Glycolysis

ATP (adenosine triphosphate) is the high-energy compound for cells. We have a limited storage of immediately available ATP, so it must be generated quickly during exercise. The higher the exercise intensity, the faster the ATP must be regenerated. In steady-state, low-intensity activity, ATP can be adequately produced aerobically from the oxidation of carbohydrate and fat. However, as exercise intensity increases, athletes need a level of ATP production that cannot be fully supplied aerobically.

Glycolysis is the process through which a high volume of ATP can be produced through the breakdown of glycogen to glucose; it can occur in the presence of oxygen (aerobic glycolysis) or without oxygen (anaerobic glycolysis). Aerobic glycolysis has the capacity to produce more ATP than anaerobic glycolysis and, unlike anaerobic glycolysis, can do so without producing lactic acid. For this reason, anaerobic glycolysis is also referred to as the lactic acid system. In activities where the intensity exceeds the capacity to bring sufficient oxygen into the system to meet energy demands, anaerobic glycolysis becomes the major pathway for ATP production. However, extremely high-intensity anaerobic activities are self-limiting because the lactic acid buildup permits activity to continue for a maximum of only 1.5 to 2 minutes. It is typical for high-intensity sports to have opportunities for recovery. For instance, the artistic gymnastics floor routine is 1.5 minutes long, after which the gymnast can rest and recover to prepare for the next high-intensity event; and hockey players are substituted frequently (a hockey player almost never skates continuously for more than 2 minutes) to allow for muscle recovery.

The lactic acid produced in anaerobic glycolysis can best be considered a form of stored energy, just waiting for sufficient oxygen to reenter the system. When exercise intensity is reduced and the athlete has enough oxygen in the system for aerobic metabolic processes, the lactic acid is converted to pyruvic acid and used to produce ATP aerobically.