Which system is engaged during prolonged physical activities that are not high in intensity?

The aerobic system is indeed the correct choice for prolonged physical activities that are not high in intensity. This system is designed to efficiently use oxygen to generate energy over an extended period, making it ideal for activities like long-distance running, cycling, or swimming.

During these longer, moderate-intensity activities, the body utilizes fat and carbohydrates more effectively, prolonging the duration of exercise without fatigue. The oxygen supply allows the aerobic system to produce ATP, the energy currency of the body, in a sustainable manner, which is crucial during extended periods of exercise.

The other systems, such as the anaerobic system, the phosphagen system, and the immediate energy system, primarily support high-intensity, short-duration activities. They rely on different pathways to produce energy, which are not as effective for longer-duration exercise where oxygen supply becomes essential. Therefore, for prolonged activities where intensity is low, the aerobic system stands out as the primary energy producer.