What energy system do fast twitch fibers primarily work under?

Fast twitch fibers, also known as type II fibers, predominantly operate through the anaerobic energy system. This is due to their design, which allows them to generate energy quickly for short bursts of high-intensity activity, such as sprinting or weightlifting. The anaerobic system provides energy without the need for oxygen, relying on stored energy sources like creatine phosphate and glucose through glycolysis.

These fibers are particularly suited for explosive movements because they can produce force rapidly, though they also fatigue quickly due to the accumulation of metabolic byproducts like lactic acid. In contrast, the aerobic system is more suited for endurance activities and lower-intensity exercise, which is not the primary function of fast twitch fibers. Therefore, the correct answer highlights the specific capacity of fast twitch fibers to utilize anaerobic processes to support high-power outputs over short durations.