Question

Why does lactic acid fermentation "kick in" during a sprint race?

Answer 1

Lack of oxygen

Explanation:

Normally when cellular respiration occurs cells use oxygen to break down food and make energy. Mitochondria is very efficient in that it can do this without oxygen, but the byproducts of the process will be lactic acid instead. When you exercise you're working the cells pretty hard. If they don't get enough oxygen molecules you get lactic acid and a lot of it in the blood which causes burning. Normally it's great that respiration can keep going. It's just a (literal) pain the byproduct is uncomfortable.