Chapter 5: Training for Endurance 98 It is important to emphasize that all three energy systems are always active. The extent to which each system is active and contributing to ATP production is dependent upon exercise duration and intensity. Table 5.3 depicts the percent contribution of anaerobic and aerobic metabolism based on exercise duration (Haff & Triplett, 2016). Table 5.3. Percent Contributions of Anaerobic and Aerobic Metabolism 0-5 sec. 30 sec. 60 sec. 90 sec. 150 sec. 200 sec. Anaerobic 96 75 50 35 30 22 Aerobic 4 25 50 65 70 78 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).