Why do our muscles pain after exercises?

Our body needs energy to do any work. The energy is provided to the muscles by breaking down of the carbohydrates in the food by the oxygen.

When we exercise strenuously, the blood cannot deliver oxygen fast enough and so carbohydrate is broken down without oxygen, also known as anaerobic respiration.

This produces a by-product called lactic acid. The pain in the muscles is due to this lactic acid.