type I to II or vice versa, but you may be able to change between type IIa and IIb with enough training and persistence.

Some differences in fibre type are inherited. In 2003, for example, one research group found that a variant in a gene affecting muscle fibres, known as ACTN3, was more common among a sample of elite athletes in speed events than in those requiring endurance.


Type Respiration Colour Where found?

Type 1 (slow)


Red due to large volumes of myoglobin (carrying oxygen) and mitochondria. Very dense capillary network, therefore very rich oxygen supply.

Postural muscles (neck and spine) due to endurance capabilities. High number in endurance athletes (eg marathon runners).

Type 2a (fast)

Aerobic and anaerobic

Red due to large volumes of myoglobin (carrying oxygen) and mitochondria. Dense capillary network, therefore rich oxygen supply.

Leg muscles have large quantities of type I and type IIa muscle fibres.

Type 2b (very fast)


White due to absence of myoglobin and fewer capillaries.

Arm muscles


Lead image:

Diagram of a cross-section of muscle, showing fibres.

Miles Kelly Art Library/Wellcome Images


Further reading

About this resource

This resource was first published in ‘Exercise, Energy and Movement’ in August 2016.

Cell biology, Physiology
Exercise, Energy and Movement
Education levels:
16–19, Continuing professional development