Lactate

Milk acid

Lactate is predominantely produced during anaerobic metabolism of glucose. That is that at a certain point during exhausting activity oxygen delivery can't meet the requirements of the oxygen consuming process of glucose metabolism in the musculature. For a short period, our body is then able to produce further energy by the anaerobic metabolism (without oxygen)
of glucose (glycolysis), producing lactate. If the rate of lactate production exceeds the rate of its removal, the lactate concentration increases accompanied by a drop of the pH value within the muscle cell, due to a separate reaction (hydrolysis of ATP). This acidosis allows an easier transfer of oxygen from the blood.