Type II muscle fibers, or fast-twitch fibers, contract quickly and generate more force than Type I fibers but fatigue more rapidly. They are divided into two subtypes:
1. Type IIa fibers: These are intermediate fibers that have characteristics of both Type I and Type II fibers. They can sustain activity for a moderate duration and are involved in activities like middle-distance running. 2. Type IIb fibers: Also known as Type IIx in humans, these fibers exhibit the fastest contraction speed and highest force output but fatigue very quickly. They are prominent in activities that require short bursts of power, such as sprinting or weightlifting.
