Muscle fibers that primarily use anaerobic energy systems are called?

Muscle fibers that primarily utilize anaerobic energy systems are known as Type II fibers. These fibers are designed for short bursts of intense activity and rely on anaerobic metabolism for energy, which enables them to generate quick force and power. Type II fibers are further classified into Type IIa and Type IIb, where Type IIb fibers are generally more fast-twitch and reliant on anaerobic pathways, leading to quick, explosive movements such as sprinting or weightlifting.

In contrast, Type I fibers are endurance-oriented and primarily rely on aerobic metabolism. They are more suited for activities that require endurance and sustained energy over longer periods. Fast-twitch fibers and slow-twitch fibers are terms that may describe variations based on speed and fatigue resistance, respectively. However, it is the Type II designation that specifically points to the fibers' reliance on anaerobic energy systems, making it the most accurate answer for this question.