Which energy system is predominantly used during low-intensity, long duration exercise, such as running a marathon?

The aerobic system is the primary energy system utilized during low-intensity, long-duration exercises, such as running a marathon. This system is efficient for activities that require sustained energy over extended periods, as it relies on the presence of oxygen to convert carbohydrates and fats into adenosine triphosphate (ATP), which is the energy currency of the body.

During a marathon, the intensity of exercise is relatively low, allowing the body to perform at an aerobic capacity where oxygen is plentiful. The aerobic system can produce a large amount of ATP through oxidative phosphorylation, making it ideal for endurance activities. It allows the body to tap into fat stores, which is crucial for long-duration activities since glycogen stores can deplete within a few hours.

In contrast, the other energy systems are designed for different contexts. The ATP-PC system and phosphocreatine system supply energy rapidly for high-intensity, short-duration efforts, such as sprinting or weightlifting, but are not sustainable for prolonged activity. The fast glycolytic system also supports higher-intensity efforts, relying on anaerobic metabolism to produce ATP quickly but leading to quicker fatigue and lactic acid buildup. As such, the aerobic system is clearly the most appropriate choice for long-duration, low-intensity exercises