What limits the duration of the ATP-PC energy system?

• A. Lactic acid
• B. Oxygen
• C. Fat stores
• D. CP stores

Answer: (D)

The main limit of the ATP-PC energy system is the amount of phosphocreatine stored in the muscles. This system uses phosphocreatine to rapidly donate a phosphate to ADP to make ATP, powering very short, high-intensity efforts. Those CP stores are finite, so once they’re exhausted the system can’t keep regenerating ATP quickly enough, and performance falls off. That’s why this system can only sustain activity for a brief period—typically around 6–10 seconds of maximal effort (with small variations for trained individuals).

Lactic acid isn’t the limiting factor here; it comes from the glycolytic system when CP runs low. Oxygen isn’t required for ATP-PC, so it doesn’t limit this pathway. Fat stores drive longer, aerobic energy production, not this immediate system.