Muscle Fibre Types
The classification of skeletal muscle fibres into slow-twitch (Type I) and fast-twitch (Type II), which contract at different speeds and suit different demands.
Definition
Skeletal muscle is made of fibres that broadly fall into two families. Slow-twitch (Type I) fibres contract more slowly, resist fatigue and rely on the aerobic system, which suits endurance and posture. Fast-twitch (Type II) fibres contract rapidly and forcefully but tire sooner, favouring sprints, jumps and other explosive actions.
Most muscles contain a mix of both, and the proportion varies between people and between muscles. This blend helps explain why some athletes gravitate toward endurance events and others toward power sports, though training, not fibre type alone, shapes performance. Fibre typing is a physiological concept described here for education, not a genetic or medical assessment.
Scope: Covers both fast-twitch (Type II) and slow-twitch (Type I) fibres as a single topic; the two are opposite ends of one classification rather than separate concepts.
Where you’ll hear “muscle fibre types”
Sports that use this term:
Running
The most accessible endurance sport — no venue, just shoes and the open road or trail.
Cycling
A low-impact endurance sport that doubles as transport, exercise and adventure.
Basketball
A fast, dynamic team sport of running, jumping and quick decisions on court.
Fitness
Strength and general fitness training — the foundation that supports every other sport.
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- Tennis ball types and specificationsThe four ITF ball types and what separates them — mass, size, rebound and deformation — plus the counterintuitive rule that fast balls are for slow courts.
- Tennis racquetA strung frame with a handle, used to hit the ball in tennis. Head size, weight and balance vary to suit different playing styles.
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- 2026-09-01
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