Noakes, T. (1986). Lore of running. Cape Town, South Africa: Oxford University Press.

Mitochondria produce ATP. When they increase in size and number as a result of aerobic training, exercise can be prolonged for a much greater period of time than in an untrained state. Changes in the mitochondria only occur in trained muscles. That results in the absolute specificity of endurance training within an activity. This phenomenon does not support any form of cross-training or transfer of peripheral training effects that result from other forms of aerobic work.

Enzymatic increases that occur within the mitochondria as a result of aerobic endurance training are as follows: 1) those associated with the Kreb's cycle and respiratory chain; 2) those associated with the shuttle systems that transfer protons developed through glycolysis into the mitochondria for use in the respiratory chain; and 3) those associated with fatty acid metabolism (by 200 to 400%). This latter feature is important because it permits the body to use more available fats for energy production, that is, more fat is extracted from normal blood to fuel exercise.

Except at the very highest levels of exercise intensity when carbohydrates serve as the main source of fuel, the oxidation of free fatty acids by the mitochondria takes precedence over all other forms of energy supply. This substitution of fats over glycogen use as fuel as a consequence of aerobic training is known as "carbohydrate-sparing." Glycolysis is reduced so the rate of pyruvate production is decreased which results in less lactic acid being produced for a given level of exercise intensity. That translates into athletes being able to perform longer and more intently without incurring any significant increase in lactic acid production. This is known as shifting the lactate turnpoint or anaerobic threshold, a phenomenon that occurs through aerobic training.

Changes in VO2max increase within a week of exposure to a constant aerobic training stimulus. After three weeks of exposure the stimulus no longer overloads the system resulting in the cessation of aerobic capacity development. It is good practice to increase the overload factor in aerobic endurance training every 7 to 14 days to allow an athlete to progress at an optimal rate of adaptation.

When training for aerobic endurance ceases, there usually is a rapid fall in capacity within the first two weeks and then the decline is more gradual.

In athletes who have an extensive history of aerobic training (a considerable number of years) the mitochondrial regression occurs at a much slower rate than that which is demonstrated by individuals without a good training background.

Once an athlete's aerobic capacity has been developed fully for a specific sport, the adaptation level can be maintained with less training. As long as the intensity of the training stimuli remain the same as that which existed in the last stage of change training, the number of aerobic training sessions can be reduced to one third of the change-training amount without any diminution in aerobic capacity.

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