While lactic acid may play a role in fatigue its supposed role in muscle soreness has been disproved and it is now being recognized as more of a positive player in metabolism. George A. Brooks has described lactic acid as a key substance used to provide energy, dispose of dietary carbohydrate, produce blood glucose and liver glycogen, and promote survival in stressful situations.
Muscle glycogen is one of the main energy sources for exercise. In order to be utilized, stored muscle glycogen must be broken down into glucose, a process known as glycolysis. During glycolysis, each glucose molecule is cleaved into two pyruvic acid molecules, and energy is released to form adenosine triphosphate (ATP). Normally, the pyruvic acid enters the mitochondria (the principal cell sites where energy is generated) and undergoes the oxidative stage of glycolysis to produce yet more ATP. However, when there is not enough oxygen present for this reaction to take place, the pyruvic acid transforms into lactic acid. From this point, lactic acid can diffuse out of the muscle cell into the blood. It is by this process (known as anaerobic glycolysis) that muscle glycogen can be converted into energy without the presence of oxygen as opposed to ATP production via aerobic glycolysis. Such a conversion allows glycolysis to proceed for minutes, when it could otherwise last only seconds. Thus, energy is supplied to promote survival in stressful times.
Once sufficient oxygen is restored, the lactic acid produced via anaerobic glycolysis can be utilized for energy or reconverted into glucose by the liver and other tissues (a process known as oxidation). This brings us full circle, and the rest of the metabolic functions as quoted earlier from Brooks have been met. This process also applies to the world of exercise.
In exercise, human bodies use energy for the purpose of muscle contraction. To accomplish this, both aerobic and anaerobic energy-producing systems need to function. Regardless of the system, lactic acid is continuously being formed and removed, even at rest. Studies show that during aerobic glycolysis lactate production seems to increase in proportion to our metabolic rate.At some point, depending on exercise duration and intensity, a workload will be reached in which lactate concentration is greatly magnified. This is known as the lactate threshold and can usually be elicited between 50-80 percent of a person's maximal oxygen consumption, VO2max.It is at this point in which the rate of lactic acid appearance becomes greater than the rate of disappearance (1, 10). This manifestation will often occur in anaerobic activities such as the 400 meter dash, 100 meter swim, tennis, or soccer. What is the significance of this fact?
When lactic acid accumulates in the cell following anaerobic glycolysis, there is potential for problems. It is necessary to maintain the proper degree of acidity in the cell because when acidity increases important contractile and metabolic functions are hindered. In the case that acidity is not regulated, the accumulation of lactic acid may be a factor in fatigue.
Although active recovery decreased lactic acid levels faster, it may also further deplete the glycogen stores that need replenishment. Therefore, a combination has been suggested whereby active and passive recovery are utilized together to decrease lactic acid levels while promoting maximal glycogen resynthesis. In other words, the athlete should warm down until normal rates of breathing return and then rest. At this time, a high carbohydrate meal should be consumed to help replace the glycogen stores, which have been depleted through exercise.
In summary, lactic acid is not a useless metabolic by-product. It can serve as a very important and useful energy source. However, if the lactate threshold is reached during exercise, excessive lactic acid can accumulate, causing fatigue. Fortunately, this negative effect can be partially offset by proper training, warm down, and a high carbohydrate diet.