The lactic acid energy system predominantly uses which fuel?

• A. Creatine phosphate
• B. Protein
• C. Fatty acids
• D. Glycogen

Answer: (D)

This question tests understanding of which fuel powers the lactic acid energy system, the anaerobic glycolysis pathway that kicks in when oxygen is limited. In this system, glucose is broken down through glycolysis to pyruvate, and to keep glycolysis running without enough oxygen, pyruvate is converted to lactate. This allows rapid ATP production for high‑intensity efforts. The fuel used here is carbohydrate, specifically glycogen stored in muscle (and also some blood glucose). So glycogen is the predominant fuel for this system. Creatine phosphate powers the immediate phosphagen system for ultra-short bursts; fatty acids are used mainly in aerobic metabolism and require oxygen; protein is not a primary energy source during short, high‑intensity work.