92 Chapter 5: Training for Endurance Phosphagen system. The phosphagen system is involved at the beginning of all activity regardless of exercise intensity and provides energy by breaking down ATP and creatine phosphate (CP) (an important compound that donates its phosphate to adenosine diphosphate (ADP) in order to make ATP) stored in the muscle cell. This process continues until exercise stops or the intensity is low enough to allow glycolysis or the oxidative system to take over. The amount of ATP and CP stored in the muscle is relatively small, which explains why the phosphagen system can only provide energy for a short period of time. The ability to store more ATP and CP in the muscle is likely one of the reasons why some individuals are able to sprint faster and longer than others. Glycolytic system. The process of breaking down carbohydrates involves multiple catabolic reactions and thus is why the glycolytic system is not as fast at producing ATP as the phosphagen system. However, because there is greater supply of glycogen and glucose in the muscle as compared to ATP and CP, the duration of energy production in the glycolytic system is significantly longer than that of the phosphagen system. The end result of glycolysis is pyruvate. Depending on exercise intensity and the availability of oxygen, pyruvate will either be converted to lactate (substance created when glucose is broken down for energy during intense exercise) or shuttled 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 5.1. depicts when the different biological energy systems are used as well as the ATP production capacity of each. Table 5.4 provides recommendations for how to train each of the different energy systems.
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