If oxygen is not available to a muscle cell, pyruvic acid is changed to and builds up in the muscle cell.

The reduction of pyruvate forms a molecule called LACTATE.  When human muscle cells doesn’t consist enough oxygen to convert organic molecules into ATP through cellular respiration, then it is called LACTIC ACID FERMENTATION. Instead of entering the mitochondria and being converted to acetyl CoA, pyruvate is reduced an electron is given, which is also a product of glycolysis.