Final answer:
The anaerobic system is most active during short bursts of high-intensity activity, relying on the glycolytic system to break down glucose for energy. Marathon running would primarily rely on the aerobic system.
Step-by-step explanation:
The most active energy system during short bursts of high-intensity activity is the anaerobic system. Anaerobic metabolism in the muscles of a sprinter allows for quick bursts of energy without the need for oxygen. This system primarily relies on the glycolytic system, which breaks down glucose for energy. Anaerobic metabolism in the muscles of a marathon runner would be less active since marathon running is a longer-duration activity that relies more on the aerobic system.