In a 60-meter sprint, which energy system primarily resynthesizes ATP to meet the initial demand of the sprint?

• A. Lactic acid system
• B. ATP-PCr system
• C. Aerobic system
• D. Glycolysis

Answer: (B)

When a sprint begins, muscles need ATP immediately, and the fastest way to supply it is the ATP-PCr system. Phosphocreatine stored in the muscle donates a phosphate to ADP to make ATP, a reaction that happens rapidly and without oxygen. This provides energy for very high-intensity contractions for roughly the first 5–10 seconds, making it the dominant source right at the start of a 60-meter sprint. PCr stores are limited, so as the sprint continues, other systems (especially anaerobic glycolysis) begin to contribute more to keep ATP production up. The aerobic system is too slow to meet the immediate energy demand in this short, explosive effort. So the ATP-PCr system is the primary source for meeting the initial ATP demand.