What is the primary energy system used during high-intensity interval training (HIIT)?

The primary energy system utilized during high-intensity interval training (HIIT) is the anaerobic system. This system is essential for activities that require quick bursts of energy and high power output over short durations, typically lasting from a few seconds up to around two minutes.

During HIIT, exercise intensity is often elevated beyond the aerobic threshold, meaning that the body's demand for energy exceeds what can be supplied via oxygen-dependent (aerobic) pathways. Instead, the body primarily relies on the anaerobic energy systems, specifically the ATP-CP (adenosine triphosphate—creatine phosphate) system for immediate energy, followed by anaerobic glycolysis for sustained high-intensity efforts.

While the aerobic system and fat oxidation play important roles in overall energy production during longer-duration, moderate-intensity exercise, they are not the primary contributors in HIIT scenarios where intensity and effort levels are much higher and shorter in duration.