Which energy system uses glycogen as its fuel?

• A. Aerobic system
• B. ATP-PC system
• C. Lactic acid system
• D. Protein system

Answer: (C)

The energy system being tested is the anaerobic glycolysis pathway, which uses glycogen stored in muscles as its fuel. When high-intensity activity is performed with limited oxygen, glycogen is rapidly broken down through glycogenolysis into glucose-6-phosphate and then processed by glycolysis to pyruvate, which is converted to lactate. This fast breakdown releases ATP quickly, supporting short, intense efforts typically lasting from about 10 seconds to a couple of minutes. Glycogen is the key carbohydrate source fueling this system, which is why it’s described as the glycogen-utilizing anaerobic pathway. In contrast, the ATP-PC system relies on phosphocreatine, and the protein energy system uses amino acids; the aerobic system can also use glycogen but is characterized by energy production with oxygen over longer durations, using a mix of fuels.