What type of depletion leads to fatigue during prolonged aerobic activities?

During prolonged aerobic activities, the body primarily relies on carbohydrate stores for energy, specifically muscle glycogen, to sustain performance and prevent fatigue. Muscle glycogen is the stored form of glucose in muscles, and it serves as a key fuel source during endurance exercise. As exercise continues, muscle glycogen becomes depleted, which can result in a significant decrease in the ability to sustain the activity, ultimately leading to fatigue.

When glycogen stores are low, the body finds it increasingly difficult to maintain the necessary intensity and performance levels, resulting in feelings of fatigue and a decrease in exercise capacity. This is especially evident as the duration of the activity extends, which emphasizes the role of carbohydrate stores in fueling prolonged exercise.

In contrast, while protein and fat can also serve as sources of energy, they are not as readily available during high-intensity or endurance-related exercise as carbohydrates. Protein depletion primarily affects recovery and muscle repair rather than immediate performance in aerobic activities, and fat depletion occurs at a slower rate, making it less of a factor in the onset of fatigue during prolonged aerobic exercises. Therefore, muscle glycogen depletion is identified as the correct answer since it is directly related to fatigue experienced during extended periods of aerobic performance.