What process causes athletes to experience a burning sensation in their muscles?
However, if there is not enough Oxygen, this Pyruvate will be converted to Lactic Acid in an alterantive pathway to keep producing energy for exercise. As Lactic Acid starts accumulating in the muscle cell, it will be expelled outside into the blood stream to be utilized by the Liver.
Depending on the intensity of the exercise and the efficiency of the person's blood circulation, clearance of Lactic Acid from the muscles' area might not be fast enough and Lactic Acid will start accumulating in that area, forming crystals.
This crystallized form of Lactic Acid with the addition of the increased blood flow into the muscles will result in the "pain burning sensation" everyone gets through exercise.
I hope that fully answered your question.