Final answer:
During intense physical activity, muscles utilize lactic acid fermentation due to a lack of oxygen which prevents aerobic respiration. This allows for continued ATP production through glycolysis. Once sufficient oxygen is available, muscles switch back to the more efficient aerobic respiration.
Step-by-step explanation:
Muscles must use lactic acid fermentation during intense physical activity because there is an insufficient oxygen supply to allow aerobic respiration to continue. This is necessary in situations where muscles are used to the point of fatigue, and the body requires high amounts of energy quickly, such as during strenuous exercise. The enzyme lactate dehydrogenase (LDH) facilitates the conversion of pyruvic acid to lactic acid, enabling the recycling of the enzyme NAD+ from NADH, which is essential for glycolysis to proceed.
The process of glycolysis can only be sustained for approximately one minute of intense muscle activity and is inefficient, with a net gain of two ATPs per molecule of glucose. The production of lactic acid, a byproduct of this process, can contribute to muscle fatigue. Muscles switch to cellular respiration when sufficient oxygen becomes available, allowing for the more efficient production of ATP and less accumulation of lactic acid.
It is important to note that while lactic acid accumulation was once thought to cause muscle stiffness, recent research disputes this claim. Instead, muscle soreness is now believed to be due to microscopic damage to muscle fibers.