When an athlete exercises too hard, there isn't enough oxygen delivered to the muscle, resulting in anaerobic respiration.
So, when glucose reacts, lactic acid is formed as a by-product.
It is a fast process.
The production of ATP is very fast.
Anaerobic respiration has the advantage of being faster than aerobic respiration and not requiring oxygen for energy conversion.
In muscles, for example, when oxygen supply is insufficient, pyruvic acid is converted to lactic acid in order to provide energy during severe activity.