What are muscle fiber types (fast-twitch and slow-twitch)?

In one line

Muscle fiber types sort the cells that make up muscle by how fast they contract: fast-twitch fibers are quick and strong, and slow-twitch fibers are slower but keep going for a long time.

What are muscle fiber types (fast-twitch and slow-twitch)?
Why people good at sprinting and at marathons are said to have different muscles
At the gym and in workout videos you often hear the words 'fast-twitch' and 'slow-twitch'. It is also common to hear that people who sprint well have more fast-twitch fibers, and people who run marathons well have more slow-twitch. Here is what these two words actually mean, step by step.
In a word
Fast-twitch contracts quickly, slow-twitch contracts slowly
When a muscle shortens and pulls, it is said to 'contract'. Muscle fiber types sort the cells that make up muscle by how fast they contract. The ones that contract fast and hard are called 'fast-twitch', and the ones that are slower but keep going for a long time are called 'slow-twitch'.
What is a muscle fiber?
A long, thread-like cell that makes up muscle
If you split a muscle into smaller and smaller pieces, you find countless long, thread-like cells. These cells are called 'muscle fibers'. When the fibers shorten, the whole muscle shortens and pulls on the bone, and that pull moves the body.
Slow-twitch
Slow, but it lasts
Slow-twitch fibers use oxygen to make plenty of energy, so they do not tire easily even when they contract for a long time. In exchange, they are thin and produce little force. That is why they are used mainly to hold posture and to keep up light movements for a long time.
Fast-twitch
Quick and strong, but tires fast
Fast-twitch fibers are thicker than slow-twitch fibers and contract quickly and hard. They make energy quickly without oxygen; they can produce a lot of force, but they tire fast. They are used for short, hard movements like an all-out sprint.
Red fibers, white fibers
The color comes from myoglobin
Slow-twitch fibers look reddish and fast-twitch fibers look closer to white. Slow-twitch fibers hold a lot of 'myoglobin', a red protein that keeps oxygen stored inside the muscle. That is why muscle rich in slow-twitch fibers is also called 'red muscle', and muscle rich in fast-twitch fibers 'white muscle'.
Fast-twitch splits in two
Type 2a in the middle, type 2x the fastest
Fast-twitch fibers come in two kinds. 'Type 2a' is a middle type: fast, but it also uses a fair amount of oxygen, so it tires relatively less. 'Type 2x' is the fastest and strongest but tires the quickest. Slow-twitch fibers are also called 'type 1'.
The order of use
Slow-twitch first for small efforts, fast-twitch added for big ones
The body does not use all its fibers at once. When only a little force is needed, it uses slow-twitch fibers first, and the more force it needs, the more fast-twitch fibers it calls in, in order. That is how the same arm muscle can pick up a feather or lift something heavy.
It differs from person to person
People strong at distance running have more slow-twitch
The share of each fiber type differs from person to person. People who reach the very top in distance running or long-distance cycling have muscles rich in slow-twitch fibers, while those at the top in sprinting or weightlifting have muscles rich in fast-twitch fibers. Nearly half of the person-to-person differences in this share were calculated to be inherited.
What training changes
The clearest change is type 2x turning into type 2a
In the first few months of training, the clearest change happens inside the fast-twitch group: type 2x fibers turn into type 2a. Both strength training and endurance training such as distance running produced this change. With strength training, the shift between slow-twitch and fast-twitch was smaller than this.
A common misconception
Are there separate 'fast-twitch muscles' and 'slow-twitch muscles'?
No. Most human muscles contain a mix of slow-twitch and fast-twitch fibers; only the share differs from muscle to muscle. Even in the same outer thigh muscle, the share of slow-twitch fibers ranged from about 15% to 85% between people. Some muscles, like the soleus deep in the calf, are mostly slow-twitch.
Summary
Two kinds of muscle fiber, sorted by contraction speed
Muscle fiber types sort muscle cells by contraction speed: fast-twitch fibers are quick and strong, and slow-twitch fibers are slower but keep going for a long time. Most human muscles hold a mix of both, and only the share differs. Related terms: myoglobin, skeletal muscle mass.