The role of lactic acid in athletic performance is one that is widely misunderstood. Lactic acid, or lactate, is a natural byproduct generated through the production of energy in the body, and is produced by the body at all times. The relationship of lactic acid to the ability of the body to perform must be assessed in two parts: the function of the lactate itself, and the adverse effects of the hydrogen ion produced in the reaction that creates lactic acid.
Lactic acid is formed through the metabolism of the carbohydrate energy source glucose during the production of energy in the cell. The ultimate energy fuel is adenosine triphosphate (ATP), which is produced in the cell. Lactic acid in the cell will itself metabolize into ATP, a process by which energy can be produced without oxygen, known as the anaerobic lactic energy system. This is the energy production mechanism utilized by the body for athletic events that are generally less than 90 seconds in duration. The method by which lactic acid is broken down to produce ATP is much quicker than the aerobic processes, those that require oxygen to be delivered by way of the cardiovascular system.
The acidic aspect of lactate is due to the presence of a hydrogen ion in the lactate molecule. The ion is the portion of an atom that creates an electric charge. This ion is itself a byproduct of the production of lactic acid, and the ion is the cause of the muscle problems frequently associated with lactic acid presence in the muscles. Lactic acid will begin to accumulate in the muscles when the athlete begins to operate above the anaerobic threshold, which is generally accepted as representing 80-90% of the maximum heart rate of the athlete. This ion will eventually make the blood sufficiently acidic that the breakdown of the energy source glucose is slowed, another negative impact on athletic performance. In these circumstances, athletes will often complain of a burning sensation in their working muscles, a condition that is sometimes accompanied by difficulties with muscle coordination or movement. These symptoms may persist after a high intensity event for as long as 48 hours. These physical consequences are attributable to the hydrogen ion and not the lactic acid itself. For this reason, lactic acid is mistakenly regarded as a waste product.
In addition to its initial role in ATP conversion, lactic acid will continue to recycle as a source of ATP. It does not remain pooled or stored in the muscles as waste, for as long as the muscles create emands for energy, lactic acid will be converted into ATP. The greater the demands for energy, to the point where the body cannot rely on stored fatty acids as a fuel source, the more readily carbohydrates will be converted, meaning the greater the amount of lactic acid available for conversion. Lactic acid is not necessarily used at the muscle location in the body where it was first generated; lactic acid can be transported through the bloodstream to a destination within the body where it is needed for ATP production.
Depending on the body's needs at a particular time, lactic acid is also capable of being converted into glycogen, the storage form of glucose, in the same fashion that blood glucose is stored, to be maintained in the liver and released into the bloodstream when required.
In endurance races such as cross-country skiing, marathon runs, or long distance cycling, the phenomenon referred to as "second wind" is a result of lactic acid effect. In the early portions of such activities, the body will obtain most of its energy from carbohydrate sources. This in turn causes the production of large amounts of lactic acid; in the latter stages of the endurance event, the lactic acid becomes an important fuel source. Certain organs of the body have a preference for lactic acid as a fuel source during endurance events, with the same bias that the brain and the central nervous system rely on carbohydrate sources for their energy needs. These organs and tissues include the heart, slow-twitch muscle fibers, and the muscle associated with breathing, the diaphragm.
Lactate is used in some sport and energy drinks due to its quality as a fast-acting fuel source. The lactic acid serves as a glycogen replacement source in the recovery phase from an endurance event. The recovery from intense exercise is also enhanced through the implementation of high intensity interval training, which serves to increase the speed with which lactic acid is recycled in the bloodstream. Interval programs will also assist the body in the speed with which the hydrogen ion associated with muscle dysfunction is flushed from the system.
