Final answer:
Lactic acid production increases when exercise intensity surpasses VO2max, as the body shifts to anaerobic metabolism due to insufficient oxygen supply, potentially leading to muscle fatigue.
Step-by-step explanation:
When intense exercise exceeds the capacity of muscles to supply oxygen for aerobic respiration, the body resorts to anaerobic metabolic processes to continue producing ATP for muscular contractions. This switch from aerobic to anaerobic metabolism results in the conversion of pyruvate into lactic acid, which is a process known as anaerobic glycolysis. If VO2max, or the maximum rate of oxygen consumption, is exceeded, the body cannot deliver oxygen quickly enough to the muscles, leading to an increase in lactic acid production.
This lactic acid can contribute to muscle fatigue and is part of the physiological response to intense exercise. While traditionally seen as a negative by-product causing muscle soreness and fatigue, lactic acid can be reutilized or converted back to pyruvate and glucose under certain conditions, serving as a temporary energy source. The body works to clear lactic acid post-exercise by converting it to glucose in the liver, a process requiring oxygen, thereby contributing to what we understand as 'oxygen debt.'