Final answer:
When oxygen is insufficient during intense exercise, lactic acid is produced as a result of anaerobic glycolysis, leading to muscle fatigue.
Step-by-step explanation:
If there is not enough oxygen available when sprinting or when first beginning to exercise, the byproduct that is created is lactic acid. During anaerobic conditions, such as high-intensity exercise, the body relies on glycolysis for energy. Glycolysis converts glucose into pyruvic acid, which is then converted into lactic acid if oxygen is not sufficient. The buildup of lactic acid may contribute to muscle fatigue. This process is a way for the body to continue producing ATP, albeit less efficiently, to maintain energy supply for muscular contractions since oxygen levels are inadequate for aerobic respiration. Aerobic respiration, on the other hand, would use pyruvic acid in the mitochondria to produce ATP, water, and carbon dioxide when oxygen is present.