The cells did not have enough oxygen, so they used fermentation as a source of energy.
Explanation:
Anaerobic respiration:
When oxygen supply is not enough to body mostly occurs during running this process is called anaerobic respiration.
During the race, body muscles do not have enough oxygen to create energy and it break down the glucose (sugar) and it produces not only CO2 and water but also excessive amount of lactate. Lactate is much difficult to reconvert back and due to lactate, hydrogen ions created and can not removed due to lack of oxygen and it causes burning feel in the muscles.
