One of the major advantages to lactate-threshold training: the intensity represents the fastest pace at which runners can train without excessive fatigue because the workouts remain aerobic. Lactate-threshold training is the best aerobic bang for your buck.
What Is Lactic Acid?
Lactic acid was first discovered in 1780 in sour milk. In our bodies, it is produced in a metabolic pathway called glycolysis, a series of nine chemical reactions that breaks down glucose to provide energy (ATP) for muscle contraction. Pyruvate, the final product of glycolysis, has two fates: (1) conversion to Acetyl-Coenzyme A and entry into the Citric Acid Cycle (Krebs Cycle) or (2) conversion to lactic acid. The latter fate occurs when oxygen is not supplied fast enough to meet the needs of the muscle cells, as has been known since the 1920s, when Nobel Prize winners A.V. Hill and Otto Meyerhof discovered that lactic acid is produced during fatiguing muscle contractions in the absence of oxygen. When lactic acid is produced at the pH of our body fluids, it immediately releases a proton and thus exists as the molecule lactate rather than as its acid form.
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Since the time of Hill's and Meyerhof's groundbreaking finding, lactic acid has been the exercising community's scapegoat for fatigue. But there has never been any experimental evidence that has shown a cause-and-effect relationship between lactate production and fatigue. Although it has been widely accepted by the scientific community for a long time that lactate is innocuous and is not the cause of fatigue or muscle burning during intense exercise, lactate still takes the blame and still is regarded by runners as the enemy. No physiologist has ever burnt himself when taking a blood sample from a subject containing a high blood lactate concentration. Indeed, lactate does not cause fatigue, and its production in muscle is vital during intense exercise, as it serves a number of roles.
Lactate production maintains the ratio of certain biochemical molecules, supporting the continued ability of glycolysis to keep working. Lactate is also used as a fuel by the heart, by the liver to make new glucose by a process called gluconeogenesis, and is converted back into glycogen (the stored form of carbohydrate) by a reversal of the chemical reactions of glycolysis. Both the new glucose and glycogen are then themselves used as fuels by muscles so high-intensity exercise can continue.