CSSD Practice Exam 2025 – Complete Study Resource

The main fuel source for ultra-endurance activities characterized by low intensity for more than 6 hours is fat oxidation. During prolonged, low-intensity exercise, the body primarily relies on fats as its main energy source due to several physiological reasons.

Firstly, in ultra-endurance scenarios, the body has the ability to tap into extensive fat stores, which can provide a substantial amount of energy without leading to fatigue as quickly as other substrates. Fats are stored in much larger quantities compared to carbohydrates in the form of triglycerides, making them a more sustainable choice for energy during long-duration activities.

Secondly, as the intensity of the activity decreases, the rate of fat oxidation increases. This is because low-intensity exercise requires a prolonged energy supply, and the mitochondria in muscle cells efficiently convert fat into ATP (adenosine triphosphate), which is the energy currency of the cell.

Carbohydrate oxidation, while important, is more suitable for higher intensity and shorter duration activities where rapid energy is required. Protein metabolism is generally not a primary energy source during moderate to low-intensity activities but may play a role in extended endurance where muscle tissue breakdown occurs for energy. Aerobic glycolysis would also play a lesser role since it primarily relies on carbohydrate metabolism