Fast twitch fibers primarily rely on which fuel source for energy?

Fast twitch fibers primarily rely on glycogen as their main fuel source for energy. These fibers are designed for short bursts of activity, such as sprinting or weightlifting, which require quick and powerful contractions. Glycogen is stored in the muscles and is readily available to be broken down into glucose, providing the rapid energy needed for high-intensity performance. This process occurs anaerobically, meaning it does not require oxygen, allowing fast twitch fibers to generate energy quickly, but it also leads to the production of lactic acid, contributing to muscle fatigue.

In contrast, other fuel sources such as oxygen are more associated with slow twitch fibers, which are designed for endurance activities and utilize aerobic metabolism. Fatigue-resistant fuel does not accurately describe the primary energy source for fast twitch fibers, as these fibers tend to fatigue more quickly due to their reliance on anaerobic metabolism. Protein is less commonly used for energy during exercise and is primarily utilized for repair and muscle building rather than as a direct fuel source during intense physical activity.