119 A Christian Guide to Body Stewardship, Diet and Exercise into the mitochondria of the muscle cell and enter the Krebs cycle (next step after glycolysis used in the production of ATP). If the energy demand is great, such as with resistance training and sprinting, then pyruvate is converted into lactate. If the energy demand is not as great and oxygen is available in sufficient quantities, such as with walking, jogging, and riding a bike, then pyruvate is shuttled from the sarcoplasm to the mitochondria where it enters the Krebs cycle. It is sometimes mistakenly said that lactic acid is formed from pyruvate during high intensity exercise. However, due to the pH in the muscle as well as some of the previous steps in the glycolysis process, lactate – not lactic acid – is produced (Haff & Triplett, 2016). Oxidative system. The oxidative system is the primary source of ATP production during rest and low-intensity activities. Although carbohydrates and fats are the preferred substrates, the oxidative system can also metabolize protein. At rest, roughly 70% of the ATP production comes from fat and 30% from carbohydrates. As exercise intensity increases, however, there is a shift from fats to carbohydrates. In fact, during high-intensity activity virtually all of the ATP produced comes from carbohydrates. During long duration low activity exercise, both fats and carbohydrates are used to produce ATP. The percentage of contribution coming from carbohydrates and fat is based on exercise intensity, duration, and substrate availability. As mentioned previously, all three energy pathways are always active and contributing to some extent to the overall production of ATP. Therefore, it is recommended to train and develop each of the different energy systems individually. Short distance sprints will help to develop the phosphagen system; longer distance sprints will help to develop the glycolytic system; and long duration, low-intensity exercise will help to develop the oxidative system. Figure 6.1. depicts when the different biological energy systems are used as well as the ATP production capacity of each. Table 6.4 provides recommendations for how to train each of the different energy systems. Figure 6.1. The 3 Biological Energy Systems Used During Exercise
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