Which energy system operates without oxygen and produces lactic acid?

The anaerobic system operates without the presence of oxygen and is primarily responsible for quick bursts of energy during high-intensity activities. When the body requires immediate energy, such as during sprinting or heavy weightlifting, the anaerobic system utilizes stored glucose in the muscles through a process called anaerobic glycolysis. This process does not involve oxygen and leads to the production of lactic acid as a byproduct.

During intense exercise, lactic acid can accumulate in the muscles, contributing to feelings of fatigue. This system is crucial for activities lasting up to about two minutes, after which the body transitions to aerobic processes that utilize oxygen for longer-duration energy needs.

The other options represent energy systems that either require oxygen (aerobic and oxidative systems) or have a different energy production mechanism that does not focus on producing lactic acid, such as the phosphocreatine system, which quickly replenishes ATP without producing lactic acid.