Course 16.2Lesson 3 of 3

Fatigue, red fibres and white fibres

Repeated contractions make lactic acid when the muscle breaks glycogen down without oxygen, and this causes fatigue. Red fibres hold much myoglobin and many mitochondria and work with oxygen. White fibres hold little myoglobin, depend on anaerobic breakdown and are tired sooner.

about 25 min1 simulation1 practice question

Lesson 3 of 3Show all lessons

Try the code and the simulations beside the lesson.

A sprinter and a marathon runner

A sprinter runs for ten seconds, and her legs burn. A marathon runner runs for hours. Both use the same kind of muscle, but not the same kind of fibre.

Fatigue

A muscle that is made to contract again and again gets tired. This is fatigue. When the muscle is used hard, the blood cannot bring all the oxygen that it needs. Then the glycogen of the muscle is broken down anaerobically, that is, without oxygen. The end product is lactic acid. The accumulation of lactic acid causes fatigue.

The reaction time of the fibres is different in different muscles. Some muscles contract faster than others.

Red fibres and white fibres

A muscle holds a red coloured pigment that stores oxygen. It is called myoglobin. Its structure is like that of haemoglobin, but it is in the muscle and not in the blood.

Two muscle fibres drawn side by side. The red fibre, on the left, is dark red, with many mitochondria drawn as small green ovals among the myofibrils, a little sarcoplasmic reticulum, and a note that it holds much myoglobin and works aerobically. The white fibre, on the right, is pale, with few mitochondria, a large network of sarcoplasmic reticulum, and a note that it holds little myoglobin and depends on anaerobic breakdown of glycogen.

Red fibres

A large amount of myoglobin. Reddish in colour.
Plenty of mitochondria. Less sarcoplasmic reticulum.
ATP comes from aerobic processes, which use the stored oxygen. They are also called aerobic muscles.

White fibres

Very little myoglobin. Pale or whitish in colour.
Fewer mitochondria. A large amount of sarcoplasmic reticulum.
ATP comes from anaerobic processes.

The red fibres hold plenty of mitochondria. The mitochondria use the large amount of oxygen that the myoglobin stores, to make ATP. So red fibres can work for a long time. The white fibres have fewer mitochondria and little myoglobin. They depend on anaerobic breakdown, and they tire sooner.

This part is not in the NEET syllabus, but A-Level and IB use other names. They call the red fibres slow twitch fibres, and the white fibres fast twitch fibres. The fast fibres give a strong, short effort, and the slow fibres give a steady, long one.

You should find that a fibre that makes most of its ATP with oxygen can make many more contractions before it is tired.

Try it in Python

The program compares a red fibre and a white fibre in the model.

Practise with new numbers

Common mistakes

Saying that red fibres have few mitochondria. They have plenty, and they use oxygen.
Saying that white fibres store oxygen in myoglobin. They hold very little myoglobin.
Saying that lactic acid is made when there is plenty of oxygen. It comes from the anaerobic breakdown of glycogen.
Mixing myoglobin and haemoglobin. Myoglobin is the pigment of the muscle, and haemoglobin is the pigment of the red blood cells.
Saying that all muscles react at the same speed. The reaction time differs from muscle to muscle.

Remember these

Fatigue: lactic acid accumulates from the anaerobic breakdown of glycogen.
Myoglobin: a red oxygen storing pigment of the muscle.
Red fibres: much myoglobin, plenty of mitochondria, aerobic.
White fibres: little myoglobin, fewer mitochondria, much sarcoplasmic reticulum, anaerobic.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

What causes the fatigue of a muscle?

What is myoglobin, and which fibres have much of it?

Why are the red fibres called aerobic muscles?

Which fibres have more sarcoplasmic reticulum?

Which kind of fibre would you expect in a muscle that must work for a long time without rest?

References

Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 10, Muscle Tissue, sections on types of muscle fibres and on exercise.
NCERT (2023). Biology, Textbook for Class XI, chapter 17.