Which physiological response occurs when exercising at high intensity for prolonged periods?

When exercising at high intensity for prolonged periods, increased lactic acid production is a physiological response that occurs due to the body’s demand for energy exceeding the availability of oxygen. During high-intensity exercise, the muscles require quick energy, which is primarily obtained through anaerobic metabolism. This process provides energy rapidly but cannot efficiently utilize oxygen to fully metabolize glucose, leading to the production of lactic acid as a byproduct.

As exercise intensity increases and is sustained, the rate of lactic acid production can surpass the body's ability to clear it, resulting in an accumulation within the muscles and bloodstream. This buildup is often associated with fatigue and can contribute to the sensation of discomfort during intense exercise.

The other options do not accurately reflect what occurs during high-intensity prolonged exercise. Oxygen consumption typically increases to meet the energy demands of the body, muscle glycogen synthesis is more associated with recovery and not during intense exercise, and heart rate variability generally decreases under stress, but this option does not directly link to the primary metabolic response happening at high intensities. Hence, increased lactic acid production is a clear and consequential response to such high-intensity workouts.