Advanced Subsidiary (AS) WJEC Physical Education (PE) Practice Test 2026 - Free PE Practice Questions and Study Guide

Fibre types differ in how they produce force and how long they can sustain that effort. The fibres designed for endurance are the slow-twitch ones. They rely mainly on aerobic metabolism, have many mitochondria and capillaries, and high myoglobin, which lets them work for long periods without tiring. Because they contract more slowly, they generate less force, but they don’t fatigue easily.

In contrast, fast-twitch fibres can produce much more force but tire quickly; Type IIa is intermediate, while Type IIx/IIb are high-force but fatigue rapidly. So the fibre type with high fatigue resistance and low force is the slow-twitch (Type I).