If heart rate stays below 85% of maximum and one energy system remains dominant for 1-2 hours, which energy system is responsible?

• A. ATP-CP
• B. Anaerobic glycolysis
• C. Aerobic glycolysis
• D. Lactic acid system

Answer: (C)

Submaximal, steady-state exercise relies on the aerobic energy system. When heart rate stays under about 85% of maximum, the body can meet ATP demands mainly with oxygen-dependent processes. Over a 1–2 hour duration, the aerobic system is efficient enough to supply energy continuously by oxidizing carbohydrates and fats in the mitochondria, which is why this scenario points to aerobic glycolysis as the dominant source.

The other systems are intended for different situations: the immediate, short bursts use the phosphagen system for up to about 10 seconds; anaerobic glycolysis kicks in for higher intensities but only lasts a couple of minutes before lactate accumulation limits performance; those cannot sustain 1–2 hours at submaximal effort. Thus, aerobic glycolysis best fits long-duration exercise at a moderate intensity.