Fast-twitch fibers are a type of muscle fiber that contract quickly and generate a lot of force. They are primarily used for short bursts of intense activity, such as sprinting or weightlifting. These fibers rely on anaerobic metabolism, which means they do not need oxygen to produce energy, allowing for rapid energy release. There are two main types of fast-twitch fibers: Type IIa and Type IIb. Type IIa fibers are more resistant to fatigue and can sustain activity longer than Type IIb fibers, which are designed for maximum power but fatigue quickly. Athletes often have a higher proportion of fast-twitch fibers, depending on their sport.