Which energy system dominates ATP production during the first 10 seconds of a maximal sprint, and what substrate provides the immediate phosphate?

• A. The glycolytic system dominates; glucose provides the immediate phosphate.
• B. The oxidative system dominates; fatty acids provide the immediate phosphate.
• C. The ATP-PCr system dominates; phosphocreatine provides the immediate phosphate.
• D. The ATP-PCr system dominates; glucose provides the immediate phosphate.

Answer: (C)

During the start of a maximal sprint, energy is supplied most rapidly by the ATP-PCr system, also known as the phosphagen system. This pathway regenerates ATP in a single, fast step: phosphocreatine donates its phosphate to ADP to form ATP, a reaction driven by the enzyme creatine kinase. Because this process happens via a direct phosphate transfer, it can meet the high energy demand in a fraction of a second, which is crucial in the very first seconds of intense effort.

The phosphate that reconstitutes ATP comes from phosphocreatine stored in the muscle, not from glucose or fatty acids. Glucose must go through glycolysis to make ATP, which takes more time, and fatty acids require even slower oxidation processes. Since phosphocreatine stores are limited, they are rapidly used up, and as the sprint continues beyond about 8–12 seconds, glycolysis begins to contribute more to maintain ATP supply. But for the first 10 seconds of a maximal sprint, the ATP-PCr system provides the majority of ATP, with phosphocreatine supplying the immediate phosphate for ATP regeneration.