Answer your question first: the burning sensation is due to the accumulation of lactic acid. Lactic acid is not the only way to produce effect, which can reduce the intensity and increase the frequency.

Reason: First of all, the human body has three energy supply systems: phosphate system, glycolysis system and oxidation system. The intense burning sensation of abdominal muscles is the reason for the rapid glycolysis of glycolysis system. When you exercise, you need a substance: ATP. Fast glycolysis decomposes carbohydrates into ATP to provide energy, and its final product acetone is converted into lactic acid. Lactic acid is the cause of pain. The more lactic acid accumulates, the more severe the pain will be. As for why lactic acid accumulation hurts, I have to talk about the nervous system and human self-protection. I won't say this for long.