118 Chapter 6: Training for Endurance The phosphagen and glycolytic systems are anaerobic systems (not requiring the presence of oxygen) that occur in the sarcoplasm of muscle cells. The oxidative system is an aerobic system (requiring the presence of oxygen) that occurs in the mitochondria of muscle cells. All three macronutrients (i.e., carbohydrates, protein, fat) can be used to produce ATP, but only carbohydrates can be metabolized without the presence of oxygen. As a result, carbohydrate availability is crucial during anaerobic metabolism. Table 6.2 depicts the characteristics of the three biological energy systems (Haff & Triplett, 2016; Walters & Byl, 2013). Table 6.2. Characteristics of the Phosphagen, Glycolytic and Oxidative Energy Systems Phosphagen Glycolytic Oxidative Exercise Duration 0-10 sec. 11-120 sec. > 2 min. Exercise Intensity Very High High Low to Moderate Rate of ATP Production Immediate Rapid Slow Fuel ATP Muscle Glycogen / Blood Glucose Stored Carbohydrate and Fat Oxygen Used? No No Yes 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 6.3 depicts the percent contribution of anaerobic and aerobic metabolism based on exercise duration (Haff & Triplett, 2016). Table 6.3. 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
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