What happens to pyruvate molecules formed in glycolysis in the absence of oxygen?

You already know the answer to this question, because when you exercise aggressively, your muscles get sore. The pyruvate is converted to lactic acid instead of shutting to Acetyl-CoA and the tricarboxylic acid cycle (also called the Krebs cycle). Anaerobic glycolysis (in the absence of oxygen) is the major fate for pyruvate in red blood cells, lens and cornea of the eye, kidney medulla, testes, and leukocytes as well.  Hope it works