Final answer:
During the first 30 minutes of heavy exercise, lactic acid accumulates due to rapid anaerobic metabolism. Muscle glycogen provides a rapid source of energy before fats become significant, and oxygen consumption is high to try to meet energy demands.
Step-by-step explanation:
For the first 30 minutes of heavy exercise, it is true that lactic acid accumulates in the muscles. This is because during high-intensity exercise, glucose is rapidly metabolized anaerobically through glycolysis, leading to the production of lactic acid when the required oxygen for aerobic respiration is not sufficiently delivered to the muscles.
Initially, the body uses creatine phosphate for immediate energy, followed by muscle glycogen to produce ATP (adenosine triphosphate). The break down of glycogen provides energy more slowly compared to creatine phosphate, but is rapid enough to sustain approximately 90 seconds of activity. Furthermore, within the first two minutes of exercise, the body starts to supply oxygen to the working muscles, allowing the onset of aerobic metabolism. However, during heavy exercise, oxygen delivery may not keep up with demand, hence continuing glycolysis leads to the accumulation of lactic acid, contributing to muscle fatigue.
While glycogen stores are not immediately depleted in the first 30 minutes, they begin to diminish as exercise continues. Fatty acids gradually become more significant as an energy source as the intensity of exercise continues at a moderate level and if glycogen stores start to fall. However, within the initial half-hour of heavy exercise, fatty acids are not yet the primary energy source, and oxygen consumption is certainly not low due to the body's efforts to meet the elevated metabolic demands.