International A Level Biology / Muscles, movement and the heart

Fast and slow twitch muscle fibres

Edexcel International A Level Biology (YBI11), chapter 21: Muscles, movement and the heart, specification point 7.10. Free lesson.

Read the revision notes · International A Level Biology past papers

Slow twitch and fast twitch muscle fibres have different structures, so they work in different ways. These differences explain why slow twitch fibres suit endurance events, while fast twitch fibres suit short bursts of speed and power.

How the fibres differ in structure

Slow twitch fibres are also called type I fibres, and fast twitch fibres are type II. Their structures support different ways of releasing energy and contracting. Slow twitch fibres have more mitochondria, myoglobin and capillaries. They are narrower, which gives oxygen a shorter distance to diffuse to the centre. Myoglobin is a red protein that stores oxygen in muscle. Slow twitch fibres contain lots of it and look red, while fast twitch fibres contain little and look pale. Fast twitch fibres are larger and contain more myofibrils, which are the structures that contract.…

How each type works

The structural differences lead to different rates of respiration and contraction. Most muscles contain both types, but the proportion differs between people. Slow twitch fibres use mainly aerobic respiration, which uses oxygen. They contract more slowly and less powerfully, but are slow to fatigue. Fast twitch fibres rely more on anaerobic respiration. They contract rapidly and powerfully, but fatigue quickly as lactate builds up. Slow twitch fibres suit long, steady activity such as a marathon. Fast twitch fibres suit short, fast bursts such as a 100 m sprint.

Why slow twitch fibres resist fatigue

A good explanation links the structures to oxygen, then to aerobic respiration and ATP. ATP supplies energy for muscle contraction.

Which fibres suit each athlete

The event determines which fibre type is most useful. In the calf muscle of an elite marathon runner, about 80% of fibres are slow twitch. In an elite sprinter, about 75% are fast twitch. A marathon lasts over two hours. Slow twitch fibres provide oxygen for aerobic respiration, and ATP is made by aerobic respiration in their many mitochondria. This helps the runner keep going without fatigue. A sprint lasts about 10 seconds at top speed. Fast twitch fibres provide rapid, powerful contractions, using ATP made quickly from creatine phosphate and anaerobic respiration. This gives speed over a…

How to describe structural differences

For a structural comparison, name the structure itself and compare the two fibre types. Blood supply describes a result, not a structure. A clear comparison is: fast twitch fibres have fewer capillaries than slow twitch fibres. Other useful structures to compare include fibre size, sarcoplasmic reticulum and glycogen stores.

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