Final answer:
During exercise, the primary energy source for the glycolysis lactic acid system is glucose, which breaks down anaerobically to produce ATP and lactic acid when oxygen levels are low, leading to muscle fatigue.
Step-by-step explanation:
The main energy source for the glycolysis lactic acid system during exercise is glucose which may come from blood glucose or the breakdown of glycogen stored in the muscle. During exercise, when the availability of oxygen is limited, glycolysis is the pathway that breaks down glucose to produce ATP anaerobically. This process results in the formation of pyruvic acid, which is then converted to lactic acid when oxygen is not sufficient. The production of lactic acid helps in the recycling of NAD+ from NADH, which is necessary for glycolysis. Lactic acid accumulation can contribute to muscle fatigue.
Upon vigorous muscular contraction, the skeletal muscle primarily undergoes anaerobic glycolysis. This is efficient for short bursts of high-intensity activity, but not for prolonged exercise as the net gain of ATP is low (2 ATPs per molecule of glucose) and the accumulation of lactic acid can lead to muscle fatigue. As exercise intensity decreases, the body shifts toward more efficient aerobic pathways, utilizing both glycogen and fat reserves, depending on the intensity and duration of the activity.