Why does serum lactate concentration increase during intense exercise in normal individuals?

Skeletal muscle has a variable requirement for energy (ATP) which depends on how hard the muscle is working. During severe exercise, when demand for ATP is high, the muscle can become temporarily starved of oxygen, which prevents ATP production through oxidative phosphorylation. Instead, the muscle depends on anaerobic glycolysis, supported by using the muscle glycogen stores. To maintain glycolytic flux, NAD+ is regenerated from NADH (produced by 3-phosphoglyceraldehyde dehydrogenase at step 6 of glycolysis) by reducing pyruvate to lactate. This produces large amounts of lactate, which is carried by the blood to the liver where it is converted to glucose via gluconeogenesis (the Cori Cycle/krebs cycle).