Fast-Twitch Muscle Fibres
You may have heard of these before. They are a part of the muscle that produces force very fast and with a lot of power. Every pedal we make in a race uses a lot of these particular fibres. Without them, we wouldn't be very fast. These fibres are quite large in size compared to slow-twitch fibres, which partially accounts for the strength that they can produce. They work without oxygen using what is called anaerobic respiration, something else you may have heard of. So that's where the name actually comes from.
More info on Wikipedia
